Why does my COP change with the outside temperature?

Q: Why does my COP change with the outside temperature?

The colder the air outside, the harder the heat pump has to work to collect heat from it, and the hotter it has to make the radiators. So COP (heat out divided by electricity in) falls on cold days and rises on mild ones. That's normal.

Updated 29 September 2026

Short answer: the colder the air outside, the harder the heat pump has to work to collect heat from it, and the hotter it has to make the radiators. So COP (heat out divided by electricity in) falls on cold days and rises on mild ones. That's normal.

Two things happen at once when it gets cold. There's less heat in the air to collect, and the house loses heat faster, so the heat pump sends hotter water to the radiators to keep up. Both make each unit of heat cost more electricity.

That's why comparing a January week with a May week says nothing about whether anything is wrong. The fair test is like-for-like: compare days with similar outdoor temperatures. If your COP is lower than it used to be at the same temperature, that's worth looking into.

In Snugdays: the Expected line in Trends is fitted to your own home's history, so you see what this house should manage at today's temperature.