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
Mar '25	450	86	162	21	13.0

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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