Time-of-Use Tariff Calculator

How to use this calculator

1. Enter your annual consumption in kWh (take it from a yearly statement, or multiply a typical month by twelve).
2. Enter the flat rate you pay now, and the peak and off-peak rates of the two-rate tariff you are considering.
3. Estimate the share you can realistically shift — the dishwasher, washing machine, water heater and EV are shiftable; the fridge, lights and cooking are not.
4. Add any extra fixed charge the two-rate tariff carries.
5. Click Calculate break-even.

The break-even formula

A two-rate tariff is a trade: you get a cheap night rate, but you pay a higher rate during the day than you would on a flat tariff. So it only wins if enough of your consumption actually moves. The point where the two tariffs cost the same is:

Below that share you are paying more, not less. This is exactly the calculation most comparison sites skip — they show you the cheap night rate and let you assume the rest, because a switch earns them a commission.

Note the second term. If the two-rate tariff carries a higher standing charge, the break-even share rises, and it rises most for households that use little electricity. A high-consumption household spreads that fixed cost thinly; a low-consumption household cannot.

What counts as shiftable consumption

Be honest here — this is where people fool themselves. Typically shiftable:

• EV charging — the single biggest and most flexible load in a modern home. An EV alone can move 20–40% of a household's consumption off-peak.
• Hot water — an immersion heater or heat-pump water tank on a timer.
• Dishwasher and washing machine — most have a delay timer.
• Storage heating and pool pumps, where present.

Rarely shiftable: cooking, lighting, television, the fridge and freezer, and the base load of everything left plugged in. In a household without an EV or electric heating, 25–35% is a realistic ceiling — which is precisely why the break-even figure is so often out of reach.

This calculator works for every national scheme

The names differ, the maths does not. It applies to Economy 7 in the UK, heures pleines / heures creuses in France, G12 and G12w in Poland, the low/high tariff (NT/VT) distribution rates in Czechia and Slovakia, tarifa bi-horária in Portugal, fasce F1/F2/F3 in Italy, discriminación horaria in Spain, night transfer (yösiirto) in Finland and time-differentiated grid tariffs in Denmark.

One warning worth stating plainly: in some markets the gap between day and night rates has narrowed so far that the honest answer is no, do not switch. This calculator will tell you that, because it has nothing to sell you.

Frequently asked questions

Is a day/night tariff worth it?

Only if you can shift enough consumption. Enter your rates and the tool gives you the exact percentage you need. As a rule of thumb, without an EV, electric heating or an electric hot-water tank, most households cannot reach it.

What percentage of electricity do I need to use at night?

It depends entirely on the spread between the rates. With a wide spread, 25–30% can be enough; with a narrow spread and a higher standing charge, you might need 50% or more, which is unrealistic for most homes.

Does an electric car change the answer?

Dramatically. EV charging is large, flexible and almost always overnight, so it can single-handedly push a household past the break-even point. If you have an EV, a two-rate or dedicated EV tariff is usually worth serious consideration.

Why does the two-rate tariff have a higher day rate?

Because the supplier has to recover the discount it gives you at night. That is the trade at the heart of every time-of-use tariff, and it is why switching without shifting your usage makes you worse off.

What about hourly spot-price tariffs?

The same logic applies but the prices change every hour (and, in the Nordics, every 15 minutes). Use the average peak and off-peak prices you actually paid over a recent month as your two rates — that gives a good approximation of the annual outcome.

Should I include the standing charge difference?

Yes, if there is one. Some two-rate tariffs need a different meter or carry a higher monthly fee. Ignoring it is the most common way people overestimate their savings.