How the comparison works
For each tariff, the calculator works out a year's cost:
It then shows the difference against the cheapest option, and the average price you'd pay per kWh on each.
The number that decides it: your off-peak share
Time-of-use tariffs usually charge more than a flat tariff at peak times, so they only pay off if enough of your usage moves into the cheap window. The calculator shows the break-even share between your first flat tariff and your first time-of-use tariff. If your usage in the cheap window is above that share, time-of-use is cheaper.
Things that push your off-peak share up:
- EV charging on a timer. This is often the biggest single block of usage you can move.
- A home battery charged overnight and used during the day.
- Hot water: heating the cylinder overnight on a timer.
- Heat pump pre-heating in the cheap hours.
- Appliances such as dishwashers and washing machines on delay timers.
Finding your figures
Your annual usage is on your bill or in your supplier's app. If you have a smart meter, many supplier apps break usage down by half-hour, so you can see how much already falls in a given window. Unit rates and standing charges are on your bill or the tariff's information sheet. The figures pre-filled here are examples only, so replace them with the tariffs you're actually comparing.
What this doesn't include
Some tariffs have more than two rates, rates that change every half-hour, or extra-cheap slots for EV charging only. For those, use a weighted average for the cheap and peak rates. Exit fees, cashback and export payments aren't included.
Frequently asked questions
When is a time-of-use tariff cheaper than a flat rate?
When enough of your usage falls in the cheap window to outweigh the higher peak rate. The calculator works out that break-even share for you. Households with an EV, a battery or a heat pump on a timer often clear it easily.
Is Economy 7 worth it?
Usually only if a large share of your electricity, often around 40% or more, is used overnight, for example with storage heaters or a hot water cylinder heated at night. Its daytime rate is normally higher than a flat tariff’s.
Do standing charges differ between tariffs?
They can, and on low usage the standing charge can decide which tariff is cheapest. Enter each tariff’s own figure.
How do I find how much electricity I use at night?
Smart meter data in your supplier’s app, or a consumer access device, can show half-hourly usage. Add up the readings in your cheap window and divide by the total.