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Economy 7 Calculator: Compare Day and Night Costs
Check annual cost, standing charges and the night-use percentage needed for Economy 7 to beat a single-rate tariff.
Checked 11 September 2026 · BillDecoded editorial team · How our tools work
Compare your two tariffs
Use prices from your current bill and a single-rate quote for the same dates. Defaults are illustrative, not live offers. Rates must include any applicable VAT. Positive savings favour Economy 7.
What the comparison includes
The calculation prices day and night usage separately and adds each tariff’s own standing charge for 365 days. That daily difference can change the result substantially for a low-use household. It assumes the same consumption and prices for the whole year; it does not forecast price-cap changes, exit fees or switching costs.
Night share means electricity recorded on the cheaper register divided by total electricity, not the percentage of hours you spend at home overnight. Check which register your supplier bills as day and night. Meter labels alone do not guarantee the correct mapping.
A worked example
With 3,000 kWh a year and 40% on the night register, the example above costs £903 on Economy 7 and £932.50 on the single-rate tariff: £29.50 less on Economy 7. The break-even night share is about 34.5%, including the 10p/day standing-charge difference. These are illustrative rates, not a recommendation to switch.
| Step | Calculation |
|---|---|
| Day use | 3,000 × 60% = 1,800 kWh |
| Night use | 3,000 × 40% = 1,200 kWh |
| Economy 7 cost | (1,800 × 30 + 1,200 × 12 + 365 × 60) ÷ 100 = £903 |
| Single-rate cost | (3,000 × 25 + 365 × 50) ÷ 100 = £932.50 |
The break-even formula
For annual use U, day price D, night price N, single price F and daily standing charges S7 and S1 (all prices in pence):
night share = [U × (D − F) + 365 × (S7 − S1)] ÷ [U × (D − N)]
Multiply by 100 for a percentage. If day and night prices are equal, changing night use cannot change the comparison. If the result falls outside 0–100%, there is no break-even point within a possible night-use share: use the annual totals to see which tariff costs less. With zero use, only standing charges matter.
Check the meter before changing when you use power
Your supplier and meter determine off-peak hours. Confirm them before relying on a timer, particularly around clock changes. Do not assume that a fixed midnight-to-7am window applies to every tariff. Moving consumption can help only if it moves onto the cheaper billed register; follow appliance safety instructions.
Ofgem Economy 7 guidance explains the tariff and metering requirements. The formulas and worked example here are BillDecoded’s own arithmetic.
If your bill still looks wrong
- Check each register from readings, adding the standing charge once.
- Check the clock-change and register issue against your tariff’s hours.
- Request a corrected reading or rebill with dated photos and a clear calculation.