# Fortyeight

> Free UK home battery calculator: run your own half-hourly smart-meter data through Agile, Go, Flux and Cosy prices, with plug-in solar. Runs in your browser.

- Fortyeight is a free UK home battery calculator: it runs a household's own half-hourly smart-meter data (Octopus API, Octopus/Bright/n3rgy CSV) or a typical-household profile through 5, 10, 13.5 kWh and custom batteries on Agile, Intelligent Octopus Go, Octopus Go, Cosy, Flux and Flexible Octopus, with or without an 800 W plug-in solar kit.
- Everything runs in the browser; CSV files and Octopus API keys are never sent to Fortyeight.
- Tariff data: Octopus Energy public API, fetched 2026-10-25; Agile actual half-hourly prices 2025-10-25 to 2026-10-25 for all 14 regions; import prices include 5% VAT.
- A battery is compared with the best tariff available without one, so a tariff switch is not credited to the battery; savings fade with capacity (default 80% after 10 years) to give payback and a price ceiling.
- Installed batteries are zero-rated for VAT until 31 March 2027, then 5% (HMRC VAT Notice 708/6); batteries bought without installation pay 20%.
- Plug-in solar up to 800 W AC is legal in Great Britain from 27 August 2026 (SI 2026/848), one per household, must be registered with the network operator, and is not permitted in Northern Ireland.
- Example (Fortyeight model, London, 2026-10-25): a typical 2,500 kWh home gains about £190–£210 a year from a battery on Agile, so a battery must cost under about £1,700–£1,900 to pay back in 10 years; homes with an EV on Intelligent Octopus Go or a heat pump on Cosy gain more.
- Free plug-in solar permission pack: landlord, freeholder/managing agent (or Scottish property factor), proposed conditions and insurer letters for England, Wales and Scotland.
- Not financial or legal advice. Not affiliated with Octopus Energy.

## Tools

- [Home battery calculator](https://home-battery-calculator.pages.dev/): Free UK home battery calculator: run your own half-hourly smart-meter data through Agile, Go, Flux and Cosy prices, with plug-in solar. Runs in your browser.
- [Plug-in solar payback and permission letters](https://home-battery-calculator.pages.dev/plug-in-solar/): PVGIS payback and landlord/freeholder/insurer letters for England, Wales and Scotland.
- [Installer question sheet](https://home-battery-calculator.pages.dev/installer-questions/): Printable questions for home battery installers.

## Guides

- [Are home batteries worth it in the UK?](https://home-battery-calculator.pages.dev/guides/are-home-batteries-worth-it-uk/): For a typical UK home using 2,500 kWh a year, usually not. In Fortyeight's model of a London home, a 5 kWh battery adds about £187 a year over the best tariff without one, so it only breaks even within 10 years if it costs under about £1,700, against the Energy Saving Trust's typical £4,600. Homes with an electric car on Intelligent Octopus Go or a heat pump on Cosy or Agile gain £380 to £700 a year and come closest. Installed batteries are zero-rated for VAT until 31 March 2027, then 5%.
- [Best Octopus tariff for a home battery](https://home-battery-calculator.pages.dev/guides/best-octopus-tariff-for-a-battery/): The best Octopus tariff for a battery depends on what else you own, because most battery-friendly tariffs have entry rules. With an electric car, Intelligent Octopus Go (6.93p/kWh from 23:30 to 05:30 in London) gave the biggest gain in Fortyeight's model: about £541 a year with 10 kWh. With MCS solar, Flux pays for battery exports at peak. With a heat pump, Cosy or Agile depending on region. With none of these, Agile is the only option, worth about £210 a year with 10 kWh.
- [Home battery without solar: does it pay?](https://home-battery-calculator.pages.dev/guides/home-battery-without-solar/): Yes, you can fit a home battery without solar panels, and since 1 February 2024 a standalone battery charged from the grid has been zero-rated for VAT when installed (until 31 March 2027, then 5%). It saves money only by charging when electricity is cheap and running your home when it is dear. Battery exports earn nothing on Outgoing or Agile Outgoing. In Fortyeight's model a typical 2,500 kWh London home on Agile gains about £187 a year from 5 kWh.
- [What size home battery do you need?](https://home-battery-calculator.pages.dev/guides/what-size-home-battery/): Size a battery to the electricity you use in the expensive hours, not to your yearly total. In Fortyeight's model a typical 2,500 kWh London home on Agile gained £187 a year from 5 kWh, £210 from 10 kWh and £211 from 13.5 kWh, because it only uses about 2.9 kWh between 16:00 and 23:00. Bigger batteries pay their way only with an electric car on Intelligent Octopus Go, a heat pump, or MCS solar on Flux.
- [Battery VAT: 0% ends on 31 March 2027](https://home-battery-calculator.pages.dev/guides/battery-vat-deadline-march-2027/): As of 25 October 2026, a home battery supplied and installed by a business in a UK home pays 0% VAT until 31 March 2027; from 1 April 2027 the rate goes to 5%, not 20%. A battery bought without installation pays the full 20%. Under HMRC's tax point rules the rate normally follows the date the installation work is completed, so a deposit paid in March does not lock in 0% on the balance of a job finished in April.
- [Heat pump and battery: is Cosy enough?](https://home-battery-calculator.pages.dev/guides/battery-with-heat-pump/): A battery adds more to a heat-pump home than to a typical one, but whether to pair it with Cosy Octopus or Agile depends on region. In Fortyeight's model of a 6,200 kWh heat-pump home, a battery on Cosy added £326 a year (5 kWh) to £499 (10 kWh) in North Scotland. In London, Agile with no battery was already £444 a year cheaper than Cosy, so the best battery gain was £384 (10 kWh on Agile). Installed heat pumps and batteries both pay 0% VAT until 31 March 2027.
- [Plug-in solar UK: the rules from August 2026](https://home-battery-calculator.pages.dev/guides/plug-in-solar-uk-rules/): Since 27 August 2026 (SI 2026/848) you can plug a solar kit of up to 800 W AC into a normal socket anywhere in Great Britain, but not in Northern Ireland. The device must meet the government's Interim Product Specification, you may have one per household, and registering it with your network operator via myplugin.solar is a legal requirement. In Fortyeight's model an 800 W kit on a south-facing balcony railing in London makes about 594 kWh a year and saves about £133.
- [Plug-in solar for renters: landlord permission](https://home-battery-calculator.pages.dev/guides/renting-plug-in-solar-landlord-permission/): Renters, leaseholders and flat owners can use plug-in solar in Great Britain (legal since 27 August 2026), but the Energy Saving Trust advises getting permission from your landlord, freeholder or building manager before you buy. A request that offers a compliant 800 W device, no drilling, a circuit check, DNO registration, insurance and removal when you leave is much easier to say yes to. In Fortyeight's model a south-facing balcony kit in London saves about £133 a year.
- [Export tariffs explained: SEG, Outgoing, Flux](https://home-battery-calculator.pages.dev/guides/export-tariffs-explained/): To be paid for exported solar in Great Britain you need MCS-certified (or equivalent) generation and a Smart Export Guarantee tariff. As of 25 October 2026 Outgoing Octopus pays a flat 12p/kWh, Agile Outgoing averaged 11.6p in London over the last year but about 10.6p for solar-shaped exports, and Flux export pays 30.3p from 16:00 to 19:00. Only Flux pays for electricity exported from a battery, and plug-in solar earns nothing.
- [Download your smart meter data (half-hourly)](https://home-battery-calculator.pages.dev/guides/download-smart-meter-data/): Octopus customers can let the Fortyeight calculator fetch a year of half-hourly readings using their account number and the API key from Developer settings, or download a CSV from the online account. Customers of other suppliers can export half-hourly data from Bright (glowmarkt.com) or n3rgy, or upload any CSV with a time column and a kWh column. The file and the key stay in your browser and are not stored.

## Battery verdicts by tariff

- [Flexible Octopus](https://home-battery-calculator.pages.dev/tariffs/flexible/): One price all day.
- [Agile Octopus](https://home-battery-calculator.pages.dev/tariffs/agile/): A new price every half-hour, set a day ahead and linked to wholesale prices.
- [Octopus Go](https://home-battery-calculator.pages.dev/tariffs/octopus-go/): Cheap 00:30–05:30.
- [Intelligent Octopus Go](https://home-battery-calculator.pages.dev/tariffs/intelligent-octopus-go/): Cheap 23:30–05:30 (smart charging can add other cheap slots for the car).
- [Cosy Octopus](https://home-battery-calculator.pages.dev/tariffs/cosy/): Cheap 04:00–07:00, 13:00–16:00 and 22:00–00:00; peak 16:00–19:00.
- [Octopus Flux](https://home-battery-calculator.pages.dev/tariffs/flux/): Off-peak 02:00–05:00; peak 16:00–19:00 for import and export.

## Agile prices by region

- [Region A: Eastern England](https://home-battery-calculator.pages.dev/regions/eastern-england/): Agile price statistics and battery value in Eastern England.
- [Region B: East Midlands](https://home-battery-calculator.pages.dev/regions/east-midlands/): Agile price statistics and battery value in East Midlands.
- [Region C: London](https://home-battery-calculator.pages.dev/regions/london/): Agile price statistics and battery value in London.
- [Region D: Merseyside and North Wales](https://home-battery-calculator.pages.dev/regions/merseyside-and-north-wales/): Agile price statistics and battery value in Merseyside and North Wales.
- [Region E: West Midlands](https://home-battery-calculator.pages.dev/regions/west-midlands/): Agile price statistics and battery value in West Midlands.
- [Region F: North East England](https://home-battery-calculator.pages.dev/regions/north-east-england/): Agile price statistics and battery value in North East England.
- [Region G: North West England](https://home-battery-calculator.pages.dev/regions/north-west-england/): Agile price statistics and battery value in North West England.
- [Region H: Southern England](https://home-battery-calculator.pages.dev/regions/southern-england/): Agile price statistics and battery value in Southern England.
- [Region J: South East England](https://home-battery-calculator.pages.dev/regions/south-east-england/): Agile price statistics and battery value in South East England.
- [Region K: South Wales](https://home-battery-calculator.pages.dev/regions/south-wales/): Agile price statistics and battery value in South Wales.
- [Region L: South West England](https://home-battery-calculator.pages.dev/regions/south-west-england/): Agile price statistics and battery value in South West England.
- [Region M: Yorkshire](https://home-battery-calculator.pages.dev/regions/yorkshire/): Agile price statistics and battery value in Yorkshire.
- [Region N: South and Central Scotland](https://home-battery-calculator.pages.dev/regions/south-and-central-scotland/): Agile price statistics and battery value in South and Central Scotland.
- [Region P: North Scotland](https://home-battery-calculator.pages.dev/regions/north-scotland/): Agile price statistics and battery value in North Scotland.

## Optional

- [Full text of all guides](https://home-battery-calculator.pages.dev/llms-full.txt): every guide as plain Markdown