Frequently asked questions

Yes. Physics does not care how old a house is; it cares how hot the water in the radiators has to be.

What changes with an older house is the cost. Poorly insulated rooms with small radiators need hotter water, and hotter water means a higher bill. Bigger radiators or better insulation bring it down.

Usually only some. Many existing radiators are already generously sized because they were fitted before the house had insulation and double glazing.

Replacements are often a matter of depth, not length — a deeper radiator on the same stretch of wall can nearly double the output.

Because they are meant to. A heat pump runs gently and continuously at a low water temperature instead of in hot bursts, and that is the efficient way to do it.

If the room is at the temperature you set, it is working. There is more in why a heat pump runs all the time.

No. Turning it off saves very little, because a warm house loses heat slowly, and getting the temperature back needs the heat pump to run hot, which is the expensive mode.

Set a temperature and leave it. A one or two degree overnight setback is fine.

Yes. Air source heat pumps operate down to about −20 °C, colder than almost anywhere in Britain gets.

Output does fall as it gets colder, which is exactly what the sizing allows for. A correctly sized system holds the house on the coldest normal day without any backup heater.

About as loud as a fridge, and quieter a few metres away. A modern unit is around 50 to 60 decibels at the machine itself.

There is a legal limit of 37 decibels at a neighbour's window, and where the unit is placed matters far more than the figure on the datasheet.

Against oil, LPG or electric heating, almost certainly. Against mains gas, it depends on the design.

Electricity costs about four times what gas does per unit, and a boiler loses some of the gas it burns, so a heat pump has to produce roughly three and a half units of heat per unit of electricity just to break even. A well designed system beats that; one running hot may not.

Yes, if you currently have a combination boiler. A heat pump cannot heat water on demand, so it heats a tank in advance.

Finding somewhere to put it is often the main obstacle in a retrofit, more than anything about the heat pump itself.

Usually not, even if it looks fine. The coil inside is sized for a boiler's very hot water, and a heat pump supplies water much cooler.

An old cylinder will reheat slowly and drag the heat pump's efficiency down while it does.

Fifteen to twenty years, much like a gas boiler, with the compressor as the part that eventually wears out.

A ground source system's buried pipework lasts far longer — fifty years or more. See what wears out, and when.

An annual service, plus keeping the outdoor unit clear of leaves and debris, which is less attention than a boiler needs.

An unvented hot water cylinder needs its own annual check by somebody qualified for unvented systems.

Some can, but not through ordinary radiators — you need fan coils or underfloor, and careful attention to condensation.

Ground source can do it more elegantly, circulating cool water from the buried loop without running the compressor at all.

That is a defrost cycle, and it is normal on a cold damp morning. Moisture from the air freezes on the outdoor coil, so the machine briefly runs backwards to melt it off.

It lasts a few minutes and then everything carries on as before.

It varies too much for a useful single figure, because it depends on your heat loss, how many radiators need changing and whether there is room for a cylinder.

Roughly half the money is the heat pump and cylinder; the rest is emitters, pipework, electrics and labour. Grants are available in England, Wales and Scotland; Northern Ireland has narrower, income-based support.

A typical ten or twelve panel array makes around 3,500 to 4,000 units a year in southern England, and around 3,000 in Scotland. For most households that is roughly what they use in a year.

The system size calculator will work out your own roof from your postcode.

Usually yes. East or west makes about 80 per cent of what a south-facing roof makes, which is a smaller penalty than most people expect.

It also spreads generation across morning and evening instead of piling it into midday, so more of it tends to get used in the house.

Barely. Anything from about 20 to 50 degrees is within a few per cent of the best, and almost every British pitched roof is in that range.

A flat roof is the exception: panels laid flat lose about 16 per cent and soil faster, so they are usually tilted on frames.

More than its share of the roof. Panels are wired in a chain, so a shadow on one drags down the others connected to it.

A chimney shadow across five per cent of the array can cost fifteen or twenty per cent of the output while it lies there.

On a shaded roof, genuinely yes — they let each panel work independently instead of being held back by its neighbour.

On an unshaded roof they add cost and more parts on the roof to fail, for very little gain.

It goes back into the grid, and licensed suppliers must offer you a payment for it. The rate is generally well below what you pay to buy electricity.

You will need a smart meter and your installation certificate before a supplier will pay you.

Without a battery, usually between a sixth and a third. Panels generate at midday and households use most electricity morning and evening.

Running the washing machine and dishwasher in the middle of the day shifts this in your favour with no equipment at all.

It roughly doubles how much of your own generation you use, which is a real improvement. Whether it pays depends on the gap between what you pay for electricity and what you are paid for it.

On typical tariffs that comes to a few hundred pounds a year against a cost of several thousand.

Twenty-five to thirty years, losing about half a per cent of their output each year. After two decades a panel is still doing around ninety per cent.

The inverter is the part that needs replacing sooner, usually around year twelve.

In Britain, rarely. Rain does the job provided the roof has enough pitch for water to run off.

Worth a look once a year for moss creeping up the bottom edge, or bird droppings, which are opaque and can affect more than one panel.

Most will. An array adds roughly 12 to 15 kilograms per square metre, which a normal British roof carries comfortably.

Older roofs with thin timbers, or ones already carrying a heavy covering, need somebody to check properly, and your installer has to arrange that check.

No. Waiting a year costs you a year of savings, and panels improve by far less than that in a year.

More in should you wait for better panels?

Yes. Plug-in solar became legal in the UK on 27 August 2026, which opens it up to renters and flats for the first time.

It is much smaller than a roof array and works differently. See plug-in balcony solar.

It moves electricity in time. Solar generates at midday; a household uses most in the morning and evening. The battery catches the midday surplus and gives it back at teatime.

What it earns you is the gap between what you would have paid to buy that unit and what you would have been paid to sell it.

It roughly doubles how much of your own generation you use. For a typical household that is a jump from around a quarter to somewhere near two thirds.

The self-consumption calculator will do it for your own figures.

For a typical house with a 4 kW array, somewhere between 5 and 10 kWh of usable capacity.

Bigger is not proportionally better: there is only so much surplus in a day to catch, so the gains fall away quickly above about 10 kWh.

What the battery will actually give back, which is less than the figure printed on it. Some is held in reserve to protect the cells.

A battery sold as 10 kWh might have 8 usable, and the usable figure is the one to compare on.

Only if it was specifically built to. By default a solar system shuts down in an outage, for the safety of anyone working on the lines.

Backup needs extra equipment and has to be asked for before installation. See solar, batteries and power cuts.

Yes, and on a tariff with a cheap night rate this can be worth more than the solar shifting.

It works every day of the year, not only when the sun shines. Some people fit a battery on this basis with no solar at all.

Ten to fifteen years. It declines gradually, holding a little less each year, and does not fail outright.

Warranties are usually ten years, or a number of cycles, or a total throughput — whichever comes first, so check which of those limits applies to you.

Usually wall-mounted in a garage or utility room. It needs a sensible temperature range and some ventilation.

Domestic batteries are overwhelmingly lithium iron phosphate, a chemistry chosen partly because it is thermally stable. Lofts are the least good location: too hot in summer, too cold in winter.

Not planning permission, if it is inside a building. But it does need telling the company that owns the wires in your street, because a battery can export.

Adding one to existing solar can push the combined system over a threshold and turn a notification into an application, so establish that before ordering.

Generally no. Home batteries are not short of space, so the density improvements that drive phone and car batteries buy you little.

What matters is cost per usable kilowatt-hour, cycle life and safety. More in is silicon-carbon suited to home batteries?

Not by law. But you will lose access to the grant schemes, and to being paid for electricity you export.

A buyer's solicitor will also ask for the certificate when you sell.

The company has been audited against a published standard, and can be audited again. It is not a guarantee that every job is good.

What it changes is that there is somebody with the power to make them put it right.

Not MCS itself. Independent certification bodies do the audits, and they in turn are accredited by UKAS, the national accreditation body.

So there are three layers: the installer, the body that audits them, and the body that accredits the auditor.

The company. It covers their quality management, their record keeping and the qualifications their people hold.

Worth checking when work is subcontracted: the certificate belongs to whoever registers the installation, and that has to be whoever did the work.

Look up the certificate number from the quotation in the MCS directory on their website.

Check two things: that it is current, and that it covers the technology being installed, since solar certification does not cover heat pumps.

That is what the insurance-backed guarantee is for. It is a separate policy from the workmanship warranty, and certified installers have to provide one.

Find the actual policy document before you need it. Many people assume they have this cover without ever seeing it.

The installer first, in writing. If that does not resolve it, there are two routes and they do different jobs.

The consumer code handles the contract, the selling and redress. The certification body handles the technical work. Your paperwork names both, so the second letter goes to whichever of them covers the problem.

Yes, and it is the most useful thing about the scheme. Anyone can read exactly what an installer has agreed to do, free, on the MCS website.

That means the calculations behind a quotation are checkable and do not have to be taken on trust. The guide on what certification covers explains how the documents are organised.

Get three quotes, and compare what is in them and not only the price. The one that surveyed your house properly will be obvious from the paperwork.

The guide on what a good quote looks like sets out what to expect, and there is a list of questions to ask before you sign.

No. Two certified installers can quote figures that differ substantially and both be complying fully.

What the standards constrain is the method behind the numbers, which is what makes two quotations comparable.

Usually not. Most installations fall under permitted development, which means no application.

The condition people most often fall foul of is noise: it must not exceed 37 decibels at a neighbour's window. Listed buildings and conservation areas have their own rules.

Most roof arrays do not. The usual conditions are that panels sit close to the roof plane and not above the ridge.

Listed buildings, conservation areas and flats all have their own rules, which your council can confirm.

A limit on how loud a heat pump may be at a neighbour's window or door — not at the boundary, and not at your own house.

It is worked out by calculation, not measured, and your installer has to do it before relying on permitted development.

No. It means the current position does not work, not that the house cannot have one.

Moving it further away is worth about six decibels per doubling of distance. Bringing it out of a corner is worth three per wall, and a solid barrier in the line of sight can be worth ten.

Yes, but your installer handles it. Both heat pumps and the electrical side of solar are notifiable.

You should receive a compliance certificate. If an installer suggests you make the application yourself, ask why.

Telling the company that owns the wires in your street that you have connected something that generates electricity.

Small systems are connected first and notified within 28 days. Larger ones need approval before connection, and that approval is not guaranteed.

Where the local network cannot take your full output, you may be allowed to install anyway on condition that a device caps how much you send back.

The annual cost is usually small, a couple of per cent, but it should be disclosed in the quotation.

The certificate above all — it is what grant schemes, export payments and your buyer's solicitor will want.

Then the design calculations, the commissioning records, and the insurance-backed guarantee policy.

Ask the installer, in writing. Registering the installation is their job and there is a time limit on it.

If that does not work, their certification body is the next step. Missing paperwork is a straightforward complaint for them to act on.

Yes. Both solar and a heat pump feed into the calculation, though how much depends on the assessment method in force at the time.

The assessor will want to see the certificate as evidence, so keep it somewhere findable.