What it is and how it works

Solar thermal collectors on the roof absorb sunlight as heat rather than converting it to electricity. A fluid circulates through them, picks that heat up and gives it to a coil inside a hot water cylinder. A pump station and a controller move the fluid whenever the collectors are hotter than the store.

Two collector types are in common use. Flat plate collectors are an absorber sheet behind glass, robust and usually cheaper per square metre. Evacuated tube collectors are rows of glass tubes with the air drawn out, which cuts heat loss and helps them perform better on cold, bright days. Which suits a building depends on the roof, the temperature the system has to reach and the budget.

Because it skips the step of making electricity, solar thermal harvests more energy per square metre of roof than photovoltaic panels do. That is the honest case for it. The catch is what it can do with that energy, which is hot water and nothing else.

Should it be thermal or photovoltaic?

This is the first question, and on most commercial buildings the answer is photovoltaic.

Solar PV makes electricity, which offsets the full price you pay per unit and can run anything in the building. Whatever you do not use can be sold under the Smart Export Guarantee. Solar thermal makes hot water, which is only worth something if you were going to buy that heat anyway, and it earns nothing once the cylinder is hot.

So solar thermal is worth the roof when three things are true at once. Hot water demand is large. It is steady through the year, rather than collapsing in summer when the collectors perform best. And there is more roof than a sensibly sized PV array would use, so the two are not competing for the same space.

That combination is real, and it is what a hotel, a gym, a care home or a commercial laundry looks like. It is not what most offices and warehouses look like, and we will say so.

Cost, VAT and allowances

We quote a fixed price after the survey, itemised across the collectors, the cylinder, the pipework and pump station, the controls, the access and the integration with your existing heat source.

No grant. The Boiler Upgrade Scheme covers heat pumps and biomass, not solar thermal. The Non-Domestic Renewable Heat Incentive closed to new applications on 31 March 2021 and has no successor for this technology.
VAT. The 0% rate for energy-saving materials covers residential accommodation and buildings used solely for a relevant charitable purpose. Commercial premises are charged the standard rate, which a VAT registered business usually recovers.
Capital allowances. HMRC treats solar thermal the same as solar PV, as special rate expenditure. A company can claim a 50% first-year allowance and carry the balance into the special rate pool at 6% a year, or use the Annual Investment Allowance of £1 million.

Figures reviewed 20 September 2026.

Hot water, safely

A solar cylinder is a pre-heat store. On a dull February week it will sit well below the temperature a hot water system has to reach, so the design always keeps a backup heat source and a control strategy behind it.

HSE guidance on controlling legionella in hot and cold water systems is that hot water is stored at least at 60°C, and distributed so that it reaches 50°C, or 55°C in healthcare premises, within one minute at the outlets. Meeting that is not optional, and it is why the cylinder, the controls and the existing boiler or heat pump are designed as one system rather than bolted together.

Solar thermal with the rest of the building

Where hot water demand is genuinely large, solar thermal and a commercial heat pump can share a cylinder, with the collectors doing the summer work and the heat pump covering the rest. Where the demand is modest, the honest answer is usually solar PV with a diverter sending surplus generation to an immersion instead. Our design and consultancy work settles that question on your own hot water figures before anything is bought.

Pros and cons

Pros

Makes hot water directly, with none of the conversion losses of making electricity first
Harvests more energy per square metre of roof than photovoltaic panels do
Works best in summer, which suits a building whose hot water demand does not drop in July
Solar thermal equipment is special rate expenditure for capital allowances, the same as solar PV
Microgeneration solar thermal on a non-domestic roof is usually permitted development

Cons

There is no grant. The Boiler Upgrade Scheme covers heat pumps and biomass, not solar thermal, and the Non-Domestic Renewable Heat Incentive closed to new applications on 31 March 2021
The 0% VAT rate for energy-saving materials does not reach commercial premises, so the standard rate applies
It competes with solar PV for the same roof, and on many buildings the PV is worth more
A solar cylinder is a pre-heat store, so you still need a backup heat source to meet the legionella temperatures
It only makes hot water, so on a building with modest hot water demand there is very little for it to do
It needs plant space for a cylinder that is usually larger than the one it replaces

Who it suits

Hotels, guest houses and student accommodation with year-round hot water demand
Gyms, leisure centres and swimming pools
Care homes and surgeries, where hot water demand is steady and does not follow the working week
Commercial kitchens, laundries and food production with a continuous hot water load
Buildings with more roof than a sensibly sized PV array would use

What is included in a Bespoke installation

A survey of the roof, the plant room, the existing hot water system and the cylinder space
A hot water demand profile built from meter readings and occupancy, not from floor area
An honest comparison against putting solar PV on the same roof instead
Supply and installation of the collectors, pipework, pump station, controls and cylinder
Integration with the existing heat source, including the legionella control strategy
Commissioning, monitoring and the operation and maintenance file

Frequently asked questions

Is there a grant for commercial solar thermal?

No. The Boiler Upgrade Scheme pays towards air source and ground source heat pumps, biomass boilers and air-to-air heat pumps, and solar thermal is not on that list. The Non-Domestic Renewable Heat Incentive, which did support commercial heat, closed to new applications on 31 March 2021. Any case for solar thermal has to stand on the fuel it displaces.

Should we fit solar thermal or solar PV?

On most commercial buildings, PV. It offsets electricity at the full price you pay, it feeds anything in the building rather than only the hot water, and spare output can be sold under the Smart Export Guarantee. Solar thermal wins where hot water demand is large, steady and year-round, and where there is more roof than the PV array would sensibly use. We model both on your own figures and tell you which one your building should have.

How does it work with legionella control?

A solar cylinder is a pre-heat store, and it will often sit below the temperature the regulations expect. HSE guidance is that hot water is stored at least at 60°C and distributed so it reaches 50°C, or 55°C in healthcare premises, within one minute at the outlets. So the design keeps a backup heat source and a control strategy that brings the store up to temperature regardless of what the sun did that day. This is part of the design, not an afterthought.

Do we need planning permission?

Usually not. Microgeneration solar thermal equipment on the roof of a non-domestic building in England is permitted development, subject to dimensional limits. It must not project more than 0.2 m beyond the plane of a pitched roof, must not sit more than 1 m above the highest part of a flat roof excluding chimneys, and must not go within 1 m of the external edge of the roof. Listed buildings and scheduled monuments are outside the right.

What can we claim against tax?

Solar equipment is special rate expenditure for capital allowances, and HMRC's manual treats solar thermal systems providing hot water the same way as photovoltaic systems. A company can claim a 50% first-year allowance on special rate spend and add the balance to the special rate pool, which writes down at 6% a year. The Annual Investment Allowance of £1 million a year is the other route. Ask your accountant which fits your position.