Solar thermal delivers the most heat per square metre — 400–500 kWh/m² a year against 150–200 kWh/m² from PV — so it wins wherever roof area is limited or hot water demand is high. A hot water heat pump gives the lowest running cost on grid or PV power, turning each kWh into 2.5–3.5 kWh of heat year-round. A PV diverter is a low-cost add-on if you already own PV and a cylinder, recovering 700–1,200 kWh a year of surplus, mostly in summer.
What are the three options actually doing?
They solve one problem with three different physics. Solar thermal converts sunlight into heat inside the collector at 50–70% efficiency and stores it in a cylinder. PV plus a diverter converts sunlight into electricity at 20–22%, then converts surplus electricity into heat in an immersion element at 1:1. PV plus a hot water heat pump takes that same electricity and multiplies it by a COP of 2.5–3.5, so one kilowatt-hour becomes 2.5–3.5 kWh of hot water — and it keeps doing so at night, in December, on grid power.
The three are not really competing for the same buyer. They are answers to three different constraints: roof area, capital cost and running cost.
How much heat does each deliver per square metre of roof?
| Route | Conversion path | Annual heat per m² of roof | Works without sun? |
|---|---|---|---|
| Solar thermal, flat plate | 50–70% straight to heat | 400–500 kWh | No |
| PV + immersion diverter | 20–22% to electricity, then 1:1 to heat | 150–200 kWh | No |
| PV + hot water heat pump | 20–22% to electricity, then ×2.5–3.5 | 375–700 kWh equivalent | Yes — also runs on grid power |
Read the third row carefully. The heat pump's advantage does not come from the roof; it comes from the compressor. That is why it is the only one of the three that still delivers on a dark December morning — and also why its output is tied to an electricity price rather than to sunshine.
Which is cheapest to install, and which to run?
A diverter is by a wide margin the cheapest item on this page: one controller and a current clamp, fitted in a morning to a cylinder that already exists, typically recovering 700–1,200 kWh a year from a 4–5 kWp array. Its limits are honest ones. It harvests only surplus that already exists, it contributes almost nothing between November and February, and every diverted kilowatt-hour is one no longer exported.
A hot water heat pump costs several times more and needs a plant space with air flow and a drain, but it produces the lowest cost per kilowatt-hour of hot water across a full year and is the only option on this list that improves a winter bill.
Solar thermal sits between them on capital and demands the most engineering — glycol, expansion vessel, annual pressure check, a fluid change every 5–8 years — in exchange for the highest yield per square metre and a collector with no moving parts and a 25–30 year life.
Where does each option genuinely win?
- You already have PV and a cylinder and want a low-cost first step → diverter, which uses summer surplus but contributes little from November to February.
- The roof is small or full and hot water demand is large — family of five, guesthouse, gym → solar thermal. It is the only route that turns limited area into large quantities of heat.
- All-electric house, cool climate, year-round demand, no gas → PV plus a hot water heat pump. Here the COP is doing more work than the roof.
- Export is unpaid, capped or curtailed → solar thermal or a heat pump. Surplus PV you cannot sell changes the arithmetic sharply.
- One small roof must supply both electricity and heat → PVT hybrid panels, which is a fourth answer rather than a compromise between these three.
How do you decide?
Answer three questions. Is hot water demand high, or roof area the binding constraint? Then solar thermal or PVT. Is the house all-electric with year-round demand? Then a hot water heat pump — on its own or with solar thermal — usually gives the lowest lifetime cost. Already own PV and a cylinder? A diverter is a cheap way to use summer surplus alongside either.
Solimpeks manufactures solar thermal collectors, PVT panels and heat pumps, so these routes can be combined around one store — the solar coil low, the heat pump coil above it — rather than chosen as rivals.
Frequently asked questions
Is a solar diverter better than solar thermal?
They do different jobs. A diverter is a cheap add-on for a household that already owns PV and a cylinder, but it only uses summer surplus. Solar thermal delivers two to three times more heat per square metre and covers 50–90% of annual hot water, which decides it wherever hot water demand is high or roof area is limited.
Can I use a diverter and solar thermal together?
Technically yes, if the cylinder has both a solar coil and an immersion element. In practice the two compete for the same tank capacity on the same sunny days, so the combination rarely earns its cost.
Does a hot water heat pump work well with PV?
Very well. Each surplus kilowatt-hour becomes 2.5–3.5 kWh of hot water instead of one, so a small PV array goes much further. Use a timer or PV-linked control so the unit runs during the midday surplus rather than overnight.
Which of the three lasts longest?
PV modules typically run 30–40 years, solar thermal collectors 25–30, hot water heat pumps 12–15 and diverter controllers 10–15. Lifetime matters most for the components that are difficult to reach: the roof-mounted ones.
What if I have no cylinder at all?
Then a diverter is not an option, because there is nothing to heat. The realistic choices become a hot water heat pump with an integral cylinder, or a solar pre-heat store working alongside the existing combi boiler.