How the savings model and price checks work
One model produces every saving, payback and worked example on this site, from the calculator to the regional pages. These are its assumptions and its sources, so you can judge the numbers for yourself.
Prices and rules checked 27 September 2026
What the model compares
It works out a year of electricity costs for the same home twice: once with no battery on a flat-rate tariff (Flexible Octopus) with flat-rate export (Outgoing Octopus) if the home has solar, and once with the battery on the tariff you choose. The saving is the difference. Standing charges are the same in both cases and are left out.
It runs month by month, using an average day in each month, so that winter (little solar, higher use) and summer (lots of solar, lower use) are both represented rather than blended into one annual figure.
Assumptions
| Assumption | Value | Why |
|---|---|---|
| Share of a home's use during daylight solar hours | 40% | A home that is empty in the working day uses most of its electricity in the morning and evening. Change it by changing your usage pattern with an installer's survey. |
| Share of the rest that falls in the 4pm to 7pm peak | 45% | Cooking, lighting and appliances cluster in the early evening, which is when peak-rate tariffs charge most. |
| Round-trip efficiency | 90% | Energy is lost charging and discharging. The model buys or stores 1.11 kWh for every 1 kWh the battery gives back. |
| Cycles | One a day at most | The battery's total charging in a day (from solar and grid together) never exceeds its usable capacity. This stops the model counting the same capacity twice. |
| Export limit in the Flux peak | 11.04 kWh | 3.68 kW, the single-phase G98 limit, for the three-hour peak window. |
| Monthly use pattern | Winter about 40% above summer | Relative monthly weights 1.18, 1.1, 1.02, 0.93, 0.87, 0.83, 0.84, 0.86, 0.92, 1.01, 1.12, 1.2 (January to December), applied to the yearly use you enter. |
| Capacity fade for the 10-year figure | Average 90% of new | Most warranties promise 70% to 80% of capacity after ten years; the model averages the decline. |
| Future prices | Flat | No price rises are assumed. If electricity gets dearer, savings rise; if the peak-to-off-peak gap narrows, they fall. |
How the battery is used on each tariff
- Flat tariff: the battery only stores solar that would otherwise be exported, and gives it back in the evening. Charging from the grid gains nothing when every hour costs the same, so the model does not do it.
- Octopus Go: as above, then the battery tops up in the cheap overnight window to cover the rest of the day's use, as long as the overnight rate (after losses) is below the daytime rate.
- Octopus Flux: solar and the cheap 2am to 5am window charge the battery, which covers the home's use with the 4pm to 7pm peak first. If "sell spare stored energy" is ticked, any capacity left after the home's needs is used to export in the peak window, from spare solar first and then from cheap-rate charging, up to the export limit and the one-cycle cap. Flux requires solar panels, so the model only runs it, and only exports from the battery, when the home has solar.
- Agile Octopus: the same time-of-use logic as Flux, using 30-day averages of Agile's half-hourly prices (UK days 28 Aug to 26 Sep 2026): the cheapest three hours of each day for charging, the 4pm to 7pm average for the peak, and the rest of the day's average otherwise. Exports are paid at the flat Outgoing Octopus rate, so there is no peak export. Agile prices change every day, so the real result will vary more than on a fixed-window tariff.
Tariff eligibility is your supplier's decision, not the model's. Octopus Flux needs solar panels, an export MPAN and the MCS certificate; Intelligent Octopus Flux is currently limited to customers who bought solar and a battery from Octopus; Octopus Go is aimed at electric car owners. The tariff guide sets out the published conditions.
Data sources
- Tariff rates: Octopus Energy's public API, for all fourteen electricity regions, 26 September 2026. Import rates include 5% VAT; export rates carry no VAT. VAT on household electricity is 0% from 1 October 2026 to 31 March 2027, so import costs in that window are about 4.8% lower if the underlying price is unchanged. Octopus is used because it publishes every rate in machine-readable form; it is not an endorsement.
- Solar yield: the European Commission's PVGIS 5.3, for a 4 kWp south-facing array at 35°, 14% system loss, no shading, at a reference city in each region. The calculator scales it to the array size you enter.
- Electricity region by postcode: Octopus Energy's public grid-supply-point lookup.
- Price checks: the median installed price per kWh of MCS-certified batteries in existing homes in 2025/26, from DESNZ's statistics, in three size bands: battery under 6 kWh £1,300; battery of 6 to 11 kWh £890; battery of 11 kWh or more £630. A quote within 20% of the median for its band counts as typical. That is a rule of thumb, not a valuation of any quote; large suppliers' standard packages often sit below it. Published supplier prices are listed on the cost guide.
Update schedule
- Tariff rates: every quarter, when the Ofgem price cap changes, and whenever Octopus changes a battery tariff.
- VAT and funding rules: after every Budget and whenever a scheme changes.
- Battery prices and warranties: when a manufacturer publishes a change, and at least twice a year.
Found a figure that is out of date? Tell us and it will be corrected at the source, so every page that uses it changes together.
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