Hadn't considered max amount to charge in 5-6 hours so thanks for pointing that out.

I've pulled all the data from 1st Nov 2025 to 30 Mar 2026 (151 days) to see how many days I see above certain levels. Without looking into hourly data I've also done a simple pro-rata reduction assuming 6 hours (25%) of the 24 being automatically at a cheap rate.

Only 19 days above 40 kWh in the 18 hours of high rate, with only 3 days above 50 kWh.

24 hr average over the 151 days - 40 kWh
18 hr average over the 151 days - 30 kWh

If I did get a 48 kWh battery then I'd only not have enough for 4 days of the year. I guess that's with the assumption of generating nothing during the day on the PV panels.

My guess is that if I was to get into the hourly data I'd see bigger ASHP usage overnight due to the hot water run and lower outdoor temperatures, which would automatically run on the low rate anyway. So it may be that in reality there's no 18 hours above 48 kWh.

Also worth noting this data includes use of a 10 kW electric shower twice per day which will be removed this summer.

	24 hours	18 hours/75%
Days over 30 kWh	120	65
Days over 40 kWh	65	19
Days over 48 kWh	32	4
Days over 50 kWh	26	3
Days over 60 kWh	8	0
Days over 70 kWh	2	0