Blewbury Energy Initiative - Domestic Energy Generation

There are other ways of gaining energy from renewable sources that might be considered for domestic installations in a village such as Blewbury.

Small wind turbines are appropriate in some locations. Although subsidised under the Feed-In Tariff, their use in Blewbury is not likely to be attractive because of low average wind speeds, often reduced further due to screening by trees – the energy available depends on the cube of the wind speed. There are also planning restrictions. Any viable wind turbine would need to be high on the downs.

The power provided by water turbines depends on the flow of water and the distance it falls to drive the turbine. The only places where generating electricity from such a turbine might be viable is where there used to be water mills. Even there the energy that could be generated, for example by the Mill Stream, is quite low. Small-scale hydropower is included in the Feed-In Tariff.

In the UK, the most common option for small-scale electrical generation which is not needed immediately is to export it to the grid. Other possibilities are to charge batteries or heat water. A wide variety of other ways of storing energy are not viable for small installations.

Thus solar thermal and solar photovoltaic installations, and to a lesser extent wood burning boilers and heat pumps, are likely to be the best options for domestic renewable energy generation for houses in Blewbury at present.

In evaluating the economic results of an investment in green energy it is necessary to consider the cost of the capital used. If an installation lasts 20 years and the money invested could have earned interest after tax of 3%, an appropriate annual cost of the capital is about 6.7%. If one takes the view that the current low interest rates will not increase much over 20 years, and interest after tax is likely to average 1%, the annual cost of capital reduces to 5.5%. If we also assume the lifetime of the system is 30 years the cost of the capital becomes 3.9%.

The new Feed-In Tariff will pay different rates for different types of system, but the overall picture seems generous – most notably because unlike such tariffs in other countries it pays for all the electricity generated, not just what is exported to the grid. The highest rate goes to solar photovoltaic systems on existing houses, where the rate is 41.3 pence per kilowatt-hour (kWh). In addition, electricity exported to the grid receives 3 pence per kWh for all types of systems.

It is also necessary to assume a tariff for electricity imported from the mains. On this page a figure of 13 pence per kWh is assumed. In 2009 the current price has dropped to around 12 pence, but many people think this cost will increase significantly in future.

The last figure required for assessing the economic return from electricity generation is the price paid for electricity exported to the grid. The Feed-In Tariff sets a figure of 3 pence per kWh on top of the generation tariff.

The programme of grants that was run by the Low Carbon Buildings Programme for renewable microgeneration of electricity and for heating (e.g. solar thermal panels) has now ended.

The boiler scrappage scheme run by the Energy Saving Trust aims to replace the least efficient central-heating boilers (G-rated) by A-rated boilers or renewable heating such as biomass boilers or heat pumps. There is more information here.

This section covers the options for the generation of energy from renewable sources on a domestic scale.

Wood requires more space for storage than other fuels. Obviously it is necessary to obtain the wood and have room to store it. The fuel options for domestic use are logs, wood chips or wood pellets. It is important to avoid burning painted wood, chipboard or other processed timber as these give off noxious gases.

Log fires are attractive but labour intensive. The work required can be minimised by using the fire to heat a large quantity of water, which then supplies the heat required for the dwelling.

Wood chips can be produced fairly easily from wood. Their advantage over logs is that they can be fed automatically into the boiler.

Wood pellets provide a consistent fuel, with a price per kilowatt similar to that of mains gas. They can be used in automatic boilers which take fuel from a hopper, and only require the ash removed every few days, or in some cases a few times a year. They provide more energy per unit weight than wood chips or logs, partly because they have been dried to reduce the water content. This is typically 8% against the 22% or so normal for untreated wood.

As wood is a fairly rare choice for heating, it is important to arrange for supply before committing to it for fuel. If you are using wood because of its green credentials it should come from a sustainable source rather than, for example, virgin forest. It is also desirable to ensure that the wood does not have to be transported too far. Wood pellets are common in Scandinavia and the USA, but they are much rarer in the UK. Welsh Biofuels is a well established supplier but rather far away. The main customers of local suppliers TV Bio Energy Ltd and Biojoule Ltd are power stations, industrial units and schools. Logpile provides a list of suppliers in Britain. However some of these import their supplies.

Although wood for burning is competitive in cost with other fuels such as oil and gas for the same heat output, the stoves and boilers are about twice the price. Home Sources provide a list of stove suppliers.

The National Energy Foundation provided a good summary: "Wood is a renewable energy source because the carbon dioxide emitted when the wood is burned has been taken out of the atmosphere by the growing plant. Even allowing for emissions of fossil carbon dioxide in planting, harvesting, processing and transporting the fuel, replacing fossil fuel with wood fuel will typically reduce net carbon dioxide emissions by over 90%."

Wood stoves and boilers can be a good alternative to fossil fuels, provided you can store the fuel and manage the extra work. The installation is more expensive, but the running costs are likely to be similar to those of fossil fuels. Boilers, but not stoves, may be subsidised from 2011. However, it is important to establish a reliable and renewable source – ideally local – for the fuel chosen.

Solar thermal panels are panels facing the sun which heat water, most commonly for domestic hot water. When the panels are hot enough, water or another fluid is pumped through them and then through a coil in a hot water tank. Installations are designed to provide nearly all of the hot water in summer (except on the greyest days), and to contribute to water heating during the rest of the year. By storing the hot water in an insulated tank, for use at night or the following day, the system provides a way to store solar energy. The panels are not practical for central heating in the UK as the panels do little in winter.

There are two main varieties of solar thermal panels being installed on houses at present. Evacuated tubes are the most efficient (45–50%). However, the alternative flat plate panels are less expensive, and a larger area of flat plates with lower efficiency (35–40%) may be preferable if roof space allows.

In Oxfordshire the solar energy falling on a square metre of roof, facing south and angled at 30 degrees, is about 1100 kWh per year. A typical dwelling uses perhaps 2500–3500 kWh a year for water heating. Four square metres of solar panels, without any screening from trees or other obstacles, would receive about 4400 kWh of solar energy. It is difficult to capture more than 35–45% of this usefully because of the heat collection efficiency of the panels, heat loss from the storage tank, and the fact that in summer some of the heat will be produced when it is not needed. If the solar panels must face east or west, a larger area of solar thermal panels is needed for the same effect.

If the water would otherwise have been heated by electricity at 13 pence per kWh, a saving of 1750 kWh would be worth about £225. If the alternative heating was by mains gas, the saving might be around £75. Under the proposed Renewable Heat Incentive there would be an annual subsidy of perhaps £300 – details are not yet final.

The cost of an installation in a modern house with a normal boiler is likely to be £3500–£4000 for a commercially installed flat-plate system, and perhaps £1000 more for evacuated tubes.

Where the building and site make them practical, solar thermal panels provide an effective way of saving carbon dioxide in supplying domestic hot water. Under the proposed government subsidy they are also a reasonable investment, with a payback time on the order of ten years or less.

Standard solar thermal panels do not normally need planning permission. However, permission is needed for all installations on listed buildings, and is also needed within a conservation area if visible from a public road or path. Always check with the Vale of White Horse planning department for such installations.

Solar heating is also used for warming swimming pools. This requires less costly solar thermal panels as the output temperature required is lower.

Solar photovoltaic (PV) systems, or solar cells, convert the energy of sunlight into electricity. The output of each cell is a low-voltage direct current, but this is usually converted by a device called an inverter into 240 volt alternating current to supplement the electricity mains. Since domestic demand for electricity does not usually match bright sunshine, in the UK surplus electricity not needed in the building is typically fed into the grid rather than trying to store it locally. An export meter enables payment for this.

Solar cells are provided in panels which ideally should be mounted at about a 30–35 degree angle facing south, with no shading by trees or other buildings.

Photovoltaic cells are less efficient in extracting solar energy than thermal panels. The current silicon-based cells have a theoretical maximum efficiency of about 28%, partly because they only extract energy from part of the sun's spectrum. Practical efficiencies at present are about 12.5–18%, so the area covered needs to be about three times that of thermal panels collecting the same amount of useful energy. In addition, the cost per unit area is greater. As a result, in the past domestic PV installations have been harder to justify on strictly financial grounds than solar thermal systems. But the new Feed-In Tariff will pay a generous subsidy that changes this. The justification is that the energy generated is electrical, which is more valuable than heat. Also note that a PV installation does not need a pump and special plumbing, and so is likely to need little maintenance.

A domestic-sized solar PV installation might be rated to generate from 1 to 3 kilowatts of energy in full summer sun of 1000 watts per sq. metre; this rating is called kilowatts-peak, or kWp. This typically requires about 7–8 sq. metres of solar cells per kilowatt. On an open south-facing location in Britain a 1.8 kWp installation is officially estimated to collect on average 1440 kWh a year. However, Oxfordshire is relatively sunny and southern. A local installation company estimates 1710 kWh per year, and a 1.8 kWp installation in Blewbury has generated 1770 kWh, 25% more than the official estimate, in a year (2009) that was not especially sunny.

A 1.8 kWp system costs roughly £8000 at present. Results from a full year of generation at the house mentioned above, which has two people living in it, are shown in the diagram at right. More than half the locally generated electricty, 1050 kWh, was exported to the grid, and 2170 kWh was imported. The 720 kWh generated and used locally saves £93. Under the new Feed-In Tariff the household would receive 41.3 pence per kWh for its entire generation of 1770 kWh, so £730. In addition, the 1050 kWh exported would receive a further 3 pence per kWh or £31. Thus the total of electricity savings and subsidy would be about £850 per year. A household with more people in it, and perhaps less frugal in its electricity usage, would export less but this makes little difference.

Where the building and site make them practical, solar photovoltaic cells provide an effective way saving carbon dioxide in generating electricity. Under the new government subsidy they are also a fairly reasonable investment, with a payback time of perhaps as little as ten years.

A solar PV system will have a lifetime of perhaps 25 years. Solar cells use considerable energy to make, so in the UK they need to be run for about 3–5 years before they have generated the energy used in their construction. However, thereafter they are benefiting the environment.

Wind turbines extract energy from the wind and are normally designed to generate electricity. However, similar devices are used for pumping water, and for the direct generation of hot water.

With small devices a fairly high wind speed is required for electricity generation applications. They are thus most suitable for remote, fairly windy locations free of trees.

Small wind turbines to generate electricity for household use might generate between 1 and 3 kilowatts at maximum. They cost from £3000 to £10,000 and require regular maintenance.

Average wind speeds in central England are lower than in the rest of the United Kingdom, and Blewbury is in a location sheltered by trees, and by the downs in the south. The lower wind speed is particularly important as the energy available increases as the cube of the wind speed. Thus the energy available from a wind installation is likely to be a third of that provided by the same installation in open country elsewhere in the UK, and an eighth of that available on a windy hill or ridge.

Wind turbines work best in a steady breeze. If a small turbine is mounted on a roof, the turbulence generated by the roof may significantly reduce the power available, vibration can lead to structural problems, and there may be problems with noise. The need for occasional access to the turbine for servicing should also be considered. In some cases the turbine will generate an unacceptable flicker with the sun in some directions.

It is unlikely that a wind turbine would be a sensible investment in a Blewbury village garden or roof.

Water power has been used for over a thousand years for milling grain, but recently has fallen into disuse. Now water turbines can be used to generate electricity provided there is a suitable flow and head of water.

The energy available from water flowing at 1 cubic metre a second through a head of 1 metre is about 10 kW. As the system cannot be 100% efficient a more typical output would be 5–7 kW. Systems are likely to be most viable if the energy available is much larger than this, say 50 kW. This either requires the high heads available in mountainous areas or large flows from rivers. As both kinds of site require sensitive treatment, systems are usually individually designed and require official approval, for example from the National Rivers Authority. This makes such installations take a long time and to be fairly expensive to develop. An example is the proposed installation at Goring-on-Thames.

Many mills, like those that existed in Blewbury, were on streams with a smaller flow and only a limited head. The Mill Stream leaving Blewbury carries water which fell as rain over perhaps 10 sq. km. With a rainfall of 600 mm a year, and allowing for evaporation, the average flow may be around 100 litres per second. It gathers into a single stream at the edge of the Thames flood plain and then falls about 8 metres over 3 miles. Small turbines are not as efficient as large ones. If a turbine could capture the whole flow with a drop of 2 metres it might generate on average around 1 kW.

The cost for such an installation depends significantly on the work required to manage the water at the specific site. The system would also require ongoing attention to clear blockages. It is unlikely that a 1 kW system could be justified financially.

The number of places where small water turbines might possibly be installed in the parish of Blewbury are much fewer than for alternative options for renewable energy. Even if they are installed wherever possible their contribution of renewable energy can only be very limited. The case for larger systems, e.g. where there were old mills on the Thames, is more attractive. Small-scale hydro systems are eligible for support under the new Feed-In Tariff.

It is unlikely that a water turbine would be a sensible investment in a Blewbury mill as the energy available is too low. Generating electricity at sites where there are larger flows and greater heads of water is more likely to be viable.

This section covers some options for increasing the effectiveness of the energy used in the home.

The design of a house or extension makes a significant difference to its energy efficiency. Good insulation is now required. Designs can capture more of the sun's energy through appropriate south-facing windows.

More complex schemes arrange that air carries heat from warmer areas, for example under tiles on a south-facing roof, to other parts of the building.

Domestic combined heat and power (CHP) installations use a conventional fuel – normally mains gas – to generate electricity and also to supply heating and hot water. While the efficiency of electrical generation does not approach that of a large power station, the fact that the heat is also used means that overall the domestic installation would use the fossil fuel more effectively.

The design aim of a domestic CHP unit is to replace a conventional gas boiler, and to require no special installation skills. The electricity generated reduces the demand from the mains, and feeds electricity back into the mains when local demand is lower. This requires an export meter.

The Feed-In Tariff includes a pilot to subsidise electricity generation by up to 30,000 micro-CHP systems, each rated at up to 2 kW of electricity. The initial rate is 10 pence per kWh generated, plus 3 pence for export to the grid, for a period of 10 years. Under the proposed Renewable Heat Incentive, CHP systems using renewable fuel (i.e. not gas) would also be subsidised for heat generation.

A major advantage of such systems could be that they can continue to power essential services such as freezers and a few lights if there is a power cut. This requires special design features, because for safety reasons such systems must not put voltage on the mains if the mains fails.

The most appropriate comparison for a CHP system would be with a modern condensing boiler. With a typical condensing boiler about 88% of the energy in the incoming gas is converted to the energy in the heated water. If the CHP system does not reach a similar efficiency the benefit of generating some of the output as electricity does not compensate for the poorer performance.

CHP installations can be sensible for factories, large offices or whole estates. CHP for domestic installations is less attractive, partly because the technology for domestic use is only just becoming available.

Domestic CHP is potentially a very significant development. If such units become available in a suitable range of sizes, at reasonable prices, and with efficiencies similar to that of condensing boilers they could become a normal installation for central heating. However, the few models available are expensive – it will be interesting to see how the first domestic-size units widely on sale in the UK perform.

Heat pumps act as refrigerators in reverse, cooling the "heat sink" while heating the house. The advantage is that, particularly if only a modest temperature is required, the amount of heat supplied can be 3 or 4 times greater than the electrical energy used to run the system. Some installations can also be used to cool the house during the summer, though this uses some of the energy that might have been saved during the winter. Heat pumps of all types (ground source, air source, the less common water source, and even geothermal heating) are included in the Renewable Heat Incentive, but their use for cooling is specifically excluded.

Ground source heat pumps extract the heat from underground, while air source heat pumps cool outside air using fan-assisted radiators like the more widely available air conditioning units.

The heat sink outside the house requires a network of pipes, with about 10 metres of pipe per kilowatt required. This can be achieved either by burying the pipes horizontally at perhaps 1.5 to 2 metres below the surface, or by using a vertical bore hole.

A typical 8 kW system costs £6400–£9600 plus the price of the heat distribution system. This can vary with property and location.

There are two types of air source heating systems. Air-to-air systems provide warm air, which is circulated to heat the building. The other type, air-to-water, heat water to provide heating to a building through radiators or an underfloor system.

A typical 6 kW system costs £7000–£10,000, but unlike ground source heat pumps there are no additional costs for the heat collection system. An air source heat pump is obviously only feasible where there is a suitable place to put the radiator unit.

The efficient delivery of warmth rather than hot water makes them most appropriate for heating swimming pools and for underfloor heating. They are less suitable for the more common heating systems that circulate very hot water to radiators, and therefore the most likely application of heat pumps is for new houses specifically designed to use them.

Where mains gas is available for heating, a heat pump relying on mains electricity may be a marginal economic choice as the fourfold efficiency gains only cancel out the fourfold higher price of electricity. The balance may be tipped by efficient use of the electricity by the pump, and by payments under the proposed Renewable Heat Incentive. The heat pump should also save about half the greenhouse gases generated by an alternative gas boiler. The carbon dioxide generated per unit of electricity in the UK is estimated to be about twice that of the same heat energy generated from gas, and the heat pump can generate about four units of heat energy for each unit of electricity used.

The best role for heat pumps is in supplying warm water, typically for underfloor heating, to new developments. Subsidies and the resulting lower prices of the equipment may turn them into an attractive option. Otherwise, the advantage in investing in a heat pump is in saving greenhouse gases.

In this section we present information on the UK's new system for supporting small-scale energy generation: the Feed-In Tariff that begins in April 2010 and pays for electricity generation, and the proposed Renewable Heat Incentive that is planned to begin in April 2011 to pay for producing heat and hot water. More detailed information may be found on the Department of Energy and Climate Change (DECC) website.

The Feed-In Tariff (FIT) aims to increase small-scale (up to 5 megawatts) electricity generation in the UK. It is hoped that by expanding the market, technologies such as solar photovoltaics which are currently quite expensive will get cheaper. The scheme runs from 1 April 2010 for at least 20 years (25 for solar photovoltaics), and covers a range of technologies.

What makes the FIT unusual, compared to schemes in other countries, is that it rewards all generation, not just what is exported to the grid. The reasoning behind this is that electricity used locally does replace energy from the grid, and is efficient because it reduces losses in transmission. It also makes consumers more aware of how they use their energy and so, hopefully, leads to lower consumption.

The FIT replaces the up-front grants from the Low Carbon Buildings Programme, which are now closed for electricity generation. The Renewable Obligations programme, which is primarily aimed at large-scale generation, will no longer cover domestic systems. Systems installed before 15 July 2009 will be transferred to the FIT, but will receive a uniform low tariff because DECC states that the FIT aims to encourage new systems, not reward existing ones.

The tariffs differ between different technologies, and for different-size systems – they are higher per kWh for smaller systems. The tariff for a particular system will be fixed at the time of installation (though later indexed for inflation), but as time goes on and the cost of the systems (hopefully) decreases the tariffs for new installations will be reduced – this is called "degression". There will be periodic reviews of the operation of the scheme and the tariffs being paid. Payments will be tax free.

As with the previous up-front grants, both the system and the installer must be approved under the Microgeneration Certification Scheme. But unlike those grants, a range of other measures to reduce energy consumption (insulation, heating controls, lighting) will not be a strict requirement for receiving the FIT.

The register of installed systems will be held by OFGEM. Payments will be via electricity suppliers.

A condensed table of Feed-In Tariffs for domestic-scale new installations follows. Exports from all systems to the grid get an extra 3 pence per kWh. Systems on output boundaries will get the higher rate. For systems installed after March 2012 the rates will start to decrease. Full details are available on the Department of Energy and Climate Change website.

Technology	Output	Tariff (p/kWh)	Tariff lifetime (years)
Hydro	≤ 15 kW	19.9	20
Micro-CHP pilot	≤ 2 kW	10	10
Solar PV (retrofit)	≤ 4 kW	41.3	25
Solar PV (new build)	≤ 4 kW	36.1	25
Solar PV	4–10 kW	36.1	25
Wind	≤ 1.5 kW	34.5	20
Wind	1.5–5 kW	26.7	20
All installed before 15/7/09	–	9.0	To 2027

The Renewable Heat Initiative (RHI) is aimed at a wide range of technologies and systems, small and large, from individual owners and landlords in both private and social housing to community groups and businesses of all sizes. The short-term target is for 15% of all UK energy consumption to be from renewable sources by 2020. The RHI will try to help with that by making renewable heating a reasonable investment. Other goals include lowering the prices for such systems by expanding their markets, and increasing the UK's energy security by reducing dependence on imported fossil fuels.

This RHI is said to be a world first. This means that, unlike the Feed-In Tariff, there were no models elsewhere from which to gain experience. Therefore some of the proposals are tentative in nature and may be modified. A public consultation on the RHI proposals is open until 26 April 2010. The proposed start-up date for the scheme is April 2011, and it is proposed that the RHI will be open until at least 2020.

This scheme will replace the up-front grants from the Low Carbon Buildings Programme, which has now ended. Systems installed before 15 July 2009 are not eligible – DECC states that the FIT aims to encourage new systems, not reward existing ones.

The tariffs differ between different technologies, and for different-size systems – they are higher per kWh for smaller systems. The tariff for a particular system will be fixed at the time of installation (though perhaps indexed for inflation), but as time goes on and the cost of the systems (hopefully) decreases the tariffs for new installations will be reduced ("degressed"). There will be reviews from time to time to evaluate progress in various technologies and to adapt to changes in their costs. Payments may or may not be tax free.

A serious problem is that, unlike electricity, .heat output is difficult to measure accurately. Therefore, payments for smaller systems will be based on what the installer estimates ("deems") the annual output of the system will be. Payments will be fixed amounts based on that estimated value – this is to encourage low energy consumption and to discourage wasting heat, as well as avoiding the difficulty in metering heat output. A robust way to establish the "deemed" output must be developed, and that is not easy.

The proposals mention fuel poverty: many people can’t afford the up-front costs of new systems so there should be ways to help. The "green loans" recently announced are a part of this.

Equipment must be new, not second-hand. As with the previous up-front grants, both the system and the installer must be approved under the Microgeneration Certification Scheme. But unlike those grants, a range of other measures to reduce energy consumption (insulation, heating controls, lighting) will not be a strict requirement for receiving the FIT.

The register of installed systems will be held by OFGEM. Small systems will be paid annually, although the details are not yet decided.

For small-scale biomass (e.g. wood pellets) there is a problem with air quality because few boilers meet current proposals. The RHI proposes to relax the standards for particulate and nitrogen oxide emissions.

Bioliquids (initially a mixture of heating oil and bioliquid) must save at least 35% of greenhouse gas emissions. For biomass a standard needs to be developed. Biogas and biomethane must be derived from bioliquids or solids that satisfy the criteria.

Heat pumps must exceed a minimum efficiency standard, and air conditioning based on using heat pumps in reverse is not supported

A condensed table of Renewable Heat Tariffs for domestic-scale new installations follows – these are proposed, and subject to change. For systems installed after March 2013 the rates may start to decrease. The annual output used for payments will be nominal (deemed) values set at the time of installation. Full details are available on the Department of Energy and Climate Change website.

Technology	Output	Proposed tariff (p/kWh)	Tariff lifetime (years)
Solid biomass (e.g. wood)	≤ 45 kW	9.0	15
Bioliquids	≤ 45 kW	6.5	15
Biogas on-site combustion	≤ 45 kW	5.5	10
Ground source heat pumps	≤ 45 kW	7.0	23
Air source heat pumps	≤ 45 kW	7.5	18
Solar thermal	≤ 20 kW	18.0	20

Green energy may be generated at a time when there is no domestic use for it. If it is not to be wasted it should be exported to the grid, or stored. This section covers the options for the storage of locally generated energy – for large-scale renewable sources see our green energy page.

If the energy for hot water is to be obtained by a daily collection of solar energy, the storage tank must be able to hold enough water for all normal uses between say 5 pm and 10 am.

The normal water usage in the Thames valley is about 168 litres per person a day – a bath may use about 80 litres of water, a 3 minute shower might use 50 litres. Washing up by hand can use 25 litres. About a third of all water usage is of hot water – say 220 litres a day for a four person household. Hot water tanks typically hold 100–300 litres.

Solar thermal systems will normally be sized so that they can meet the normal requirements for hot water over the course of a day for six to eight months each the year.

A large hot water tank of diameter 50 cm and height 120 cm holds about 235 litres of water. To heat this volume of water to 60 degrees Celsius from a cold water supply at 10 degrees takes about 13.5 kWh. To generate this in a normal summer day, solar panels occupying at least 4 sq. metres are desirable. Doubling the area of panels would have no benefit on many summer days, but would help in autumn or spring. If the tank is insulated by 75 mm of foam with a U value of 0.1, a further 1.5 kWh of energy may be lost by convection each day. This would cool the tank by about 4 degrees overnight. Thicker insulation would be an advantage.

It is very desirable that the supply of hot water should use the locally generated heat when it is available. This should be as automatic as possible, without people having to remember to switch off conventional water heaters when solar energy is available. For most of the year, even if the solar panels do not heat the water fully they provide some pre-heating. The extra heating from solar panels is largely wasted if the conventional heating were to come on in the morning if the hot water tank is not hot enough. The usual arrangement is to have a single large tank with two or three kinds of heating: one coil from solar panels, one from a boiler, and an electric immersion heater. Heating from the boiler or immersion heater can be controlled to run only in the evening. A common arrangement is that the boiler or immersion heater only heat the top part of the tank, while the solar heating is controlled by a sensor at the very bottom and therefore heats more water.

A system can be configured to try to store enough hot water to support two days' usage. This allows continuing use of solar energy for hot water if a sunny day is followed by a cloudy one. This would be a more expensive system with a larger area of solar panels and a larger hot water tank.

Storing solar energy as hot water is a practical option for the hot water usage of a dwelling.

A separate, larger hot water tank could be used to store Economy 7 electricity to drive a flexible central heating system, but a gas boiler without storage is usually a better solution. It might be thought that Economy 7 electricity generates less carbon dioxide than the direct use of gas, because of the greater proportion of nuclear and renewable energy in the electricity used at night. However the overnight consumption of electricity in England currently far exceeds the available nuclear and renewable generation capacity so any additional load would be supplied by fossil fuel power stations. The carbon dioxide produced by these for each kWh of useful heating far exceeds that from the direct use of gas.

The use of hot water to store energy from other sources such as wind turbines fails to attract because the other energy sources are not viable.

Electrical energy can be stored in rechargeable batteries and used later, converted to supplement the mains supply. Special batteries are available designed for the storage of solar or wind generated energy, but their general characteristics are similar to those of a conventional car battery, which can hold about 0.5 kWh. Batteries will only return a proportion of the energy used to charge them. The proportion depends on the charging and discharging rates and the depth of discharge, but it is difficult to achieve more than 80% efficiency. Batteries will only last a limited number of charge/discharge cycles. For the fairly deep discharge cycles likely in this kind of application this might be around 400 cycles. If so the cost of the battery must be spread over the storage of 200 kWh. This is likely to be too high to be viable. For example a new Trojan deep cycle battery L16H intended for this purpose holds about 3 kWh and the makers state it can supply 1004 kWh during its life. This costs about £220, making the battery cost per kWh 22 pence. In addition, end-of-life disposal of batteries is a significant environmental problem.

For greatest efficiency and lifetime batteries should be charged and discharged fairly slowly, e.g. over a period of 10 hours. Heavy short-term loads such as electric showers and clothes dryers should be met from the grid. This can be achieved by limiting the maximum energy output from the batteries. It is not sensible for a battery to feed electricity into the grid at times of low demand, so a sensor of the metered electricity input is required.

A practical installation might use 20 batteries able to hold say 8 kWh of electrical energy. These would be able to accept the full output of a 1 kW generator such as a solar cell panel or wind turbine. On an average day such a generator might provide a total of 8 kWh, and the system might then be able to replace about half the 12 kWh electrical energy used by a typical household.

Any system able to feed the grid must switch off if the mains is not present. This is to protect engineers working on the public lines. Thus such systems do not normally provide backup power for mains failure.

Battery storage may also be used to accept energy from the mains on a low overnight tariff and return it during the day. This is used in Japan, where there is a large difference between the two tariffs. It is less viable in the UK, where the tariff reduction is about 50% and part of the benefit of this is lost during the storage process. This process of smoothing demand has some environmental benefits as the power stations providing the base load at night are typically more efficient, or use nuclear fuel.

Battery storage does not seem likely to be an effective or economic method of storing energy domestically.

Systems are available for storing energy in a variety of other ways. However, at present none is competitive in small installations. We cover storage of energy from large-scale renewable sources (many of which are intermittent) on our green energy page.

Energy can be stored as hydrogen by electrolysing water. With a suitable system the hydrogen could be created at high pressure, avoiding the need to compress it to minimise the volume needed to store it. If atmospheric pressure storage was used, an ability to store about 5 cubic metres of hydrogen would be needed to deliver 10 kWh. The hydrogen could be converted back into electricity when required using a fuel cell. This is the most promising of the alternative solutions and is beginning to be employed in demonstration systems, for example in a house relying entirely on solar photovoltaic energy.

Energy can be stored by raising water to a high reservoir, and allowing the energy to be released through a turbine when is needed. This is used very effectively in large scale pumped-storage power stations, but the concept does not scale down well. If a domestic installation pumped water between two reservoirs each 1 metre cube, with a vertical separation of 10 metres, the maximum energy stored would be 0.03 kWh worth perhaps 0.4 pence.

A more dramatic installation might imagine a reservoir on Churn Hill of say 10 x 10 x 10 metres, capable of holding 1000 tonnes of water, connected to a similar reservoir 50 metres lower down. This installation would be able to store a potential energy 5000 times greater than the domestic option or 150 kWh. Such a system might act as the backup storage for a wind turbine or solar cell installation rated at 15 kW, and the combination might meet most of the electrical needs of 10 houses. However it would be very expensive to build and maintain. A system that fed the excess energy back to the mains would be much less expensive and more efficient.

Another way of storing energy is in the rotation of a flywheel. Existing flywheel technology can store energy for a year or more. See for example the Active Power company. Compared with batteries, the materials used are more benign, the power extracted is closer to the power stored, and the system does not deteriorate after a limited number of cycles. However a flywheel installation with the same energy storage as a normal car battery is expensive and weighs a quarter of a tonne.

Utilities can store energy as compressed air in airtight underground caverns. When required the air expands through a turbine to create electricity. The decisive problem with much smaller systems is the limited power that can be generated. A container holding a cubic metre of air at 100 atmospheres would only have a potential energy of about 0.1 kWh, worth about 1 penny.

Supercapacitors can store a modest amount of electrical energy very efficiently in enhanced versions of standard electronic components. They can provide very large bursts of power. Their ideal application is as a means of providing bursts of energy for road vehicles when overtaking or climbing hills, thus allowing the main engine to be sized for the normal load, making it more efficient. However they are not appropriate for storing the energy needed in domestic installations.

Storage of energy in superconducting magnetic fields is practicable for power utilities, aiming to improve the stability of supplies with rapidly varying loads. However, introducing very large magnetic fields and cryogenic temperatures into a domestic installation would be as unwelcome as it is impractical.

TV Energy page on renewable energy sources provides more information on the technologies.

The yougen website has very clear descriptions of all the domestic renewable technologies – including pros and cons, lists of installers (some with reviews), and other information.