Every saving on this site comes from running Orza’s planner through a home’s electricity, half hour by half hour, knowing only what was known at the time. The same battery then runs on its own schedule, and the saving is the difference between the two bills.
Days counted on 29 October 2026 from every result we kept, re-runs included, and the runs that built and tuned the planner as well as the tests below. Runs whose results were not kept aren’t counted, so these are the least it could be.
Different pages quote different tests. These are the words they use.
Each figure on the site says which of these it comes from.
Twenty households, each with a 9.5 kWh battery and 4 kWp of panels and again with 13.5 kWh and 6 kWp, through 1 October 2025 to 30 September 2026 on the Octopus tariffs a battery owner might choose. More than 280,000 days planned. Most guides use it.
How the tariffs comparedThe same homes at 5, 13.5 and 20 kWh on Agile with Outgoing Octopus, and six of them at 5 kWh on all four export tariffs.
What size battery?Six households using 21 to 63 kWh a day, one of them already in the twenty, at 20 and 30 kWh and at a size matched to each home, 25 to 45 kWh, on all four export tariffs. That makes 25 homes and 316 whole years, the runs behind the calculator.
Try the calculatorBefore those six homes’ matched runs started, the calculator’s curve predicted what each would save. Then we ran them. The typical miss was 6.1%, and 22 of 24 landed within 15%.
See every predictionThe year test’s homes on Go with Outgoing Octopus with a simple routine: fill in the cheap window, then let the battery run the house. It saved £263 a year on average against the battery left alone, where Orza’s plan saved £404.
How that sits with Charge PackA backtest on the real home’s own readings, 6 October 2025 to 1 September 2026, with its 27 kWh battery, on four tariffs: £500 to £970 ahead of the same battery left alone. The home page’s large figure.
Its real half hours of sun and use, 3 September to 7 October 2026, planned on each tariff and export limit. The guides that say “one real home” use it.
The export limit guideTwenty modelled homes with and without a car charging, 3 September to 7 October 2026, on three tariffs.
A battery with an electric carTheir electricity use is: real households’ smart meter readings, half hour by half hour. The panels and batteries are modelled, except for the one real home, whose figures come from its own readings. The prices are the ones Octopus published, and the weather is the weather that came.
They are quoting different tests. The year test is 20 households, and on two sizes of battery and panels those make 40 modelled homes. Six bigger households, one of them already in the 20, bring the calculator to 25.
Each home is run on several battery sizes and tariffs, so the same date is planned many times. The count also includes every other backtest we have kept, re-runs and the planner’s development runs among them. It counts days of planning, not different dates.
Not exactly, since your home, tariff, battery and roof are your own. The calculator gives a figure and a range for your use and battery. Once you connect, Savings compares your battery with a copy of it following Orza’s plan, from your own readings.
Free, with no card. Look round an example home first, and connect your battery when you’re ready.
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