Buying advice for solar panels

Well-chosen solar panels can provide a reliable source of renewable electricity for decades, helping to slash your electricity bills and cut your carbon footprint. 

But buying an inappropriate solar PV system for your home could leave you out of pocket. 

Use our expert advice to help you decide what's most suitable for your home and the features to look out for before you buy.

Read more: what owners think of their solar panel brands.

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How many solar panels do I need?

The number of solar panels you install (or the size of your system) will depend on how much electricity you need to generate and the amount of space available on your roof.

It's important to install the right size for your household. Too large a system may be a waste of money if you generate energy that you can’t use, although installing a solar battery and/or exporting energy to the grid can help make the most of any excess.

Solar panel system sizes are normally expressed in kilowatt peaks (kWp), which is the maximum output of the system.

Household solar panel systems are typically up to 4kWp.

We spoke to more than 2,000 solar panel owners about the size of their system and how much of their electricity it provides in summer and in winter.

Which? members can log in to see this data. If you're not a member, join Which? to unlock it.

Find out how much solar panels cost.

What do you want from your solar panels?

Once they're installed, you may discover unforeseen benefits in your solar panels, but it's worth considering what you are expecting upfront. 

Solar panel owners we've spoken to have had some quite different motivations:

• Wanting to charge their electric vehicle from solar panel electricity so that it's guaranteed 'green energy'.
• A long-term investment that's future-proofing against electricity price rises.
• Trying to reduce their carbon footprint and do their bit for the environment.
• Using 'free' electricity generated by their panels.
• Wanting to be less reliant on grid electricity (and its price changes).

In the past, solar panels were sometimes seen as a money-making opportunity. Some of the homeowners who bought solar panels recently told us that, while they don't expect to benefit financially from their system and weren't driven by that as a motivation, a future owner of their house might well profit from lower bills.

Read more: are solar panels worth it?

What to know before you buy: solar panel owners' tips

We spoke to hundreds of solar panel owners who bought their systems in the past few years, and asked them to share their top tips on choosing and buying solar panel systems.

Which? members can log in to see tips from current solar panel owners. If you're not a member, join Which? to unlock these.

Do solar panels need direct sunlight?

No. Solar panels can still produce electricity in winter, or on days when it's cloudy.

That's because they use particles of light – or photons – to generate electricity. These are found in both direct and indirect sunlight.

But solar panels work best when the sun is shining on them, and they can't produce electricity at night.

Is your home suitable for solar panels?

You’ll need to consider the following factors to know whether solar panels will work for your home:

• Location: Southern areas of the UK receive around 20% more solar energy than those furthest north.
• Roof slope: A 30-40-degree slope is ideal. The average UK home's roof slopes at 30 degrees – use this in a calculation if you’re not sure.
• Shading: A roof with 20% shading or less is best. Shading can heavily affect energy output – a roof that’s more than 80% shaded can reduce output by as much as 50%.
• Roof direction: A south-facing roof is optimal. An east or west-facing roof will produce 15% less energy, and a north-facing roof around 30% less.
• System size: The average size is around 4kWp, which requires at least 20 square metres of roof space. Adjust this down, if your roof is smaller; or up if you want to cover the energy usage of a larger household.

To get a good idea of the potential savings of panels based on your home, you can enter this information into the Energy Saving Trust's solar panel calculator.

 Find out more about solar panel installation.

Finding a good solar panel installer

We recommend that you get at least three quotes from different installers. This will help give you an idea of the going rate for the type of system you want in your area. 

Also check solar panel costs for an initial guide.

As with any building work, compare quotes and make sure they include itemised details of what you'll get for your money. Make sure you get a breakdown of how any claimed energy savings are calculated too. Query any projections that seem too good to be true.

Use our Which? Trusted Traders search tool below to find reliable solar panel installers near you.

Getting solar panel quotes: expert checklist

Follow our tips and advice on what you should do, plus the questions to ask, before, during and after a visit from a solar PV installer.

Before the visit:

• Check local planning regulations to make sure you're allowed to install a solar PV system (see above).
• Check that the solar PV company is MCS certified. This is often required for government support schemes or to get Smart Export Guarantee (SEG) payments.
• Check if your installer is a member of RECC. Members agree to uphold high standards of consumer protection. RECC runs dispute resolution if you need it. 
• Don't accept a price over the phone before the company has visited. 
• Get an Energy Performance Certificate for your home. 
• Find your energy use for the past year. Here's how to estimate your energy use.
• Check with your home insurer that it will still insure you with solar panels.

During the visit:

• Check if the company representative is a salesperson or surveyor. Only a surveyor can give an accurate quote after doing a survey. Don't sign a contract based only on a salesperson's visit.
• A good surveyor should do a technical site survey including looking at roof orientation, measuring roof tilt, assessing roof quality (inside and out), recording any risks of shade on the panels (taking into account how this will change over time and at different times of the day and year), looking at your consumer unit and metering, asking about your electricity use (and when you use it), and discussing where to locate cables and equipment.

After the visit:

• You should get an in-depth quote from the company, including information on the PV panels, inverter, warranties, terms and conditions, and how it has calculated the payback, rate of return and savings.
• Ask for a breakdown of the quote and check it includes full costs of the installation (eg scaffolding) and any applicable VAT.
• Check whether the quote is tailored to your property and energy use (rather than being based on averages).
• Did the company use SAP (the government-recommended method) to estimate performance, or its own method?
• Check that the system comes with a guarantee.
• Ask who will be doing the installation. Is it is the same company or a sub-contractor?
• What maintenance would the panels need? Is this included within a package or would you need to pay separately?

The company shouldn't do anything to pressure you into buying the system that day. It shouldn't offer large time-limited discounts to tempt you, or use other pressure-selling techniques. 

Types of solar panels

The most common type of solar panel system used for domestic homes is PV – photovoltaic – panels. They collect energy from the sun in photovoltaic cells, which is then passed through an inverter to generate electricity.

Each photovoltaic cell is made up of a series of layers of conductive material. Silicon is the most common.

Before you invite any solar panel firms to give you a quote, consider what type of solar PV you want. 

Monocrystalline:

• The most popular, most efficient and most expensive option.
• Very space efficient, so they need the smallest amount of roof space.
• Made from a very pure form of silicon.
• High power output.
• Very long-lasting – current life expectancy is around 50 years.
• Perform better than other types in low levels of sunlight.
• Dark black in colour.

Polycrystalline:

• A less efficient but cheaper option.
• Made of multiple layers of silicon crystals, which makes them quicker and cheaper to produce.
• Last the same length of time as monocrystalline panels – currently estimated to be around 50 years.
• Don’t cope very well in extreme heat.
• Blue-ish in colour.

Thin film:

• Usually the cheapest option.
• Less efficient than mono or polycrystalline panels.
• Lightweight and flexible, so they can be used in alternative ways – such as on caravans and boats.
• Maintain their efficiency in weak or low light, and in high heat.
• Long-lasting.  

Hybrid (PV-T):

• Combines photovoltaic cells with solar thermal panels, so that the same panel can generate heat and electricity.
• The technology is still very new, so needs specialist installation with higher costs.
• The thermal portion of a PV-T panel doesn’t reach as high temperatures as an independent solar thermal panel, so you’ll still need a primary heating system.

Solar panels are typically fitted on top of your existing roof, but you can also choose solar tiles and slates, which blend in better. However, these are pricey and may only be practical if you're replacing your roof at the same time.

Bifacial solar panels also exist, which can generate electricity from both sides of the panel.

Choosing a solar panel inverter

To actually use the electricity generated by your solar panels, you need an inverter. This converts the direct current (DC) produced by the panels into usable alternating current (AC).

String inverters are the most common and cheapest option. They connect solar panels in series. If one of your panels fails or starts to be overshadowed by a growing tree, it could impact your whole system.

Micro-inverters 'separate' the panels so, if one panel fails, the whole system won't be affected. It should also be easier to spot problems through the power-monitoring system. These are more expensive.

Inverters are often fitted in the loft so that they're not too far from your solar panels and energy loss in cables is minimised. But they can be affected by the heat, so if your loft tends to get very hot in summer, a garage might be a better bet, if you have one.

Beware these solar panel sales tactics

At Which? we hear concerns from people approached by solar panel companies out of the blue, who put them under pressure to buy quickly. 

It's also common to get cold calls about add-ons to your existing solar panel system, which you may not need. 

Many solar panel firms are signed up to a consumer code that bans pressure-selling tactics. But you may still come across unscrupulous tactics. Here's what to watch out for:

• Time-limited or 'one-off' discounts.
• Receiving a quote from a salesperson, rather than a specialist surveyor.
• Being put under pressure to make an immediate decision.
• No information about ongoing costs, such as maintenance.
• A very low price (which might not include all necessary parts and services).
• Very optimistic savings figures or extremely short payback time estimates. 

A reputable firm will give you the time to consider your options and its quote, and will be willing to help provide the information you need to help make your decision. 

You can report pressure selling to the Renewable Energy Consumer Code by calling 020 7981 0850.

See all of our solar panel advice for more reading, or head to our energy efficiency advice for lots of tips on making your home more efficient across the board.