Day/night tariff calculator
Compare a single-rate tariff with day/night rates using your kWh split and both standing charges.
All starting rates and standing charges are hypothetical, not supplier offers. Enter your actual VAT-inclusive rates. This compares a constant single rate with exactly two constant rates over 365 days, not a dynamic half-hourly tariff.
Enter the percentage of kWh used off-peak, not the percentage of hours in the day. Confirm your supplier's exact off-peak window and meter eligibility.
Annual electricity bill comparison
1800.0 kWh daytime + 1200.0 kWh off-peak. Average monthly totals: £77.71 single-rate; £75.21 day/night.
Day/night costs £30.00 less per year under these assumptions.
Break-even off-peak share: 33.33%. Day/night is cheaper above this share; equal at the threshold.
Includes entered energy and daily standing charges only. Excludes exit fees, discounts, metering costs and future rate changes. Cheaper hours do not override appliance operating instructions; do not leave laundry appliances running while asleep or out. Electrical Safety First guidance.
Worked example: Compare rates and standing charges together
These fixed teaching inputs are separate from the editable calculator above and are not current tariff or product claims.
- Inputs
- 3,000kWh/year, 40% off-peak; single rate 25p, day 30p, night 15p; both standing charges 50p/day.
- The working
- Single energy: £750. Two-rate energy: 1,800 × £0.30 + 1,200 × £0.15 = £720. Add £182.50 standing charges to each.
- The result
- Totals: £932.50 single-rate and £902.50 day/night. Break-even off-peak share: 33.33%. This comparison is hypothetical, not an available tariff.
How this calculator works
Single total = annual kWh ? single unit rate ? 100 + 365 ? single standing charge ? 100. Day/night total = daytime kWh ? day rate ? 100 + off-peak kWh ? night rate ? 100 + 365 ? day/night standing charge ? 100. The break-even share solves equality of those totals.
Cost in pounds = energy in kWh × electricity rate in pence ÷ 100. Monthly costs are annual costs divided by 12; daily costs are divided by 365. These are averages over a year. Calculations retain precision until display rounding.
Get a more useful estimate
Read our step-by-step guide to measuring appliance electricity before treating a nameplate rating as a measured average.
Start with the rate on your bill and energy information for your exact model. A plug-in energy monitor can help measure suitable plug-in appliances; follow its safety instructions and power limits. Do not apply full-power wattage for 24 hours to a cycling fridge or freezer.
What is excluded?
The day/night comparison includes the standing charges you enter; appliance-only tools exclude them. Gas, water, detergent, maintenance, discounts and purchase costs are excluded unless the tool explicitly asks for a purchase premium. Usage and tariffs can change. The reference tariff covers Great Britain and does not represent Northern Ireland or every contract.
Read our sources and assumptions →Common questions
Is seven off-peak hours the same as using 7/24 of my electricity off-peak?
No. The split is based on kWh, not clock hours. Use your day/night meter registers or supplier consumption breakdown for a matching period. Confirm the applicable window with the supplier rather than assuming the same times for every meter.
Does a cheap night rate guarantee a cheaper annual bill?
No. Compare the day rate, night rate, actual kWh split and both standing charges. The result holds those inputs constant for 365 days and excludes exit fees, discounts and other contract conditions. It is not a supplier quote or switching recommendation.