Can a home battery run a heat pump in a power cut?

Yes, a home battery backup keeps a heat pump running in a power cut. It needs the heat pump on the backup output and enough inverter power per phase. It will not heat a large house alone through winter days, so plan hours of full comfort and days of reduced comfort with a generator.

A large house without gas: a heat pump for the floors and the hot water, an induction hob, an electric car. A Dutch owner, after six cuts in two months, woke up in a cold house with no shower and no coffee.

In a home in Wetteren the whole house runs through the battery on all three phases: 9 kVA, 15.4 kWh.

What happens to a heat pump in a power cut?

Without backup it stops, like every other heating system. A New York State programme puts it plainly: when the power goes out, heat pumps "are no different than any other HVAC system". None of them work.

A gas boiler also needs electricity for its pump and controls. But an all-electric house has no fallback at all.

One owner wrote on Reddit that his home now depends completely on electricity, because it no longer has gas. The heat stored in the floor and the hot-water tank buys some hours, then the house cools down.

Many quotes do not say it: a standard home battery does not keep the heat pump running at all. The heat pump stops, the solar inverter must switch off, and a hybrid inverter without a backup output does the same. Only a battery that forms its own grid, with a real backup output sized for the heat pump, keeps the house warm.

Which set-ups keep running, and how fast they switch over, is in our guide to a home battery with backup power.

What does a heat pump need from a home battery backup?

Two numbers must fit: the power per phase and the energy stored. The inverter must deliver what the heat pump draws at that moment; the battery must hold enough kWh for the hours you want to bridge.

How much power does the battery need to start and run the heat pump?

Enough continuous power per phase for the compressor on the coldest hour, and enough peak power for its start. Check both against the heat pump datasheet, not against the total kVA on the quote.

Most modern air-to-water heat pumps have an inverter compressor that ramps up slowly, so the start peak stays small. Heat pumps with an on/off compressor start harder. The datasheet of your model gives its maximum input and, often, its starting current.

On the battery side, a Victron MultiPlus-II 48/5000 delivers 4,000 W continuous at 25 °C and a peak of 9,000 W for a short start. A larger heat pump is often three-phase, and in a power cut each phase only gets what its own inverter delivers. So heat pump, hob and oven are spread over the phases, not stacked on one.

Our article on three-phase backup explains the power per phase; a 3x230 V connection that is too weak adds limits.

The real risk is the electric back-up heater. Energuide writes that most modern heat pumps include one.

It "kicks in when the heat pump can no longer meet the heating demand" and "consumes a lot of electricity". On battery power that is exactly what you do not want. Energuide also warns that switching it off for good can cause problems in very cold weather and may affect the warranty.

So ask your installer and the heat pump supplier how the back-up heater behaves when the grid is down.

Does the battery carry the heat pump through a winter day and a long outage?

Through an evening and a night, yes; through several grey days, rarely. The problem is the sun: in December and January, solar panels in Belgium produce a fraction of June.

A French owner (A-rated house, heat pump) logged 20 to 25 kWh a day in December and January, with peaks of 30. A Dutch owner with 8 kVA and 45 kWh of batteries wrote that his home "quickly" uses 40 kWh a day on cold days. That is two to four times the roughly 10 kWh of sun on an average December day, and a dark day gives much less.

With round numbers: a house using 30 kWh on a cold day, a 15.4 kWh battery and 10 kWp of panels. The battery covers roughly half a day, the panels about a third on an average December day; the rest must come from elsewhere. A 30.8 kWh battery, as in our project in [Kortenberg](https://www.youpower.be/en/references/home-batteries/92-batteries-3-phases-full-backup), doubles the hours but does not change the winter.

So the winter plan matters more than the battery size.

In a long outage you lower the room temperature a few degrees and block the back-up heater. You heat water only on sun and keep a generator input ready. Do nothing and the battery may run flat at night: solar panels on the backup side then cannot restart the system next morning.

Each step exists in our projects.

The switchover within 20 milliseconds is fast enough for computers, so the heat pump controls do not notice either. A GX controller can start a back-up generator automatically. In Maaseik the owner already had one; the home battery got an off-grid mode that lets it connect.

These four three-phase home batteries we installed each have 9 kVA. They show what stays on when the street fails: the whole house, a full backup in off-grid mode, or off-grid mode with a generator. Click a town for the details.

Which circuits go on the backup with a heat pump?

The heat pump, its controls and pumps, the fridge and freezer, the network and the lights. Not the car charger, and the back-up heater only under control.

Check how the back-up heater is wired. If it shares a group with the compressor in the distribution board, it comes onto the battery without anyone noticing. Its own group is part of the design, and easier on day one than after the first cold outage.

What does SG Ready change for a heat pump with a battery?

SG Ready lets a battery system tell the heat pump when to run harder and when to hold back. It is a label of the German heat pump association BWP for heat pumps with a standard control input.

The interface has four operating states. State 1 is a block of at most two hours, state 2 normal operation, state 3 a boost for room heating and hot water. State 4 is a definite start command, as far as the heat pump settings allow.

• On sunny days: the system sends a boost (state 3) when the battery is full and the panels still produce. Floor, buffer and hot-water tank then store the sun as heat, a second battery that costs nothing extra.
• During a power cut: the system can hold the heat pump back (state 1) when the battery runs low, so the fridge, network and controls keep going. The block lasts at most two hours, which fits the hours before a generator or the grid returns.

Whether it works depends on your heat pump model and how the installer wires the two contacts. Ask for it when you order the heat pump: adding it later is often possible, but not always.

Does a home battery also lower the capacity tariff of a heat pump house?

It can, because a heat pump raises your monthly peak. In Flanders the capacity tariff is based on your highest quarter-hour of each month, with a minimum of 2.5 kW.

A heat pump on a cold morning, the back-up heater, the hob and a charging car at the same time: that quarter-hour sets the bill. A battery that takes over part of that load keeps the peak lower, the peak shaving described on our home battery page. In Wallonia and Brussels the rules for homes are different.

For a business with a heat pump the calculation differs again: see what a heat pump does to a business connection.