Before you begin
What affects solar diverter payback
Use the export rate currently paid by your supplier.
Choose the heating source the diverter would genuinely displace, not the most expensive source in the house.
Where the tool asks what that heat costs, answer for a usable kWh at the tap rather than for the fuel. Divide what the fuel costs per unit by the share of it that actually reaches the water, because a boiler and a heat pump both deliver less heat than the energy they consume.
The three common cases worked through, which is also where the figures the field opens on come from: gas at 12c a unit through a boiler running at about 85% is roughly 14c of usable heat; kerosene at €1.10 a litre holds 10.35 kWh, so about 10.6c a unit, or 12.5c usable through the same boiler; and a heat pump on 36c electricity reaching a hot-water COP of 2.75 is about 13c.
Reading the answer
Understanding whether a diverter pays
A negative net value means every diverted unit is worth less than simply exporting it.
The result caps hot-water capture to reflect the mismatch between summer surplus and year-round demand.
Boundaries
Assumptions and limitations
Working assumptions
- Generation: 900 kWh per installed kWp per year.
- Surplus: 50% without a battery or 20% after a battery; capture efficiency 85%.
- Hot-water demand: 1.3 kWh per person per day, with a 55% seasonal capture cap.
- Displaced heat: whatever you enter. The figures the fields open on are broad starting points, not researched norms for your house.
Where to be careful
- Roof yield, household load timing, cylinder losses and control strategy vary.
- The verdict turns on the cost of the heat being displaced, and that figure is yours rather than a maintained one. A default left untouched is a guess, and the result says so.
Worked example
The same question, answered end to end
A 4.2 kWp array with no battery, three people, heating water by daytime immersion, exporting at 20c and importing at 36c, with a €550 diverter.
What was entered
- A 4.2 kWp array, no battery competing for the same surplus.
- Three people in the household, water currently heated by immersion during the day.
- 36c to import, 20c paid for exports.
- €550 for the diverter, installed.
How it is worked out
4.2 kWp at 900 kWh/kWp gives about 3,780 kWh of generation a year.
The part of that the household does not use as it is produced, and that the hot-water demand can actually absorb, is the divertable surplus.
Each diverted unit replaces electricity that would have cost 36c, but gives up the 20c the supplier would have paid for it.
The net value of a diverted unit is therefore 16c, not 36c — which is the number most diverter marketing leaves out.
Annual saving is the surplus multiplied by 16c, and payback is €550 divided by that.
What the tool returns
- Surplus you could divert
- 783 kWh/year
- Net value per diverted unit
- +16.0c/kWh
- Annual saving versus exporting
- €125/year
- Simple payback
- 4.4 years
Common questions
Questions about this tool
What should I put for the cost of the heat a diverter replaces?
The cost of a kWh that actually reaches the water. Take the fuel price and divide by the efficiency that turns it into hot water: gas bought at about 12c a kWh through a boiler that is 85% efficient on a summer hot-water cycle costs roughly 14c a usable kWh. A heat pump is the other direction — 36c of electricity at a hot-water COP of about 2.8 is roughly 13c. If the figure is a guess, treat a verdict close to the line as undecided.
Why does an export payment weaken the diverter case?
Sending a unit to the immersion means giving up the payment for exporting that unit. Only the difference between displaced heat cost and export value is a saving.
Does a battery make a diverter less useful?
Usually. The battery absorbs much of the surplus first, leaving fewer units for the diverter.