Heat Pump in a Power Cut: What Happens and How to Prepare

A heat pump runs on electricity, so the honest answer to the question everyone asks is simple: in a power cut, your heat pump stops, exactly like the modern gas boiler next door. The reassuring part is what happens next. Almost every system sold in the UK in the last decade restarts on its own when the power returns, your home holds its warmth for far longer than most people expect, and there are practical ways to keep the heating going through an outage if you want them.

This guide, written for UK homeowners, walks through what really happens when the lights go out, how long a well-insulated home stays comfortable, the backup options worth considering, and a clear step-by-step routine for preparing before and recovering after an outage. I have commissioned and serviced hundreds of air source systems across Yorkshire, and the power-cut worry comes up on almost every job. It is usually overblown, but a little preparation removes it entirely.

What Really Happens to Your Heat Pump in a Power Cut?

When the grid supply drops, your heat pump loses power to three things at once: the compressor that does the heavy lifting, the circulation pumps that move warm water around your radiators or underfloor loops, and the control board that runs the whole show. All of it stops instantly. There is no reservoir of stored heat inside the unit itself, so heating and hot water production pause for as long as the outage lasts.

This is worth stating plainly because of a stubborn myth that heat pumps are uniquely vulnerable. They are not. A modern condensing gas boiler also needs mains electricity for its fan, its circulation pump and its electronic ignition. If your street loses power, the gas boiler down the road is just as dead as your heat pump. The fuel is different; the dependence on electricity is the same.

What sets a heat pump apart in a good way is its recovery behaviour. When the supply returns, the control board reboots and runs through its start-up checks. Rather than firing the compressor straight back up, it waits out a short protective delay, usually a few minutes, before resuming. This is the anti-short-cycle delay, and it exists to stop the compressor restarting against high refrigerant pressure, which would shorten its life. So a brief pause after power is restored is not a fault. It is the system protecting itself, exactly as designed.

For the vast majority of homes the whole episode is invisible. The power flickers off, comes back, the heat pump waits a few minutes, and then quietly resumes. You may never notice it happened.

How Long Will My Home Stay Warm? A Realistic Timeline

This is the question that matters most, and it is the one the top search results tend to skip. The good news is that a heat pump heats your home gently and continuously, which means your house is rarely running on the ragged edge. When the heat input pauses, the building coasts on the heat already stored in its fabric, your furniture and the air.

How fast that warmth fades depends almost entirely on insulation, not on the heat pump. As a realistic rule of thumb:

A well-insulated modern home (good loft and wall insulation, double or triple glazing, draught-proofed) typically loses around 0.5C to 1C per hour in cold weather with no heat input. It can stay genuinely comfortable for several hours.
A typical 1990s-to-2000s UK home with reasonable insulation loses roughly 1C to 1.5C per hour.
A poorly insulated older property with single glazing and gaps can shed 2C or more per hour, cooling noticeably within an hour or two.

So a comfortable home at 21C in a well-insulated property might still be sitting around 17C to 18C after four hours without heat, which is uncomfortable but far from an emergency. This is precisely why fabric improvements matter more than backup gadgets for most people: a warmer, tighter house simply does not care about a two-hour outage. If you want to understand how your system copes when temperatures plunge, our guide to heat pump winter performance covers cold-weather behaviour in detail.

It is also worth knowing how long outages typically last. According to Ofgem's guidance on planning for a power cut, most unplanned cuts are restored within a few hours, and network operators are required to keep customers informed and, in longer outages, provide welfare support. The single nationwide number to remember is 105, the free call that connects you to your local network operator wherever you are in England, Scotland and Wales.

Your Hot Water Is a Hidden Backup

One detail that calms a lot of nerves: your hot water cylinder is, in effect, a thermal battery. A typical 180 to 250 litre cylinder heated to 50C holds a substantial store of usable hot water, enough for washing up, hand-washing and a wash even if the outage stretches on. As long as the cylinder was up to temperature before the cut, you will not be left scrambling for hot water in a short outage. Space heating pauses, but the comfort essentials hold up surprisingly well.

Your Backup Power Options: A UK Homeowner's Comparison

If you live somewhere prone to longer outages, run medical equipment, or simply want peace of mind, there are three broad routes to keeping the heating alive during a cut. They differ enormously in cost, practicality and how much resilience they actually buy you.

Option 1: Home Battery Storage (with or without solar)

A home battery such as a Tesla Powerwall, GivEnergy or Sonnen unit, paired with a backup-capable inverter, can keep your heat pump running through an outage. The catch many people miss is that a standard solar battery does not automatically provide backup power. Most are designed to optimise self-consumption and only supply the house while the grid is up. To run loads during an outage you need a battery and inverter specifically configured with a backup or "island" function and the right protective switchgear, which an MCS-certified installer fits.

The other reality is sizing. A heat pump's compressor can draw a meaningful surge on start-up, and running full heating from a battery drains it fast. A more realistic approach for most homes is partial resilience: a backup-enabled battery keeps the controls, circulation pumps and essential circuits alive and can run the heat pump in shorter bursts, which dramatically slows how fast the house cools. The day-to-day payback comes not from outages but from cheaper running costs, which our guide to combining a heat pump with solar panels and battery storage explores in full.

Upfront cost: high (roughly £5,000 to £12,000+ for a quality battery and backup-capable inverter, before any solar)
Running cost: low, and can cut everyday bills via off-peak charging
Resilience delivered: moderate to high, depending on size and configuration
Best for: homeowners who want lower bills anyway and treat outage resilience as a bonus

Option 2: A Standby Generator

A petrol, diesel or LPG generator can power a heat pump, but it is a blunt instrument for this job. To run a heat pump cleanly you generally need an inverter generator that produces a stable sine-wave output, sized for the compressor's surge, plus a properly installed manual or automatic transfer switch so it can never back-feed the grid and endanger engineers. This is not a job for a cheap building-site generator and an extension lead.

Upfront cost: moderate to high once you include a suitable inverter generator and safe transfer switching (often £1,500 to £4,000+ installed)
Running cost: high while running, plus fuel storage, noise and maintenance
Resilience delivered: high for long outages, if you have fuel
Best for: rural properties with frequent, lengthy cuts where battery capacity falls short

Option 3: A Simple Resilience Kit (no power for the heat pump)

For most UK households, the proportionate answer is not to power the heat pump at all, but to prepare to stay warm and safe for a few hours without it. That means good blankets and warm layers, a charged power bank for phones, a torch, and a plan for where you will go if a cut ever ran into days. Citizens Advice's "Prepare for a power cut" guidance is an excellent, free checklist for exactly this.

Upfront cost: negligible
Running cost: none
Resilience delivered: enough for the short outages that make up the overwhelming majority of cases
Best for: almost everyone, especially well-insulated homes

The honest takeaway is that backup hardware is a genuine luxury or a niche necessity, not a requirement for owning a heat pump. Insulation plus a basic kit covers the realistic risk for the typical UK home.

Can the Boiler Upgrade Scheme Help Fund Resilience?

A common hope is that grants will pay for a battery as part of a heat pump install. To be clear: the Boiler Upgrade Scheme (BUS) provides a grant towards the heat pump itself, applied for by your MCS-certified installer, but it does not fund batteries, generators or standalone backup kit. If resilience matters to you, factor the battery in as a separate investment, ideally one that also lowers your everyday bills. For a full breakdown of what is and is not covered, see our dedicated heat pump grant guide.

Step-by-Step: Preparing For and Recovering From an Outage

Resilience is mostly about routine. Here is the practical sequence I give every customer.

Before a Power Cut

Check your insulation and draught-proofing. The cheapest resilience upgrade there is. A tighter home stays warm for hours.
Sign up to the Priority Services Register if anyone in the home is elderly, disabled, has a long-term health condition or relies on powered medical equipment. It is free, and as Ofgem explains, it can mean advance warning of planned cuts and priority support, including welfare provision in long outages.
Save 105 in your phone, the free number for reporting and checking power cuts anywhere on the GB network.
Prepare a simple kit: blankets, warm layers, a torch, a charged power bank and some food that needs no cooking.
Know where your consumer unit is and which breaker serves the heat pump, so you can reset it confidently if needed.

During a Power Cut

Do not touch the heat pump's controls or outdoor unit. Leave it exactly as it is.
Report the cut by calling 105 if it is not obviously a local trip in your own home.
Conserve warmth: close internal doors, draw curtains, and gather in one room.
Resist the urge to fiddle. The system is designed to restart itself; intervention only risks confusing the recovery.

After Power Is Restored

Wait. Give the controls a few minutes for the anti-short-cycle delay to clear. A brief pause before the compressor restarts is normal.
Check the controller for the home screen and confirm there are no fault codes.
Confirm heating and hot water resume over the following 15 to 30 minutes.
If the system has not restarted after a long outage, carry out a simple reset (below) before calling anyone.

How to Reset a Heat Pump After a Power Cut

If your heat pump does not come back on its own after the power returns, a basic reset clears most niggles:

Turn the thermostat or controller off.
Locate your home's consumer unit (fuse box).
Switch off the circuit breaker that serves the heat pump.
Wait at least 10 minutes so internal pressures equalise and the controls fully power down.
Switch the breaker back on.
Turn the thermostat back on and set it to heating.

If it still will not run, or you see a persistent fault code, stop there and contact your installer rather than digging through engineer-level menus. Ongoing care, including knowing when a reset is appropriate versus when to call a professional, is part of routine upkeep, which our heat pump maintenance costs guide sets in context.

Are Some Heat Pumps Better Than Others in a Power Cut?

In terms of surviving the outage, no, they are all equally off while the power is out. The meaningful differences are about recovery and resilience-readiness:

Clean automatic restart. Almost all modern units self-recover, but reliability of the restart varies a little between models and firmware. A well-commissioned system on current firmware is your best insurance.
Battery and backup compatibility. If resilience is a priority, choose a system and inverter that an installer can integrate with a backup-capable battery, rather than retrofitting it awkwardly later.
Sensible controls. Weather compensation and steady low-temperature running keep the house warmer for longer between heat inputs, which indirectly improves how a home rides out a cut. Our heat pump COP explained guide covers why steady, low-temperature operation matters.

For most buyers, power-cut behaviour should not drive the choice of machine. Insulation, correct sizing and good commissioning matter far more, and they happen to also make outages a non-event.

Frequently asked questions

Does a heat pump work in a power cut?

No. A heat pump needs mains electricity to run its compressor, circulation pumps and controls, so it stops when the power goes out. The same is true of a modern gas or oil boiler, which also relies on electricity for its fan, pump and ignition. The key difference is that almost all modern heat pumps restart automatically once the supply returns.

How do I reset my heat pump after a power cut?

If it does not restart on its own, turn off the thermostat, switch off the heat pump's circuit breaker at your consumer unit, wait at least 10 minutes, then switch the breaker back on and set the thermostat to heating. If it still will not run or shows a fault code, contact your installer rather than altering engineer-level settings yourself.

Why does my heat pump not restart immediately when the power comes back?

That short delay is deliberate. It is the anti-short-cycle protection, which holds the compressor off for a few minutes so it never restarts against high refrigerant pressure. A brief pause before heating resumes is normal and is the system protecting itself, not a fault.

How long will my house stay warm during a power cut?

It depends on insulation, not the heat pump. A well-insulated home typically loses around 0.5C to 1C per hour with no heat input and stays comfortable for several hours, while a poorly insulated property can lose 2C or more per hour. Your hot water cylinder also holds a usable store of hot water through a short outage.

Can I run my heat pump on a battery or generator during an outage?

Yes, but it needs the right setup. A standard solar battery does not provide backup unless it has a backup-capable inverter and the correct switchgear; a generator must be an inverter type sized for the compressor and wired through a proper transfer switch. Both should be specified and installed by an MCS-certified professional. For most homes, good insulation and a basic resilience kit are a more proportionate answer than powering the heat pump.

The Bottom Line

A heat pump in a power cut behaves much like any modern, electrically dependent heating system: it pauses, then almost always restarts itself when the power returns, often without you noticing. Your home holds its warmth for hours if it is reasonably insulated, your hot water cylinder carries you through, and the only number you really need is 105. Backup batteries and generators are worthwhile for some households, but for the typical UK home the smartest preparation is a tighter, better-insulated building, a simple resilience kit, and a place on the Priority Services Register if anyone is vulnerable. Get those right and a power cut stops being a heat pump worry at all.

Reviewed by James Mitchell, Renewable Energy Consultant. Updated June 2026. This guide is general information, not a substitute for advice from your MCS-certified installer.