Solar Thermal vs Solar PV: Which Is Better?

Solar thermal heats water directly using the sun. Solar PV generates electricity. Twenty years ago, thermal was the obvious choice for hot water. In 2026, PV has largely won the argument — but thermal still has its niche.

The Fundamental Difference

Solar thermal panels contain fluid that absorbs heat from the sun. This hot fluid passes through a coil in your hot water cylinder, heating the water directly. It's a plumbing system.

Solar PV panels convert sunlight into electricity. That electricity can power anything — appliances, lighting, EV charging, or an immersion heater to heat water. It's an electrical system.

Solar thermal is a specialist: it does one thing (heat water) very well. Solar PV is a generalist: it does everything adequately, including heating water.

Head-to-Head Comparison

Why PV Has Won for Most Homes

Flexibility

PV electricity can do anything. On a sunny day, your panels power your appliances, charge your car, and heat your water. Solar thermal only heats water — if your cylinder is already hot, the excess heat is wasted (or the system stagnates).

Economics

A 4kW PV system costs roughly £6,500 and saves £600–£900/year across all electricity uses. A solar thermal system costs roughly £4,000 and saves £100–£200/year on hot water alone. The payback is faster for PV because it offsets more expensive grid electricity across all uses.

PV + Diverter = Hot Water Too

Products like the myenergi Eddi and Solar iBoost+ divert surplus PV electricity to your immersion heater. Instead of exporting excess generation at 12p/kWh, you heat water with it — saving the 26p/kWh you'd have spent on grid electricity for heating.

myenergi Eddi Solar Diverter

£185

max power w

3000

modes

power_divert,timed_boost

outputs

2

priority

configurable

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This gives you the best of both worlds: electricity when you need it, hot water when you don't.

Lower Maintenance

PV panels have no moving parts and need minimal maintenance. Solar thermal systems have a pump, fluid, valves, and expansion vessel — all of which need periodic checking and eventual replacement.

Longer Lifespan

PV panels are warranted for 25 years and typically last 30+. Solar thermal panels are warranted for 10–15 years, and the associated pump and fluid need attention before then.

When Solar Thermal Still Makes Sense

Very Limited Roof Space

If you only have 3–4 m² of suitable roof, you can't fit enough PV panels to be worthwhile. Two solar thermal panels in that space could provide 50–70% of your hot water.

High Hot Water Demand

Households using lots of hot water (large families, frequent bathing rather than showering) get more from thermal because the panels are more efficient per square metre at converting sunlight to usable heat.

No Immersion Heater or Cylinder

If your home doesn't have a hot water cylinder (common with combi boilers) and you want solar hot water, thermal panels with a new cylinder is a viable option. PV + diverter also needs a cylinder, so neither option avoids this cost.

Existing System Replacement

If you already have a solar thermal system that's reached end of life, replacing like-for-like may be simpler and cheaper than converting to PV + diverter.

The PV + Diverter Setup Explained

Here's how solar PV heats water in practice:

1. Solar panels generate electricity
2. Your home uses what it needs for appliances, lights, etc.
3. Any surplus is detected by the diverter (Eddi, iBoost+, or similar)
4. The diverter sends surplus electricity to the immersion heater in your hot water cylinder
5. Only genuine surplus after home use and hot water goes to grid export

This is intelligent: it prioritises your immediate electricity needs, then hot water, then export. On a good summer day, your cylinder will be fully heated by solar by mid-afternoon. In winter, the immersion tops up less but you're still generating some electricity for other uses.

Solar iBoost+ Immersion Heater Controller

£150

max power w

3000

modes

auto_divert,manual_boost

outputs

1

buddy unit available

true

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What About Solar Thermal + PV Together?

Some homes have both — typically where thermal was installed years ago and PV has been added later. This works but there are diminishing returns: both systems compete for the same solar resource (roof space and sunlight), and in summer both may try to heat water that's already hot.

If you're starting from scratch, PV + diverter is almost always better value than PV + thermal.

The Efficiency Myth

Solar thermal advocates correctly point out that thermal panels are more efficient per square metre at converting sunlight to usable heat (typically 60–70% efficiency vs PV's 20–22%).

But this comparison is misleading for two reasons:

1. PV panels are cheaper per square metre, so you can install more of them for the same budget
2. PV electricity is more valuable because it's versatile — the 26p/kWh you save on electricity is worth more than the gas you'd save heating water (~7p/kWh equivalent)

When you compare value per pound spent rather than efficiency per square metre, PV wins decisively.

Our Recommendation

For the vast majority of UK homes in 2026:

• Install solar PV as your primary solar technology
• Add a diverter (Eddi or iBoost+) to handle hot water with surplus generation
• Skip solar thermal unless you have very limited roof space and high hot water demand

The PV + diverter combination costs less, lasts longer, earns export income, and gives you both electricity and hot water. Solar thermal's niche has shrunk to the point where it's rarely the best choice for a new installation.

For more on heating water with solar, see our solar hot water guide.

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