Immersion diverters have traditionally offered a simple way for UK homes to use spare rooftop solar power. A meter sensor detects electricity being exported and sends it to a cylinder’s immersion element, which turns electricity directly into heat. But for households replacing gas boilers with air-to-water heat pumps, using surplus power to run the heat pump can provide more hot water from each kilowatt-hour.
An immersion element produces one kilowatt-hour of heat from one kilowatt-hour of electricity. A heat pump transfers heat from the air, so it can deliver several kilowatt-hours of heat per kilowatt-hour consumed. The UK Electrification of Heat demonstration project, analysed by Energy Systems Catapult, found a median seasonal performance factor of 2.80 across 291 air-source heat pumps. The figure includes heating, hot water, backup heaters and immersion use; the report cautions that summer hot-water operation can be less efficient than the annual average.
The article compares the options using an electricity price of 26.32 pence per kilowatt-hour under Ofgem’s price cap for October to December 2026, alongside the trial’s 2.80 median performance factor. On that basis, heat produced by a heat pump using grid electricity costs about 9.4 pence per kilowatt-hour. Surplus solar sent to an immersion element avoids that cost, while surplus sent to the heat pump displaces a full unit of grid electricity. The resulting value is close to three times higher for the heat pump, the article says.
Export rates and household tariffs can change the calculation. Under the Smart Export Guarantee, suppliers set their own rates, which Ofgem requires to be above zero. If a home’s export payment is higher than the value of using electricity in its immersion heater, exporting may be preferable. A cheap overnight electricity tariff can also make it more economical to sell daytime solar power and heat water later. Households should therefore consider their tariff before setting up their controls.
Immersion diverters nevertheless have a practical advantage: they can use small or fluctuating amounts of surplus power. Heat pumps have a minimum power draw and generally work better over longer runs, so variable sunshine may not provide enough steady power for the compressor to operate. The hot-water cylinder also puts a limit on how much surplus can be stored as heat.
The article gives the example of raising the temperature of 200 litres of water by 10°C, which stores about 2.3 kilowatt-hours of heat. A heat pump operating at an efficiency of 2.8 would need around 0.8 kilowatt-hours of electricity for this, compared with 2.3 kilowatt-hours for an immersion element. The remaining solar generation could then be directed to a battery or exported.
An immersion element may still be useful for periodic high-temperature water treatment. The article cites Health and Safety Executive guidance recommending that hot water be stored at 60°C or above to kill Legionella bacteria. Scheduling such a cycle for a sunny period can allow solar generation to power it.
Coordinating solar, batteries and heat pumps requires a control link. Germany’s BWP SG Ready label defines operating states that external controls can send to a heat pump. In one state, the pump runs in boosted mode and raises the target temperature in a hot-water store or buffer; another provides a start command. An inverter, energy manager or battery system with a compatible contact can signal the heat pump when export exceeds an installer-set threshold.
The system’s priorities matter too. A household may direct surplus first to its battery, then to hot water through the heat pump, and finally to an immersion element. The article notes that a battery filled by midday may leave subsequent solar generation available for the heat pump. Seasonal conditions also affect the opportunity: surplus is generally greatest in summer, when hot water may be the heat pump’s main load, and smallest in winter, when space-heating demand is higher and solar output lower.
The article also notes that installations of heat pumps, solar panels and batteries in Great Britain are subject to 0% VAT until 31 March 2027, rising to 5% from 1 April 2027. It says the 28 October Budget added heat batteries to the energy-saving materials list at 5% from that date. A heat battery charged by an electric element follows the same basic one-to-one electricity-to-heat calculation as an immersion heater, rather than the heat pump’s efficiency multiplier.
Sources: pv magazine.