FAQs for Households


Solar PV FAQs

Both Solar PV and Solar Thermal act to capture energy from the sun and convert it into a form which we can use in our homes. On a simple level, a PV system converts energy into electricity, whilst a thermal system uses energy directly to heat air or water. Both technologies are highly effective and are great tools for carbon offsetting; whichever you choose is purely down to your preferences and energy requirements.
The majority of roofs are suitable for solar power. The only roof that we would never recommend is north facing. South-facing roofs are best and the angle should be between 30 to 40 degrees for the best performance. However, east and west roofs produce only up to 15% less power than a roof facing due south. Flat roofs produce around 10% less than the most optimal southern roof. Your roof should not be shaded.

You may also need room – usually in your loft space – for your inverter, depending on the product you choose. The inverter converts the power produced by your solar panels into power you can use in your home. It needs cool air to circulate around it, and usually requires just over a cubic meter of space.

Finally, your roof needs to be structurally sound because solar modules are heavy. The great majority of roofs are strong, but this is certainly a factor your installer should check.


A good price for a straight-forward 4kW system is around £5,000. A 4kW system is suitable for a large family home. As the cost of solar PV has fallen, average scheme size has increased.

Your costs will vary depending on the components you choose; if you want high efficiency panels they will be more expensive, for instance. If you want solar modules integrated into roofing slates these may be more expensive still, but they are very attractive and can make good economic sense if you need to replace your existing roof slates.

There are a number of good places to start. See the websites of Renewable Energy Consumer Code and HIES, check the MCS register, or check the STA membership list. All members of the STA are fully accredited- something we strongly advise. See our section on What to Ask Your Installer.
As above, this depends on where you are, you roof orientation, what kind of solar power you have installed and how big your system is. However, a typical system of around 2.5kW – 3kW(p) will readily meet half the average electricity needs of the average UK home. This can be increased to around 70% using battery storage, the systems for which becoming increasingly available on the market. See FAQs on Battery Storage.
Maps produced by the Met Office show that different regions of the UK benefit from different levels of solar radiation. The region with the highest solar resource is the south east. Each kW(p) of solar installed in the south east will generate 1132kWh of solar per annum. The lowest generation is in the Shetlands at 714kWh per annum. Cornwall receives 1093kWh per annum. The most recent map is available here: solar radiation map. Check out how
There are three basic types of solar panel available. Solar is also available as ‘Building Integrated PV’ (BIPV), where it is embedded in roof tiles. Solar cells can also be embedded in glass. If you have only a small roof space it makes sense to go for the highest efficiency solar panels to optimise how much solar power you can generate.
• Monocrystalline solar is made from a single crystal of silicon. These are the most efficient, but also the most expensive cells to produce.
• Polycrystalline solar is made from blocks of silicon made up of many crystals. Both forms of crystalline solar are brittle and must be mounted in rigid frames.
• Thin-film solar requires only a thin film of silicon on a surface that can be flexible – for example on plastic. It is cheaper, but less efficient, so it needs a larger area. Some thin-film solar panels look lovely on historic properties with dark slate roofs in conservation areas.
• Bi-facial modules produce power from both sides of the panel. This is particularly useful for panels on/near highly reflective surfaces, such as light coloured roofs or stones. Bifacials can increase annual energy production by a considerable amount.
The size of your system will be expressed in terms of kW(p). This refers to how much electricity your system can generate at peak performance (p stands for peak). This is the optimum output your system could technically achieve under optimum conditions.
Solar PV is often known as a ‘fit and forget’ technology because it requires so little maintenance. Apart from keeping it relatively clean, there is nothing more to do. Solar PV systems are usually guaranteed to last over 25 years. During this time you may need to replace the inverter.
Solar generates the most power when it is sunny and cold. Heat can actually reduce the performance of some solar modules. However, even on an overcast day your system will generate about a third of the power it would generate in direct sunlight.
There is a widespread perception that solar suits only sunny countries. Actually a study by Oxford University’s Environmental Change Institute showed that there is a surprisingly small difference between the output of solar panels in summer in the UK and solar panels in Mediterranean countries. This is because heat can reduce performance and the longer daylight hours in northerly latitudes compensates for lower output.
Solar panels have no moving parts which makes them unique amongst power generation technologies. It means little wear and tear. They are therefore exceptionally long lasting. Solar panels manufactured in the 1970s are still generating power today, so we don’t really know how long they last – certainly 40 years or more. Over time the output of solar panels reduces. At the Solar Trade Association we assume that panels lose around 10% of their output after 10 years. Most solar panels are guaranteed to lose only 1% efficiency per annum and most are guaranteed to last for 25 years.
Consumer bodies do not consider it advisable to make this claim outright. However, solar can be considered to add value to homes in a variety of nuanced ways.
Firstly, solar improves a building’s energy efficiency rating, which has a direct impact on valuation. Also, research by mortgage provider ING Direct shows that solar panels are the number one ‘non-essential’ item in clinching house sales, so having panels on your roof is likely to be an asset when it comes to securing a sale. However, installing panels badly is unlikely to help the value of your home. See our solar expert Ray Noble’s gallery of shame for examples of how NOT to install solar pv.
If you cannot afford a whole system, or if you like the idea of clubbing together with like-minded people, there is now exciting growth in community solar. This means you could have a share in a much larger local solar scheme. Often these community schemes will offer a fair rate of return but invest a chunk of the profits locally for wider community benefit. See the website of Community Energy England if you are interested in community solar.
VAT ranges from 0% to 20% for different types of installation. Please download our VAT guidance for more information.

Solar Thermal FAQs

The orientation of your roof, the slope of your roof and any shading are vital factors in deciding whether solar thermal is a good choice for your home regardless of the type of panel you are interested in. Maximum energy is collected by a system tilted 35 degrees from horizontal.
We would never recommend a solar collector facing north. However, many people are surprised to learn that there is not a great deal of difference on the total energy collected over a given year for all other directions. This is especially true for panels on a roof tilted at around 35 degrees, where the difference in performance between facing east or west, and facing due south, is only around 12%. It is important to be aware that if the output of the collector is reduced on an east or west orientation the collector area should be increased accordingly to ensure hot water needs can be met.
The Microgeneration Certification Scheme (MCS) is an independent scheme that certifies products and installers to a professional set of standards. To qualify for Government grants both the product and the installer need to meet MCS standards.
Your installer should also be a member of the RECC or HIES.
There are 2 key types of solar thermal collector:

• Flat plate collectors. These are based on a thin heat absorber sheet, usually copper, backed by a tubing system to carry fluids. They are relatively thin, highly insulated and encased in glass. They are cost effective, with a range of mounting options and can reach efficiencies of 75-80%. They can be fitted on top of existing roof tiles, or in new-build or roof replacements where they can be integrated into roof tiles themselves.
• Evacuated tube collectors. A vacuum between glass tubing provides extremely efficient insulation and evactuated tubes can reach very high temperatures. There are two different categories of evacuated tube (direct flow and heat pipe). Both are similar in appearance but work in different ways. Heat pipe evacuated tubes can only be installed vertically but direct flow tubes can be installed vertically or horizontally.

Solar thermal is supported under the Renewable Heat Incentive. In domestic systems the Tariff will be ‘deemed’ – this means that an estimate will be made of the output of your system based on typical domestic use and the number of people living in your home. The RHI will pay a minimum of 19.2p/kWh for 7 years, with payment rising with RPI. However, the total level of support you will get from the RHI will depend on the number of people living in your home. The logic of this is that while everyone can enjoy the same space heating, the more people living in your home the more hot water you will use. See the Financial Incentives section for more details on the RHI and the STA’s estimates of the financial returns for homes of different occupancy levels.
You will not need to meter your solar thermal system, however, in order to be eligible for these incentives, the installer and product must be MCS registered.
To be eligible for the Renewable Heat Incentive your property needs to meet a minimum energy performance standard EPC C and have basic measures installed i.e. your loft must be lagged to at least 250mm and any cavity walls must be filled. You must also use MCS-certified equipment and installers.
The cost of systems varies, but the average installed cost across the UK currently is £4200. This is for a system made up of a 4m2 collector area, a dual-coil hot water cylinder of around 200-250 litres, a controller and pipework etc.
Please note that average system cost is only indicative. Costs can vary significantly depending on your hot water delivery system.
On average water heating makes up 20-30% of total domestic gas bills. Normally you would expect to save between 30-70% of your annual water heating costs with a solar thermal system. If the occupant carefully follows the installer’s advice, higher savings can be obtained.
In order to ensure proper efficiency of your water heating system a solar thermal system must be used in conjunction with a traditional gas or electric heating system.
Combination (combi) boilers that provide hot water on demand have become increasingly prevalent in the UK. 80% of all new boilers sold in the UK are combis. According to the English Housing Survey 2012, 32% of all homes now have a combi-boiler. It is a common misconception that solar thermal cannot be combined with combi boilers.
Solar Trade Association manufacturer members have developed novel and unique solutions to this UK specific challenge that will allow a wider adoption of solar thermal with combi-boilers already installed throughout the UK. Major manufacturers like Worcester Bosch and Veissmann offer combi boilers that can accept preheated water from solar thermal systems.
Solar collectors can work even if it’s freezing cold. All they need is daylight and we get enough of that even on a cloudy day. An antifreeze solution circulates around the loop of an indirect solar thermal system protecting it from sub-zero temperatures.
The UK receives 60% of the solar energy received at the equator. Each square meter of the UK receives ample energy to operate an efficient solar panel system as long as the collectors are sized according to the required output. The prevalence of solar thermal in Scandinavia proves it can work effectively in colder climates than the UK.
The collector size you’ll need depends on a number of factors; estimated hot water usage; how large a contribution you want to annual hot water load; geographical location; inclination of the roof and type of collector.
For domestic solar hot water systems it has been found that installing 1m2 of flat plate collector, or 3/4m2 evacuated tubes per person will give satisfactory results. Heat loss through the system will be larger the smaller the collector is, therefore it is unusual to install a collector of less than 2.5m2 in the UK.
So it is important to beware of underestimating the size of the collector needed, as any cost savings may be outweighed by reduced performance if a system is too small.
Solar thermal usually has a life expectancy greater than 20 years, and systems are usually supplied with a manufacturer’s warranty of 10 years.
Negligible maintenance is needed. The fluid circulating in the system should be topped up or even drained and replaced every 5 years or so. However, an annual assessment of the solar thermal system is recommended to help maintain efficiency levels and to help avoid potential long-term problems. This could happen alongside your annual boiler service.
No, it is not our role to recommend certain products, installers and companies. Please see the resources section for independent sources of advice. You can search the Renewable Energy Consumer Code and the Microgeneration Certification Scheme websites for installers. You can also search the Solar Trade Association database, as all our members have to be certified by both schemes.
Common installation mistakes can include incorrectly specified components (such as pumps, valves and joints) that aren’t rated for the temperatures and pressures that can be present in a solar circuit. Lack of insulation (or use of low temperature insulation which then melts) on pipes is also a common mistake. Undersizing the hot water cylinder is often also an issue which limits system performance.
They should work automatically together. A conventional solar thermal system heats the water in a hot water cylinder. The boiler’s input should be thermostatically controlled so if the water in the cylinder is below the desired target temperature then the boiler simply tops up the solar. If the solar has heated the cylinder to or beyond the target temperature then the boiler won’t have anything to do. Either way the boiler doesn’t have to work as hard and fuel is saved. Integration with heat pumps is similar. Integration with underfloor heating is a little too complex to quickly describe and is better illustrated with a diagrammatic explanation.
Ideally use most of the hot water in an evening and morning so the hot water cylinder is cool at the start of the day. That way the solar thermal system has a lot of “space” to add free solar heat.
Most importantly, a hot water cylinder; ideally one which is a little larger than that of the average airing cupboard. It is possible to add solar thermal to a combination boiler (i.e. no hot water cylinder) and there are cylinders designed to supply solar pre-heated water to the inlet of combination boilers. If necessary some of these products can be located in the loft space. Other components, pumps, valves pipes etc take up very little space and will go in the loft or airing cupboard quite easily.
VAT ranges from 0% to 20% for different types of installation. Please download our VAT guidance for more information.

Battery Storage FAQs

Battery storage will enable you to store more of the solar power that you generate so you can use it when the sun has gone down instead of drawing on grid electricity from your electricity supplier. Storage is particularly useful and most valuable economically if you have solar but are out most of the day & can’t use all of your solar power. Storage can also be used to draw power from the grid when it is especially cheap, such as through Economy 7 tariffs, which millions of customers already have. There are also now a couple of electricity suppliers who offer other, smarter Time of Use Tariffs (TOUTs) which means you can buy power from them much more cheaply at certain times of the day or night.

New guidance is coming soon from MCS, based on empirical data from Loughborough University and Advance Further Energy informed by real world data from our members. This guidance will help to ensure you receive accurate information on how your battery storage system will perform with a solar system from your MCS registered installer.

Various types of lithium-ion batteries are increasingly common and recommended for domestic solar storage system. They often incorporate integrated software that manages when the system charges or discharges to maximise potential savings for you. They have a long lifetime at around ten years (and some are warranted for more than 10 years), and are light, efficient and compact. You are likely to have to replace your battery storage unit during the lifetime of your solar system, as solar panels are often warranteed for 25 years. Other types of storage exist such as heat batteries and flow batteries which each have their own merits.


Some storage systems incorporate a range of technologies already in one case, such as an inverter, writing and smart controls. Other systems are built up for each customer out of separate units, typically; a battery, inverter and separate controls. So be sure to ask what you are getting.

The savings associated with battery storage depend on a number of variables and are not always the right investment for all people. As a general rule, battery storage is more valuable when you are out during the day and want to save solar power for evenings when you are home. It is also more valuable if you have a large solar PV system and spill lots of power to the grid during the day. Battery storage also makes it easier to consume power from solar as you don’t need to worry about timing your appliance use to when there is bright daylight.

We recommend comparing the benefits you could get with a battery and solar storage system to a solar only system to make your decision, bearing in mind the VAT benefits of installing both together.

VAT ranges from 0% to 5% depending on the installation. Please see our VAT guidance for more information.
The average household uses around 10-12kWh of electricity a day. Battery storage units are available in the market ranging from 2kWh to over 20kWh. Which? members typically paid between £4,000 and £7,000 for their battery storage system but prices are decreasing. The size of battery unit you need will depend on how much of your solar power you use during the day and your typical home energy consumption. If your levels of self-consumption are high, for example if you are home all day and run appliances during the day, then you will have less solar power to store in your battery unit. So in this example it would make little sense to splash out on a bigger storage unit unless you also plan to charge the battery from, say, cheap night-time electricity. If you have an electric vehicle you may also want to think about whether you want to be able to charge it at night from your stored solar power, in which case you may benefit from a larger solar and larger battery storage system.

Reputable installers will offer advice on the size of the battery for your system. If you are in all day, a 5kWh battery should be sufficient, although to get maximum savings you might wish to opt for a 10kWh battery or larger.

The capacity rating of batteries can be confusing, since there is a difference between ‘total’ capacity and ‘usable’ capacity because a battery will never fully discharge to avoid damage. Be sure to understand the usable capacity of any unit – this is what is used to save energy from solar power. Furthermore, you will also need to understand the battery’s power output. Consider if the battery will be powerful enough to meet the load of the electrical equipment that you want to run in your home simultaneously. Your kettle, for example, may need to be able to draw 2kW of power from your battery alongside your fridge and other electrical items.

So be sure to understand both the power output as well as the usable capacity of your storage unit.

As a rough guide, a fully-charged 6-7kWh system could usually store sufficient energy to power your lights, fridge-freezer, TV and computer over at least eight hours. Bear in mind appliances such as washing machines can use 2-3kWh in one go. And during the winter time your solar system will produce less power to store, though you should be able to top it up from the grid when power is cheap. Economy 7 and other Time of Use Tariffs (TOUTs) will enable you to do this. Data from Elexon on real world domestic consumer consumption patterns on weekdays in Spring suggests, roughly, a 2kWh battery would support average domestic consumption levels between 5.30pm and 8.00pm (or up to 7 hours overnight).

Incomplete charging can reduce the life of some batteries so some systems will from time to time be programmed to fully charge from the grid in order to complete the charging cycle and maintain your system.

Battery storage fits perfectly with solar PV because it allows you to use the power your solar panels have generated during the day when you need it, even when the sun has gone down. So you can use more of the clean power you generate. And it can be particularly satisfying to drive an electric vehicle powered only by the sun.

At the technical level, solar PV generates direct current (DC) and battery storage units store and discharge DC, so both require power to be converted to alternating current (AC) to be used in the home or to export onto the grid. DC-Coupled systems put the solar PV modules and battery storage unit on the same side of the inverter. Alternatively, AC-Coupled systems put the battery storage unit on the grid side of the inverter and use a separate inverter to convert the power back to AC when the battery discharges. If you already have solar PV and now want to install battery storage you are likely to be offered an AC-Coupled system. However, if you are installing a new solar and storage system from scratch you are more likely to be offered a DC-Coupled system. It is not uncommon for households to want to operate in power cut conditions. However, this is not strictly necessary given power cuts are rare in the UK. Some battery systems can operate in a power cut and in some cases this would require changes to the wiring in your home – you will need to check with the battery manufacturer. Note that complex changes to your electrical wiring can be costly.

No. A recent ruling by Ofgem clarifying the regulations means that if you install a battery system you will still be able to deem the output from your solar PV system at 50%, even if you install a smart meter. This is because of the difficulty determining whether exports from your house to the grid are coming from your solar system or from grid-drawn power passing through your storage unit.

If you do not have solar already and are installing a new solar and storage system from scratch, the good news is that you will only pay 5% VAT on both the solar and storage. For new installations what you will be paid for your exported electricity remains as yet unclear as details have not been published on how the new Smart Export Guarantees will work or what the price per unit may be. As well as seeing our SEG pages, you can also see our League table where we list suppliers who are offering the best SEGs. The STA would like to see all exports from homes with solar and storage qualifying for the SEG.