About Fortyeight

Fortyeight is a free calculator for UK households deciding whether a home battery — and an 800 W plug-in solar kit — is worth buying. It is named after the 48 half-hours in every electricity day: your meter records them, Octopus prices them, and a battery earns its keep by moving energy between them. Everything runs in your browser.

How the calculation works

1. Your year. Readings from the Octopus API or a CSV export are placed on a 365-day × 48-half-hour grid in UK clock time. Gaps are filled from the nearest day with data. Without smart data, a typical year is built from the Elexon Profile Class 1 domestic shape, made peakier to resemble one home rather than an average of thousands, plus an optional heat pump (shape from the RHPP field trial, monthly split by GB heating degree days) and electric car charging in the cheapest overnight slots.
2. Prices. Agile uses its actual half-hourly prices for your region over the last 12 months. Go, Intelligent Go, Cosy, Flux and Flexible use the rates published on the day the data was fetched (25 October 2026). Import prices include the normal 5% VAT throughout, although VAT is off bills from 1 October 2026 to 31 March 2027.
3. The battery. Each day the battery plans 36 hours ahead: the most valuable half-hours are served first from the cheapest earlier energy — what’s already stored, spare solar, or grid charging — within capacity, power and round-trip losses. It knows the next day’s use exactly, so real controllers usually do a little worse.
4. Export rules. Battery exports are paid only on Flux. Solar exports are paid only with MCS-certified generation (Outgoing or Agile Outgoing; the better is chosen). Plug-in solar exports earn nothing.
5. The verdict. Every battery is compared with the best tariff you could have without one, so a tariff switch is never credited to the battery. Savings fade with the battery’s capacity (default 80% after 10 years) to give a payback year and a price ceiling.

Sources

• Tariffs: Octopus Energy public API, fetched 25 October 2026; Agile prices 25/10/2025–25/10/2026.
• Solar: PVGIS 5.3, European Commission Joint Research Centre (PVGIS-SARAH3 2005–2023), one representative town per region.
• Load shape: Elexon Profile Class 1 via the UKERC Energy Data Centre; heat pump shape from Love et al. (2017); degree days from DESNZ DUKES table 1.1.9.
• Rules: HMRC VAT Notice 708/6, SI 2026/848, SI 2026/896, ESQCR reg 22, Ofgem price cap, Octopus export terms.

The tariff and solar data are refreshed by re-running the scripts in the project and rebuilding the site; each page states when its data was fetched.

Limits

• It is a model, not financial, legal or electrical advice. Check quotes, warranties and tariff terms before you buy.
• Future prices will differ from the last 12 months. Use the sensitivity table.
• It does not model backup power, battery wear beyond a linear capacity fade, outages, or tariffs from suppliers other than Octopus (enter your current tariff by hand).
• Typical-household profiles are averages; your own half-hourly data is always better.
• Fortyeight is independent and not affiliated with Octopus Energy, any installer or any manufacturer. Product names are used only to describe compatibility.

Contact

Join the beta list on any page and reply to the welcome email: it reaches the people who build Fortyeight.