How the systems work
Roof-mounted collectors absorb solar heat and transfer it to water, either directly or through a heat-transfer fluid in a closed loop. The heated water goes to a storage tank, and a conventional heater covers the shortfall.
Two broad families. Direct systems circulate the household water itself, which is simple and only viable where freezing is not a concern. Indirect systems circulate an antifreeze fluid through a heat exchanger, which is what any cold climate requires.
Within those, circulation is either active (a pump, and therefore a component that can fail) or passive (thermosiphon, relying on hot water rising, with nothing to break but more constraints on layout).
Why photovoltaics won
Not on physics. On economics and flexibility.
A thermal collector is more efficient at capturing energy per square metre than a PV panel. But it produces heat, and only heat, and only when you want hot water. Surplus heat in July has nowhere to go.
PV produces electricity, which heats water, runs the air conditioning, charges a car, or goes back to the grid for credit. As PV prices fell, that flexibility stopped being a luxury.
A heat pump water heater running on PV electricity now covers the same job for many households, with one technology on the roof instead of two.
Where thermal still makes sense
Specific, real cases — not "everywhere", and not "nowhere".
- Large, constant hot water demand. Big households, laundries, and anywhere the tank is genuinely being emptied daily and year-round.
- Pool heating. The strongest residential case by some distance. Pools want exactly what a collector produces: a lot of low-grade heat.
- Expensive electric or propane water heating. Where the incumbent fuel is costly, the offset is worth more.
- Limited roof space with high hot water needs. A collector array delivers more hot water per square metre than a PV array powering a heater.
What it demands that PV does not
PV has no moving parts. Solar thermal has pumps, valves, expansion tanks, sensors and often a glycol fluid that degrades and needs replacing on a schedule.
Freeze protection is not optional in a cold climate and it is the most common failure point. So is stagnation — an unused collector in midsummer gets extremely hot, and the system has to handle that.
Budget for maintenance in a way you would not for PV.
How the work runs
- Assess hot water demandActual daily usage and its seasonality. This decides whether the answer is thermal at all.
- Choose direct or indirectFreezing conditions make this decision for you.
- Choose active or passive circulationPump reliability and efficiency against simplicity and roof loading.
- Size collectors and tankMatched to demand. Oversizing creates stagnation problems rather than more hot water.
- Structural checkA full solar storage tank is heavy, and roof-mounted tanks in passive systems especially so.
- Permit and installPlumbing and sometimes structural permits. Different trade from PV.
- Commission and set the backupThe conventional heater should fill the gap without fighting the solar contribution.
What moves the price
We do not publish a national average and present it as your price. These are the variables that separate two quotes for the same job — run your own numbers with the cost guides and calculators, and check what incentives you actually qualify for.
| Factor | Why it moves the number |
|---|---|
| Direct vs indirect | A closed-loop system with a heat exchanger and glycol costs more than a direct one. |
| Active vs passive | Pumps, controllers and sensors against a simpler thermosiphon layout. |
| Collector area | Driven by demand, not by roof size. |
| Tank and plumbing | Retrofitting into existing plumbing can be a significant part of the job. |
| Freeze protection | Required in cold climates and non-negotiable. |
| Ongoing maintenance | Fluid replacement and pump service are real recurring costs. PV has no equivalent. |
What to ask before you commit
Take this list to the conversation. A professional who answers these directly is telling you a lot; one who deflects is telling you more.
- What fraction of my annual hot water will this actually cover?
- Would PV plus a heat pump water heater do this job for less, on the same roof?
- Direct or indirect, and what is the freeze protection strategy?
- What is the maintenance schedule and what does it cost per year?
- How does the system handle stagnation in midsummer?
- Can my roof and my plumbing take the tank where you propose to put it?
Common questions
Is solar water heating better than solar panels?
Better at heating water, worse at everything else. A thermal collector captures more energy per square metre, but produces only heat. PV produces electricity, which is flexible, and has become cheap enough that it usually wins overall.
Does solar water heating work in winter?
It works, at reduced output, and only if it has proper freeze protection — an indirect closed-loop system in any climate that freezes. A conventional heater covers the shortfall.
Can I have both solar water heating and solar panels?
Yes, roof space permitting. In practice most households now find PV plus an efficient water heater simpler than maintaining two roof technologies.
What maintenance does it need?
More than PV. Pump and controller servicing on active systems, periodic heat-transfer fluid replacement on closed-loop systems, and checks on valves and expansion tanks.