typical 4kW system exports 1,200-2,000
Advanced options
What The Solar Diverter Calculator Works Out
- The trade per kWh diverted: each kilowatt-hour of solar sent to the immersion heater saves the hot-water fuel it replaces but sacrifices the export payment, so the net is gas rate minus export rate. The gas rate defaults to the current Ofgem price cap: at 15p export against gas at around 8p, diverting loses about 7p a unit.
- Verdict: the net per kWh multiplied by the divertable kilowatt-hours, capped at the roughly 1,200 kWh a year a cylinder can absorb. A positive figure shows the payback on a GBP 425 fitted diverter; a negative one says keep exporting. In the zero-export-payment years it was a two-year payback; for a gas household on a decent SEG tariff it is now a money-loser.
- The era check: the answer flips at export equals fuel rate, and both sides reprice annually. Oil, LPG and electric hot water households should enter their own fuel rate: at the price cap electricity rate (about 26p) the diverter wins decisively.
- Cylinder capacity (advanced): how much hot water the tank can actually take each year. Most of a 4kW system's export happens in summer midday, and a combi household has nothing to divert into.
- Diverter cost (advanced): the fitted price, typically GBP 350 to GBP 500, used for payback and the net position.
- Hot water boiler efficiency (advanced): a gas boiler heating a cylinder runs at 80% to 90%, so each kWh of solar heat replaces slightly more than one kWh of gas. Defaults to 100% so the basic answer is unchanged.
- Legacy FIT with deemed export (advanced): early Feed-in Tariff households are paid an assumed export regardless of what they actually send out, so diverting loses nothing and every kWh is pure gain: free hot water with the cheque intact.
- Break-even export rate (advanced): the export price at which diverting stops paying, equal to the fuel saved per kWh. Compare it with the best SEG rates on offer, some of which now approach import rates.
- Net position and export income (advanced): the yearly figure added up over your chosen years less the diverter's cost, and the export income you keep on the undiverted kWh against the income the diverted kWh give up.
- Free alternative: a timer (advanced): moving the existing hot-water schedule to midday uses solar you would otherwise export, with no hardware. Enter the share you could shift to see its value; registering for SEG at all is often the better first fix.