Fortyeight

Will a home battery pay for itself on your meter?

Run a year of your own half-hourly readings through every Octopus smart tariff, battery size and an 800 W plug-in panel. You get the best set-up, the payback, and the most you should pay an installer.

156 days of 0% VAT left on installed batteries

Agile prices in London, Oct 2025–Oct 2026: every half-hour of the year. The brown band is the 16:00–19:00 peak a battery avoids.

How Fortyeight decides

A battery earns money in one way: it lets you buy electricity in a cheap half-hour and use it in a dear one. How much that is worth depends on your own pattern of use and the tariffs open to you, so Fortyeight works on all 17,520 half-hours of your year rather than an annual total.

It compares every battery against the best tariff you could switch to without one, so a tariff change is never credited to the battery. That gives an honest ceiling: the most a battery can cost and still pay back.

Prices come from the public Octopus Energy API (fetched 25 October 2026): the last 12 months of real Agile prices for your region and today’s rates for Go, Intelligent Go, Flux, Cosy and Flexible. Solar output comes from the European Commission’s PVGIS database.

The printable report Free during beta carries the numbers, charts and a sheet of questions for installers. Read the method and its limits.

Frequently asked questions

Is a home battery worth it in the UK?

It depends on when you use electricity and which tariff you can get. In Fortyeight’s model a typical 2,500 kWh home in London gains about £190–£210 a year from a battery on Agile Octopus, so it should cost under about £1,700–£1,900 to pay back in 10 years. Homes with an electric car on Intelligent Octopus Go or a heat pump on Cosy can save two to three times as much. Run your own meter data to see where you sit.

Do I need solar panels to benefit from a battery?

No. A battery can charge from the grid at cheap times and power the home at dear times. Standalone batteries have been zero-rated for VAT when installed since 1 February 2024. Without MCS-certified solar you can’t be paid for exports, so the saving comes only from shifting your own use.

When does 0% VAT on home batteries end?

HMRC’s zero rate for installed energy-saving materials, including batteries, ends on 31 March 2027. From 1 April 2027 installed batteries pay the reduced rate of 5%. A battery bought without installation is charged the standard 20%.

How do I get my half-hourly smart meter data?

Octopus customers can paste their account number and API key (from Developer settings on the Octopus dashboard) and Fortyeight fetches a year of readings directly from Octopus in your browser. Anyone can upload a CSV from the Octopus dashboard, the Bright app’s web download (Glowmarkt) or n3rgy. No smart meter? Use the typical-household option.

Is my data sent anywhere?

No. CSV files are read in your browser, and with the Octopus option your browser talks to the Octopus API directly; the key is never stored or sent to Fortyeight. We only count anonymous events such as “a simulation was run”.

Which Octopus tariff is best with a battery?

With an electric car, Intelligent Octopus Go usually wins because its 23:30–05:30 rate is a fraction of the day rate. With a heat pump, Cosy’s three cheap windows suit a battery well. Without either, Agile is the main option; Flux needs MCS solar and a battery. Fortyeight runs all of them on your data.

Can I add an 800 W plug-in solar panel?

Yes, in Great Britain since 27 August 2026: plug-in solar up to 800 W that meets the government’s product specification, one per home, registered with the network operator. It is not permitted in Northern Ireland. Renters and flat owners should get permission first; Fortyeight writes the letters.

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