Short answer: if you are with Octopus, give the Fortyeight calculator your account number and the API key from your account’s Developer settings. Your browser then fetches up to a year of half-hourly readings straight from Octopus. Otherwise, download a CSV: from your Octopus account, from Bright/Glowmarkt (glowmarkt.com) or from n3rgy. Any other supplier’s file also works if it has a time column and a kWh column. The data is processed in your browser, and the key is not stored.
Why half-hourly data matters
A battery earns money by moving electricity from cheap half-hours to expensive ones. Annual kWh cannot tell you how much of your use falls in the 16:00–19:00 peak, how much happens overnight when Octopus Go or Flux is cheap, or how big your winter evenings are. Two homes that both use 2,500 kWh a year, Ofgem’s typical figure from 1 July 2026, can get very different results from the same battery.
Without your data, Fortyeight uses a standard load shape (Elexon Profile Class 1). That shape describes the average home, not yours. Your own readings take the guesswork out of battery sizing and of whether a battery is worth it at all.
Route 1: Octopus API key (quickest)
Octopus publishes a REST API that returns half-hourly consumption for each meter, identified by MPAN and meter serial number.
- Account number. It looks like A-1234ABCD and is on your bills and at the top of your online account.
- API key. Log in, open Developer settings, and copy the key. It starts with sk_live_.
- Paste both into the calculator. It reads your account to find the electricity meter points, and an export meter if you have one. It works out your region and current tariff from the tariff code, then downloads the half-hourly readings page by page.
How the key is handled: the request goes from your browser directly to api.octopus.energy. The key is used only in that request’s authorisation header. It is not stored and not sent to Fortyeight or anyone else. The key can read your account details and consumption, so treat it like a password.
If a meter was replaced, the readings still live under the same MPAN. The calculator merges the old and new serial numbers and keeps whichever has more data for each half-hour.
If the API returns little or no data, your meter may be sending only daily or monthly readings, or not communicating at all. Ask Octopus whether half-hourly readings are being collected for your meter.
Route 2: Octopus dashboard CSV
You can also download your usage from the Octopus online account as a CSV. The file has these columns:
Consumption (kwh), Estimated Cost Inc. Tax (p), Start, End
The timestamps are UK local time with an offset, and Start marks the beginning of each half-hour. Upload the file and the calculator recognises it automatically. This route is useful if you would rather not paste an API key anywhere.
Route 3: Bright app / Glowmarkt
If your supplier is not Octopus, the free Bright app reads smart meter data through the Glowmarkt platform. Log in at glowmarkt.com with your Bright details and use the energy data page to download a CSV. Menu names may differ from what we describe. The file looks like this:
epochTimestamp,kWh,dateTime
Timestamps are UTC and mark the start of each interval. The epoch can be in seconds or milliseconds, and both are handled. Some exports come at 15-minute or hourly resolution. The calculator adds 15-minute readings up into half-hours and splits hourly readings in two.
Route 4: n3rgy
n3rgy (data.n3rgy.com) offers consumer access to smart meter data. As far as we have been able to check, you register with your MPAN (on your bill, a 13-digit number) and the MAC or GUID of your in-home display,. Data comes as a ZIP of CSV files in batches. The sign-up steps may have changed, so follow n3rgy’s own instructions.
Two things to know about the files:
- The columns are
"timestamp (UTC)","energyConsumption (kWh)", and the timestamp marks the end of each half-hour, in UTC. A reading stamped 00:30 covers 00:00 to 00:30. The calculator shifts these so they line up with other sources. - A value of 16777.215 is a meter error code, not real consumption. Those rows are skipped.
Route 5: any other CSV
Many suppliers offer a usage download in their online account, and their formats vary. The calculator looks for a header row, then guesses which column is the time and which is the kWh. You confirm or correct the guess. Check whether your supplier’s timestamps mark the start or the end of each interval. Rows that cannot be read, negative values and implausibly large readings are counted and skipped, so one bad line does not reject the file.
| Source | Header | Timezone | Timestamp marks |
|---|---|---|---|
| Octopus dashboard | Consumption (kwh), Estimated Cost Inc. Tax (p), Start, End | UK local, with offset | Start |
| Bright / Glowmarkt | epochTimestamp, kWh, dateTime | UTC | Start |
| n3rgy | “timestamp (UTC)”, “energyConsumption (kWh)” | UTC | End |
| Other | Any time column plus a kWh column | You confirm | You confirm |
How much data, and what happens to gaps
Twelve months is best. A battery that looks good on summer data can look very different in January, and the reverse. The calculator uses the most recent 365 days in your file or API response. It fills any missing half-hours from the same half-hour on the nearest day that has data. The fewer gaps, the better the result. Six months of data gives a result, but treat it as rougher.
If you have solar with an export meter, the Octopus route also fetches your export readings. That matters for export tariff choices and for modelling how much surplus a battery can soak up.
Common problems
| Symptom | Likely cause | What to try |
|---|---|---|
| “Octopus rejected the API key” | Key copied with a missing character, or an old key | Copy it again from Developer settings. It should start with sk_live_. |
| “Could not find that account number” | Typo, or a gas-only or old account | Check the A- number on a recent electricity bill. |
| Very low annual total | Several months missing, or the file covers only part of the year | Check the date range in the file and download a longer period. |
| Usage appears an hour out | Timezone mismatch in a generic CSV | Check whether the times are UTC or UK local, and start or end of interval. |
| Totals twice as high as your bill | Import and export, or two meters, in one file | Upload the import meter’s data only. |
As a sense check, compare the annual kWh the calculator reports with the annual usage on your bill. Ofgem’s typical values for single-rate electricity run from 1,600 kWh (low) to 3,800 kWh (high). Heat pumps and electric cars push that much higher. If the two figures are far apart, the file is probably incomplete or mapped wrongly.
Privacy
Fortyeight does the processing in your browser. Uploaded CSVs are parsed on your device, and API calls go straight from your browser to Octopus. Fortyeight does not receive or store your meter readings or your key. The privacy page has the details.
Once your data is loaded, the calculator compares Flexible, Agile, Go, Intelligent Octopus Go, Cosy and Flux, with and without a battery and plug-in solar, using your actual half-hours. To understand the results, see the best Octopus tariff for a battery.