The usual answer to surplus rooftop PV in a British home has been an immersion diverter. A current clamp on the meter tails detects export, and the diverter feeds that spare power into the immersion element in the hot water cylinder, varying the power to match whatever is spare. The kit is cheap, it works with almost any cylinder, and it suited homes heated by gas, where the cylinder’s hot water would otherwise have come from the boiler.
Homes in England and Wales are replacing gas boilers with air-to-water heat pumps under the Boiler Upgrade Scheme, and some of them already have panels on the roof or add them at the same time. In those homes the diverter’s arithmetic stops working the way it used to.
An immersion element is a resistance heater. One kWh of electricity in gives one kWh of heat in the water. A heat pump moves heat from outside air into the water, so the same kWh of electricity delivers several kWh of heat.
The government’s Electrification of Heat demonstration project ran one of the largest UK field trials of heat pump performance. Its interim analysis by Energy Systems Catapult measured a median seasonal performance factor of 2.80 across 291 air source heat pumps, using a boundary that counts space heating, hot water, back-up heaters and immersion use together. The report notes that summer hot water runs are less efficient than the annual picture, because the heat pump runs less often and works against a higher target temperature.
The comparison does not need a precise summer figure. A heat pump in hot water mode beats the immersion for every kWh of surplus as long as it delivers more than one kWh of heat per kWh of electricity, and a working air-to-water unit clears that bar by a wide margin.
The value of a kWh of surplus PV depends on what it displaces. Take the Ofgem price cap for October to December 2026, 26.32 p per kWh of electricity for a direct debit customer, and the EoH median of 2.80 p.
In a heat pump home, the heat pump would otherwise make the cylinder’s hot water from grid power at about 9.4 p per kWh of heat. A kWh of surplus sent through a diverter into the immersion makes one kWh of heat, so it saves about 9.4 p. If the heat pump itself runs on that same kWh of surplus, the electricity it would have bought from the grid comes off the roof instead, and the saving is the full 26.32 p. On these figures, pointing surplus at the heat pump is worth close to three times as much as pointing it at the immersion.
The same arithmetic changes how the diverter compares with export. Under the Smart Export Guarantee, each supplier sets its own export rate, and Ofgem requires only that it stays above zero. In a gas-heated home the diverter competed against the gas price. In a heat pump home it competes against 9.4 p, so a household on an export tariff paying more than that would do better selling the power than heating water with the element.
Time-of-use tariffs move the numbers. A household that heats its cylinder overnight on a cheap rate pays far less than the cap for that heat, which shrinks the value of any daytime surplus spent on hot water. The tariff should be chosen before the controls are set. “Somebody on a cheap overnight rate and a good export rate may be better off exporting at midday and heating water at 2am. Somebody on a flat rate should run the cylinder in the solar window. You cannot set the controls without knowing the tariff,” reads Eco Renewables’ guide to time-of-use tariffs for heat pump homes.
The diverter keeps one practical advantage. It can use any surplus, however small, because it can run the element at any power level. A heat pump compressor has a minimum power draw and works best on longer runs, so it needs a steady surplus through a stretch of the day. A British June afternoon with broken cloud gives a diverter something to use and may give a heat pump controller nothing it can act on.
The cylinder also limits the gain. Lifting 200 litres of water by 10 C stores about 2.3 kWh of heat. A heat pump at an efficiency of 2.8 needs about 0.8 kWh of electricity to do it, where an immersion needs the full 2.3 kWh. The heat pump fills the same tank from a third of the surplus, which leaves the rest of the afternoon’s generation for a battery or for export.
The immersion has a second job in a heat pump home: a periodic pasteurisation cycle. HSE guidance on legionella says hot water should be stored at 60 C or above to kill the bacteria, a temperature that costs some heat pumps more electricity to reach than their normal hot water setpoint. Scheduling that cycle in the middle of a sunny day lets the PV pay for it, and it gives the diverter a role in a heat pump home without competing with the compressor.
The control link is the harder part. Germany’s heat pump association BWP runs the SG Ready label, which defines the operating states a heat pump controller must accept from outside. In state 3 the heat pump runs in boosted mode and raises the setpoint in the buffer or hot water store to soak up surplus electricity. State 4 is a definite start command. An inverter, energy manager or battery system with a free contact can signal the heat pump when export passes a threshold the installer sets.
The decision about priority belongs at the design stage. “We work out which load takes the surplus first: the battery, the cylinder through the heat pump, or the immersion. If the battery fills by lunchtime in summer, the heat pump should get the next kilowatt-hour, and the immersion only the scraps,” reads Eco Renewables’ guide to running a heat pump alongside rooftop solar and its explainer on how a heat pump heats a hot water cylinder covers cylinder sizing and reheat times.
The seasons limit all of this. The surplus is largest in summer, when the heat pump’s only load is hot water, and smallest in December, when space heating demand peaks and a south-facing array in Berkshire produces a fraction of its June output. Hot water is the one heat load that runs all year, which is why it is the natural first home for spare PV.
Installing heat pumps, solar panels and batteries in homes in Great Britain carries 0% VAT until 31 March 2027 under HMRC notice 708/6, rising to 5% from 1 April 2027. The 28 October Budget confirmed that date and added heat batteries, the phase-change stores sold as compact cylinder replacements, to the energy-saving materials list at 5% from the same day. A heat battery charged by its own electric element sits on the immersion side of the arithmetic above, so a home choosing one over a cylinder gives up the heat pump’s multiplier on that store.
Author: Adam Dare
Adam Dare is Managing Director of Eco Renewables, an MCS certified installer of heat pumps, solar panels and home batteries in Binfield, Berkshire. He is Heat Geek trained and verified, with a Gas Safe background.
The views and opinions expressed in this article are the author’s own, and do not necessarily reflect those held by pv magazine.
This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: [email protected].
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