Solar Thermal Calculator

A well-sized solar thermal system typically provides 50-70% of a household's annual hot water demand. In summer it can provide close to 100%, while in winter it may only pre-heat the water. The annual solar fraction depends on collector size, orientation and household demand.

Flat plate collectors are cheaper, more durable and integrate well with roofs. Evacuated tubes perform slightly better in cold/overcast conditions and produce more per m², but cost more and can be fragile. For most UK domestic installations, flat plates are the best value.

A typical domestic solar thermal system costs £3,500-£5,500 including collector panel, twin-coil cylinder, controls and installation. Evacuated tube systems cost 10-20% more than flat plate. A new twin-coil cylinder adds £500-£800 if not included.

Solar thermal requires a twin-coil cylinder (or a thermal store) with a dedicated solar coil. Size should be approximately 50 litres per person, minimum 150 litres. The solar coil must be at the bottom of the cylinder with the boiler coil above. Unvented cylinders are preferred.

Solar PV is generally considered better value now because surplus electricity can be exported (SEG payments), used for other appliances, or stored in batteries. Solar thermal only heats water. However, solar thermal produces more energy per m² of roof and is efficient for homes with high hot water demand.