Before you begin
What this tool needs
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.
Reading the answer
What your result means
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.
Where to be careful
- Roof yield, household load timing, cylinder losses and control strategy vary.
- The cost of boiler and heat-pump heat is fixed by the option you choose and cannot be typed in directly.
Common questions
Questions about this tool
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.