Thanks! The house needs around 800W to 1,000W of heat when it's very cold, around -5°C outside, to keep the room temperature at around 21.5°C. 900W of heating is under 300W of electrical power into the heat pump, although it won't modulate down that low, plus we only usually run it overnight during the off-peak period. The heat pump draws about 1.5kW for a few minutes when it first fires up, then settles back to run at around 800W or so, cycling on and off periodically once it's up to temperature. I have seen it run all night at a steady 900W once, when it was very cold (around -4°C), but for most of the winter it seems to only come on once every two or three nights.
We could do with a smaller heat pump, really, but when I bought it 6kW was the smallest model I could find. Ideally one about half that power output would be about ideal, as it would be able to modulate down to a lower level than the one we have. A smaller heat pump would also make it easier to run the heating from the batteries, which could be useful in very cold weather.
