Split-period energy bill calculator
A bill can cross a price-cap or tariff change. Check its two sets of usage, rates and standing-charge days in one place.
Check the split
Use the bill's dates: the start and change date count, but the final end date does not. A bill from 15 September to 15 October with a change on 1 October has 16 days before and 14 after. Enter the kWh shown for each rate line. If your bill gives only one total, dividing it by days is an estimate that assumes uniform use.
How the calculation works
For each fuel and period: (billed kWh × p/kWh + days × p/day) ÷ 100. Add the four subtotals for dual fuel. Enter VAT-inclusive rates; the calculator does not add VAT again. It does not model discounts, debt, credit, off-peak rates, gas meter conversion or partial-day billing.
The national rate loader uses only the two dated Great Britain direct-debit averages in the current price-cap guide. A fixed tariff, region, payment type or supplier may have different rates. If the chosen dates do not fit both reviewed periods, enter your bill's rates manually.
If the bill differs
- Compare each billed rate and day count with this breakdown.
- Check whether the bill's gas usage is in converted kWh rather than m³.
- Take an actual meter reading and compare it with the opening and closing readings.
- Ask the supplier for the full rebill calculation if a discrepancy remains; use the complaint template and attach the downloaded breakdown.
See the wrong-reading guide if the rate is right but the usage is wrong.
Sources and limits
Rate example checked 28 September 2026 against Ofgem's unit-rate tables. HMRC's VAT guidance explains the different tax treatment from 1 October 2026. The example rates already include applicable VAT. Read our calculation methodology. This tool is a bill arithmetic check, not legal advice or an official supplier quote.