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Solar Battery Sizing Tool
Find a battery size that suits your solar generation and household demand rather than just buying the biggest unit available. See likely self-consumption uplift, extra savings, payback, and whether time-of-use arbitrage materially improves the case.
Battery recommendation
6.0 kWh
Sized to capture a good share of daytime surplus without overspending on rarely used capacity.
ROI rating
Good
Estimated new self-consumption
58%
Additional savings from battery
£294/year
Indicative installed cost
£2,700
Simple payback
9.2 years
Time-of-use arbitrage
Adds some value, but solar shifting remains the main driver.
This estimate includes battery round-trip efficiency of around 90% and assumes only a modest amount of deliberate off-peak charging.
Methodology
How the battery sizing logic works
The recommendation starts from likely solar output for the chosen PV size using a UK-average specific yield. It then estimates how much of that generation is currently exported and how much of your demand sits in the evening and overnight, when a battery is most useful.
Rather than aiming to store every spare kWh on the best summer day, the model targets the amount of routinely useful storage a typical home can cycle through on many days of the year. That helps avoid oversizing batteries that look impressive on paper but produce weak financial returns.
Extra savings come from shifting exported solar into later household use, priced at the difference between avoided import cost and lost SEG export income, with 90% round-trip efficiency. Homes with EV charging, heat pumps, or off-peak tariffs can justify larger batteries than homes with low evening demand.
Installed cost is modelled at roughly £450 per usable kWh, a reasonable current planning assumption for many UK domestic systems. Actual quotes vary with hybrid inverter choices, retrofit complexity, warranty, and whether backup or whole-home switching is included.