Costs of Running an Air-source Heat Pump

Here are my stats for electricity usage, compared to the same period a year ago (when heating and hot water were oil-fuelled):

Month Expected units Expected cost (£) Actual units Actual cost (£) Actual cost per unit (p)
Nov '24 250 48 290 37 12.8
Dec '24 450 86 340 59 17.4
Jan '25 625 120 440 77 17.5
Feb '25 625 120 320 53 16.6

Note - The expected usage is based on my rough estimates of how much oil I might have been using each month - and that's very rough as I've never kept a close measure of how much oil was in my tank.

So it's early days in monitoring ASHP electricity usage, but I'm pleasantly surprised to see it using (in terms of units of electricity) around 70% of what I was anticipating. And costing under £80 per month during Winter does seem quite reasonable!

How come the per-unit price of the extra electricity used is so low? Well, I'm on the Octopus Cosy tariff which has three very cheap periods each day - plus one more expensive one between 4pm and 7pm. So I prioritise heating/hot water during the cheap periods, but not by a lot. I literally run a schedule that bumps the desired temperature up by a couple of degrees during the cheap periods, and also have a hot water schedule that runs in the middle of the night.


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