Can excess solar power run storage heaters or underfloor heating?

The scale of the surplus is the constraint. On 21 and 22 June 2026 a typical UK rooftop solar installation generated 15 kWh, equivalent to five hours of air conditioning use, and across 1.88 million domestic-scale installations the average was 28.5 GWh per day, or 15.2 kWh per installation². Those are peak summer days. In winter the same array produces a fraction of that, which is why heating from surplus alone is a summer story unless a battery or a heat store carries the energy across.

Three routes connect panels to warmth: a battery that shifts midday generation into the evening, a diverter that sends unused electricity to an immersion heater, and a heat battery that stores warmth rather than charge. Each has a different cost, a different loss and a different effect on how independent the household actually is.

Why storage heaters and underfloor heating are a poor direct match for solar panels

The mismatch is one of timing and control, not of voltage. Storage heaters do not produce heat instantly like a regular electric heater; they rely on the heat stored overnight, so planning ahead is essential⁶. That design assumes a cheap overnight charging window, not a midday generation peak. A solar array peaks around the middle of the day, which is when a well-insulated home often needs least heat. The result is that a storage heater charged by solar in the morning can leave a home very warm in the morning and struggle to stay warm in the evening⁷.

Running costs compound the problem. Compared with liquid fuel, night storage heaters have significant disadvantages because they are expensive to use and offer limited control⁸. That judgement comes from an off-gas-grid heating body, and it applies to the appliance rather than to the fuel feeding it, so surplus solar improves the carbon position more than it improves the cost position.

Underfloor heating has its own constraints. Electric underfloor heating carries higher running costs than the water-based alternative⁹. It is also incompatible with some floor coverings: thick carpet traps too much heat and solid wood becomes warped by the temperature changes¹⁰. Where a wet system is installed, independent guidance notes that solar panels could generate the electricity a heat pump needs, or power a dry electric underfloor heating system, and that a battery to store excess solar electricity would make this more efficient¹¹.

The practical picture is that both technologies can absorb surplus solar, but neither is designed around it. They need controls that can be told when to draw power, and they need the surplus to arrive when the heat is wanted. Where those two conditions are not met, the generation is better used elsewhere or exported.

The role of a battery: turning midday surplus into evening heat

A battery is the most direct answer to the timing problem. Guidance recommends installing battery storage to store the extra energy created during the day, so it can be used at times when the panels are generating less, such as at night¹². A home battery captures surplus solar energy for later use, including overnight electric vehicle charging, whereas without it unused solar electricity is wasted or exported to the grid, often at a relatively low rate¹³.

The same principle applies to heat. Excess power produced during the day can be stored and used when demand is higher¹⁴. A battery can also be charged when a tariff is cheap and discharged at night or when prices are high, so it is not limited to solar input¹⁵. That flexibility matters for a household trying to reduce what it draws from the grid, because it decouples the moment of generation from the moment of use.

For a household already buying a battery, the guidance is explicit: consider getting battery storage if the system does not already have it, so surplus electricity is stored instead of exported¹⁶. A solar PV system can be paired with a battery storage system to store excess electricity for when it is most needed¹⁷.

There is a limit worth stating plainly. A battery shifts electricity, not heat, and it holds a finite number of kilowatt hours. A storage heater on a cold evening can draw more than a domestic battery holds, so the battery defers grid import rather than removing it. The household remains connected to the grid and to a supplier, and in winter the heating load will exceed what the array and battery can cover. Battery storage is best paired with heat pumps rather than with solar panels where the aim is savings on an appropriate tariff¹⁸, which suggests the heat source itself matters as much as the store behind it.

Smart diverters, immersion elements and heat batteries: the routes from panel to warmth

Where a hot water cylinder is present, a diverter is the simplest route. A diverter allows unused electricity to be directed to another appliance, usually a hot water cylinder¹⁹. Official guidance describes a solar diverter switch powering the immersion heater in a hot water tank for free, storing hot water for later use²⁰. Independent guidance from Scotland puts it the same way: instead of sending surplus electricity to the grid, a PV diverter switch can power the hot water tank's immersion heater, storing hot water for later use²¹.

Solar water heating offers a related but separate route. During summer, solar thermal panels can meet all of a household's hot water needs, but a backup system is still needed, especially during winter²². Some solar thermal systems use two cylinders, one dedicated to the solar collectors and another for the immersion heater²³. The Welsh Government's guidance caps a solar heated water storage volume at less than 440 litres for indirect systems supplying domestic hot water³, and the same figure appears in Approved Document L⁴.

Heat batteries take a different approach: they store energy as heat, provided by electricity from the grid or your own renewable source, or by surplus heat from a boiler, heat pump or another heating system⁵. Some can be used as a thermal store within a central heating system, typically alongside a heat pump⁵. For a household, the attraction is that heat storage is cheaper per kilowatt hour than battery storage and does not degrade in the same way, but it can only serve heating and hot water, not lighting or appliances.

Modern storage heaters use digital controls, while older heaters have manual controls⁶, and that difference decides whether a heater can be scheduled around generation at all. A heater with only a manual input dial cannot respond to a sunny afternoon.

Costs, savings and payback: what the surplus is actually worth

No published prices exist for diverters, batteries or heat batteries, so any figure here is installer-quoted. What can be stated is what the surplus is worth in energy terms and what the documented savings look like. Storing solar surplus to offset peak-rate imports rather than exporting it at a low SEG rate is put at over £320 a year for a typical household¹, and adding battery storage to rooftop solar is worth another £400 to £600 a year². A pass-through tariff with solar PV and battery storage, where the storage responds to price signals, is modelled at £1,002 a year³.

On the generation side, the summer peak is substantial: 15 kWh for a typical installation on 21 and 22 June 2026, and 15.2 kWh average across 1.88 million domestic-scale installations². Across the 1.9 million UK homes with rooftop solar, that was equivalent to 10 million solar-powered air conditioning hours per heatwave day². Those figures describe June, not the heating season, and the winter gap is where the shortfall sits.

On the heating side, high heat-retention storage heaters retain up to 45% of their heat even 24 hours after they were last charged³, which is what makes day charging viable at all. Upgrading to a modern storage heater can help reduce energy bills by about 10%⁴. For households modelling a wider system, a pass-through tariff with solar PV and battery storage produced an average annual saving of £1,002 per year in modelling where solar PV and battery storage respond to price signals and the heat pump does not⁵.

Export remains the alternative. Surplus solar electricity can be sold back to an energy supplier through a Smart Export Guarantee tariff when the battery is full¹⁵, and the scheme exists to sell surplus solar electricity to the grid²⁰. The same route is described as selling excess energy generated back to the grid via the Smart Export Guarantee²⁴. Whether using surplus for heat beats exporting depends on the export rate and the cost of the fuel displaced, and no rate is given here.

Two conditions shape the practical outcome. First, eligibility rules for some funded measures require solar PV to be installed alongside a hydronic heat pump, an electric storage heater with a SAP responsiveness of 0.8 or above, or an electric heating system with the same responsiveness, or the property to be heated by one of these already²⁵. Second, for park homes, electric storage heaters are only an option where the home has its own electricity supply²⁶. Battery storage added alongside plug-in solar panels must be permanently wired and installed by a qualified electrician²⁷.

For a household, the independence question is straightforward. Surplus solar used for heat reduces the electricity and gas drawn from a supplier, and a diverter or heat battery does so without a cloud service or an app in the loop. What remains is the grid connection, the winter shortfall, and for battery systems the manufacturer's software. The winter gap is the limit that no amount of summer diversion removes, and self-consumption is the measure that decides how much of the surplus is captured rather than exported.