Solar Thermal Electricity vs Photovoltaic Energy Output Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

Per square metre of roof, solar thermal is the more efficient harvester, converting 50 to 70 percent of solar energy into useful heat, around 300 to 450 kWh per square metre per year, while PV converts 18 to 25 percent into electricity, around 150 to 210 kWh per square metre. The catch is that a kWh of heat is worth less than a kWh of electricity, can only be used for hot water, and cannot be exported, which is why raw output alone does not settle the choice. One clarification on the name: domestic systems in the UK produce heat, not electricity; solar thermal electricity generation, known as concentrated solar power, is a utility-scale technology for desert climates and is not available for UK homes.

Roof area to compare (square metres)

Enter the roof area you could dedicate to either technology. A typical domestic solar thermal installation uses 2 to 5 square metres of collectors, while PV systems commonly occupy 15 to 25 square metres, so small areas favour comparing like for like and large areas increasingly favour PV, which can always use the extra space productively.

Region (solar resource)

The same panels generate more in Cornwall than in Caithness. The industry baseline is around 850 kWh per year for every kWp installed on an unshaded south-facing roof in an average UK location, with southern England running around 5 percent above that and Scotland 10 to 15 percent below. The calculator scales its output and savings figures accordingly.

What else affects the figures

The value comparison narrows dramatically once usage is considered: thermal heat displaces gas at roughly 6 to 8 pence per kWh in most homes, while PV electricity displaces grid supply at 25 to 26 pence or earns export income, so a square metre of PV often returns as much money as a square metre of thermal despite harvesting less energy, with far more flexibility. PV also enjoys 0 percent VAT and Smart Export Guarantee income, and surplus PV can heat water anyway through a to immersion diverter. Solar thermal remains strongest for households with very high hot water demand or roofs too small for a worthwhile PV array, and the two can happily share a roof where space allows.

Certification, planning and grid connection

Solar thermal collectors generally fall under permitted development on houses, subject to the usual 200 millimetre projection limit and stricter rules for listed buildings and conservation areas, so check with your local planning authority if either applies. Choose an MCS-certified installer for quality assurance even though solar thermal earns no Smart Export Guarantee, since it generates heat rather than electricity. VAT treatment differs from solar PV and is worth confirming on your quote, and unvented cylinder work must be carried out by a suitably qualified installer under the building regulations, with the cylinder notified to building control where required.