A 10 kWh battery added to 4 kWp of existing solar saves between about £379 and £455 a year on Octopus Flux across Britain's fourteen electricity regions, in this site's model of a home using 3,000 kWh. The same battery on a flat tariff saves £141 to £181, which is why the tariff matters as much as the battery.
What the calculator shows you
The yearly saving is the drop in your electricity cost against the same home with no battery on a flat tariff. Simple payback divides your quoted price by that saving. Price per usable kWh lets you compare quotes for different sizes on a like-for-like basis, and the verdict compares it with published installed prices for a battery of that size.
Why bigger batteries save less per kWh
A battery only earns on energy it can move each day. Once it covers the evening, and, on Flux, fills the peak export allowance, extra capacity sits unused. This chart runs the model at different sizes:
For a battery-only home on an overnight tariff the curve flattens earlier, because once the battery covers the whole day's use outside the cheap window there is nothing left to shift. That is the most common way a quote is oversized. To price the whole system, the solar panels and battery cost guide models the panels and the battery separately, and homes with solar can check their region's tariff on the Octopus Flux rates page.
Where the numbers come from
Rates are Octopus Energy's published regional prices for 26 September 2026. Solar output comes from PVGIS for a reference city in your region. The full list of assumptions, from the 90% round-trip efficiency to the one-cycle-a-day cap, is on the methodology page. If you have a quote, compare it line by line with the quote checklist, and if you want more prices to compare, the battery storage quotes service answers your request by email.
Calculator questions
How do I work out what size solar battery I need?
Start from your evening electricity use, not the panel size. A battery earns only on energy it can shift each day, so size it to what the home uses between sunset and bedtime (plus any overnight tariff charging). In this site's model a London home using 3,000 kWh a year with 4 kWp of solar on Octopus Flux gains about £414 a year from 10 kWh, but only £162 more going from 13.5 kWh to 20 kWh. The sizing guide goes step by step.
Why does the calculator compare against a flat tariff?
Because that is what most homes without a battery are on. A battery is what makes a peak-rate tariff such as Octopus Flux safe to use, so the fair comparison is the home without a battery on a flat tariff against the home with a battery on the tariff it enables.
Is the payback figure a guarantee?
No. It is a model using published rates on one date and your inputs. Real savings depend on how you use electricity, on the equipment, and on future tariffs. The assumptions are all on the methodology page so you can see what would change the answer.
What does the price check compare my quote with?
It divides the price by the usable capacity to give pounds per kWh, then compares that with the range of published installed prices this site has recorded for batteries of a similar size. A price above the range is not proof of a bad quote: backup hardware, a new consumer unit or a difficult cable run add cost. It is a prompt to ask what is included.
Does it work for a battery without solar panels?
Yes. Choose "None" for solar. Octopus Flux needs solar panels, so the calculator switches to Agile Octopus, where the battery charges in the cheapest hours of each day and covers the rest. If you have an electric car, try Octopus Go, which Octopus markets to EV owners.