How it works
Solar thermal collectors — flat plate panels or evacuated tubes — absorb solar radiation and transfer the heat via a glycol loop to a coil in a twin-coil hot water cylinder. A well-sized system provides 50–60% of a dwelling’s annual hot water, with the boiler or heat pump topping up. Per square metre, solar thermal converts sunlight to useful energy more efficiently than PV.
In the SAP calculation
SAP credits solar hot water against the dwelling’s water-heating energy, improving the DER — a useful contribution, though smaller than what a decently sized PV array achieves, because water heating is only part of a dwelling’s energy balance.
Solar thermal vs PV + diverter
The market has largely moved to PV: panel prices fell, and a PV diverter (now recognised in SAP 10) sends surplus PV generation into the immersion heater — heating water and offsetting electricity, with fewer roof penetrations, no glycol maintenance, and full credit in the calculation. Solar thermal still suits projects with high hot water demand and limited roof area.
Unsure which suits your project and planning condition? We’ll model both.