Size your battery,
right the first time

Methodology: LFP 90% usable depth of discharge assumed throughout. On Octopus Go/IOG (TOU), the cheap-rate window is only a few hours overnight, so the battery is sized to cover the entire rest of the day's house load, not just an evening-hours slice of it. On flat rate or Solar+TOU, only the evening/night peak share of house load needs battery coverage, using the "Peak period" slider above. Once you tick the heat pump box, the full daily heat pump electric use is always added in full on every tariff — a heat pump can run at any hour of the day (not just evening/night), so there's no reduced "peak portion" applied to it. Heat pump projection itself uses only the Dec–Feb winter share (~47%) of annual gas use spread over 90 days, not an annual daily average — since UK heating degree days are heavily concentrated in the coldest three months, averaging over all 365 days would understate real winter demand. This treats your full annual gas figure as space heating; if a meaningful share is water heating or cooking (which runs fairly flat year-round), this slightly overstates winter heat pump load — a deliberately conservative, safer-to-oversize assumption. All recommendations rounded up to nearest 0.5 kWh and then to nearest product size.