If you’ve noticed your heat pump using more electricity than usual, especially during a cold spell, this is nothing to worry about. Electricity use by heat pumps changes significantly depending on the outside temperature. This article explains why, and what you should expect.
Key point: Heat pumps use more electricity when it’s cold outside
Your heat pump’s efficiency depends on the outside air temperature. When it gets colder, the system has to work harder to pull heat from the air.
This means:
Your heat pump uses more electricity as temperatures drop
Your usage can rise quickly, even if your thermostat stays the same
This behaviour is normal and doesn’t mean there’s a fault
Why this happens (in simple terms)
Heat pumps have something called a Coefficient of Performance (COP). It describes how much heat you get out for each unit of electricity you put in.
On a mild day, the COP is higher → you get more heat for less electricity
On a freezing day, the COP is lower → you need more electricity to get the same heat
This relationship isn’t linear, so a small drop in temperature can mean a noticeably higher electricity draw. That's why your costs might have blown up out nowhere!
What does this look like in reality?
To show what this looks like in the real world, here is data from a typical home courtesy of Mick Wall, best know as energystats.co.uk
Here's some details about his home
• 1930s semi-detached
• Around 100 m², located in Sheffield
• Heat loss: ~4 kW
• Heat pump size: 5 kW
• Flow temperature: ~40 °C at –3 °C outside
Mick measured data from November 2023 to April 2024
Credit: Mick Wall; link here
His data shows a clear pattern:
As the outside temperature increased, the COP went up and electricity usage went down. On the coldest days, electricity usage was significantly higher, even though the home’s comfort level stayed the same.
If your usage sounds like this, there's nothing to worry about.
What does this means for your running costs?
During very cold spells:
Your heat pump may use more electricity than you expect
Costs may rise temporarily
This doesn’t mean your heat pump is inefficient or broken
Once the weather warms, usage will drop back down again
Heat pumps still outperform direct electric heating, even in cold weather — but their efficiency curve means you’ll see higher consumption on colder days. And over the year, a well set up heat pump knocks the socks off a gas boiler
How to get the best performance
Here are some steps that can help:
Use the Havenwise app
Make sure your home is well-insulated
Ensure your emitters and heat pump are adequately size
Credit
Many thanks to real-world data from Mick Wall. You can (and should) visit his website at energy-stats.uk