What Happens to a Heat Pump in a Power Cut?
If you are considering an air source heat pump, there is a simple question that often comes up, usually later in the process:
What happens if the power goes out?
Will the heating stop completely?
Will the house get cold quickly?
And do you need a backup system?
The short answer is straightforward.
But the more important question is not about the system itself. It is about how your home performs when the system stops.
That is where a whole-house retrofit approach becomes essential.
If you want to understand how to design for this properly, see our guide to preparing your home for power cuts.
Do heat pumps work during a power cut in the UK?
No.
A heat pump is an electric system. It depends entirely on power to run.
So in a power cut:
the heat pump stops
the circulation pumps stop
your heating system stops delivering heat
There is no built-in fallback.
But this is only part of the picture.
Because the real question is not:
“Does the heating stop?”
It is:
“How does your home behave when it does?”
How quickly will your home get cold?
This is what actually affects comfort.
In many UK homes, the temperature drops quickly when the heating stops. That is because heat escapes quickly, especially in homes that are not well insulated or airtight.
Even a short power cut on a winter evening can feel uncomfortable.
Why low-energy homes behave differently
A well-retrofitted home is designed to hold onto heat, not constantly replace it.
This comes down to a few key improvements to the building itself.
Insulation
Good insulation slows heat loss through the building fabric, especially when insulating a period London home.
Airtightness
Reducing draughts keeps warm air inside where it belongs, which is why improving airtightness in older London homes makes such a difference.
Thermal mass
The structure of the building absorbs heat and releases it slowly over time.
A simple way to think about it
A typical home is like wearing a thin, damp layer on a cold day. Heat is lost quickly.
A well-insulated, airtight home is like wearing a thick, dry coat. Even without active heating, you stay comfortable for much longer.
So what happens in practice?
In a well-retrofitted home:
the heating stops during a power cut
the temperature drops slowly, not suddenly
comfort is maintained for hours, sometimes longer
In homes designed using Passive House principles, internal temperatures can remain stable for extended periods without active heating.
This is what resilience looks like in a building.
Do you need a backup system for a heat pump?
Sometimes. But not always.
It is easy to assume the answer is more technology. In reality, the first step is usually to improve the building itself.
Option 1: Improve the building first
A fabric-first retrofit approach focuses on improving the performance of the building before adding systems.
This includes:
insulation
airtightness
addressing weak points where heat escapes
By reducing heat demand and slowing heat loss, the home becomes more stable and more resilient.
Option 2: Add backup power
In some situations, a backup system can make sense:
in areas with unreliable electricity supply
where outages are likely to last longer
where there are critical needs within the home
Options include:
battery storage systems
solar and battery combinations
backup generators
But these come with cost and complexity. They are most effective when combined with a low-energy building that already performs well.
This is why the question of backup power often sits alongside a bigger decision: how far to go with your retrofit in the first place.
If you are weighing this up, it helps to understand how far you should retrofit your home.
A better question to ask
Instead of asking:
“What happens if my heat pump stops?”
A better question is:
How long will my home stay comfortable without heating?
This is something you can understand more clearly when you look at how long your home stays warm without heating.
Understanding this changes how you think about comfort, risk, and resilience.
A calmer way to think about energy security
Energy security is becoming a more common concern. That is understandable.
But resilience in your home is not about preparing for extreme scenarios.
It is about:
reducing reliance on constant energy input
designing a home that performs well in all conditions
making long-term, considered decisions
A well-retrofitted home is warmer, quieter, and more comfortable day to day.
It is also more stable and predictable when things do not go to plan.
If you are thinking more broadly about this, it is worth understanding preparing your home for power cuts as part of a wider retrofit strategy.
Thinking about a heat pump or a retrofit?
If you are considering a heat pump installation, it is worth stepping back and looking at the whole system.
Our Retrofit Strategy Service helps you understand:
how your home currently performs
what level of upgrade is appropriate
how to sequence improvements
So that you can invest with clarity and confidence.
