How time of use charging windows actually work

The gap a window opens

For the price cap period running 1 July to 30 September 2026, the average capped electricity unit rate is 26.11p per kWh, with an average standing charge of 57.19p a day. Off-peak overnight rates currently cluster between 6.99p and 9p. That is a discount of roughly 65 to 73% on every unit you can move into the cheap window.

Nothing else in home energy offers that kind of margin for so little effort, because a car parked overnight is the most flexible load a house has. It does not care when it charges, only that it is ready in the morning. This page explains how the window works. The rates themselves, supplier by supplier, with the conditions attached to each, are in the tariff comparison.

What a cheap window actually is

Nothing changes physically at the wall. Your smart meter records consumption in half-hour blocks and reports them to the supplier, and the supplier prices the blocks that fall inside the window at one rate and everything else at another. That is the whole mechanism, and it is why every tariff of this shape carries the same precondition: a smart meter with half-hourly consent. E.ON Next Drive Smart states it explicitly, and EDF GoElectric Overnight requires a fully connected smart meter. Without half-hourly readings there is no window to charge in.

It also means the window is a billing boundary, not a switch. Electricity drawn at 05:55 on a window that closes at 06:00 is cheap, and the same kettle at 06:05 is not. Anything you want at the cheap rate has to be running inside the block, which is what timers, charger schedules and battery control software exist to arrange.

Whole-home and car-only are metered differently

Every off-peak charging arrangement on the market is one of two shapes, and confusing them is the most common expensive mistake.

• Whole-home two-rate structures. The meter does all the work. Every half-hour block inside the window is priced at the cheap rate regardless of what consumed it, so your dishwasher, your immersion and your home battery all benefit alongside the car. Outside the window, everything is priced at the higher rate. Intelligent Octopus Go, E.ON Next Drive and the British Gas EV tariff are all described by their suppliers as whole-home.
• Car-only add-ons. The meter cannot tell a car apart from a tumble dryer, so the separation happens in software instead. OVO Charge Anytime requires a compatible EV or charger plus a smart meter and an account, and the app decides when the car charges. The discount attaches to the charging session the app controls at 14p per kWh with no fixed window at all. The rest of the house stays on your existing rate, for better and for worse.

Smart scheduling: the window that moves

A second mechanism sits on top of the fixed window. Intelligent Octopus Go publishes a whole-home cheap window of 23:30 to 05:30 and separately allows up to six hours of smart-scheduled charging a day. The important detail is that charging dispatched outside the published window is still billed at the off-peak rate. In practice the supplier decides when your car charges, based on grid conditions, and you get the cheap price whenever it happens.

That only works if the charger or the car can take the instruction. Ohme integrates on the charger side, with the tariff configured automatically in its app and the price cap setting removed, and Andersen chargers are supported. Where the integration is car-side instead, the charger is just a dumb pipe and the car takes the schedule. Either works. Neither works if you own a unit with no tariff integration at all, which is covered in the charger comparison.

What a battery does inside the window

On a whole-home structure a battery is simply another appliance drawing power inside a cheap block. Fill it overnight, run the house from it through the expensive hours, and you have converted a car tariff into a household one. On a car-only add-on you cannot, because the discount is attached to the charging session rather than to the meter reading, so the battery pays your standard rate whenever it charges.

That distinction is worth real money, and the arithmetic behind it, including the round-trip losses most quotes leave out, is worked through in what a battery saves on cheap night rates. Which windows a battery can actually reach, and the solar and battery tariffs built around them, are covered in off peak tariffs and battery charging.

The arithmetic that decides the shape

Take a household using 2,500 kWh of electricity a year, which is the consumption value Ofgem now uses for a typical home, plus a car covering 10,000 miles a year. That mileage is an illustrative input, not an average: substitute your own.

Start with the car. A 7kW charger adds roughly 25 to 35 miles of range an hour, so a mile costs somewhere between 0.20 and 0.28 kWh. Ten thousand miles is therefore 2,000 to 2,800 kWh a year.

Unit costs only, standing charges excluded. Energy per mile derived from the 25 to 35 miles a 7kW charger adds per hour. Rate bands as at 3 August 2026 and set out tariff by tariff in the comparison linked above.

So the car alone saves roughly £340 to £535 a year against charging at the capped rate. Now the other side of the bet, which is where the shape of the tariff decides the answer.

On that assumption, a household that can move a fifth of its 2,500 kWh into the window pays £40 for the 500 kWh inside it and £622 for the 2,000 kWh outside, so £662 against £653 at the capped rate. Roughly £9 handed back, against several hundred saved on the car. That household should take the whole-home structure without much thinking about it.

Change one input and it turns. A household at home all day, using 4,500 kWh with nothing it can realistically shift, pays £1,400 on the same assumed peak rate against £1,175 at the capped rate. That is £225 handed back. If the same household drives 5,000 miles a year, the car saves roughly £171 to £268 in the cheap window, so the whole-home structure lands somewhere between losing £54 and gaining £43. A car-only add-on at 14p, which leaves the rest of the house on its existing rate, saves that household roughly £121 to £169 with no peak penalty at all, and wins on every version of the sum. This is why the answer is genuinely different house to house, and why we would rather run it for you than publish a winner.

What to check before you switch

1. Whether the cheap rate covers the whole house or only the car, because that changes the arithmetic completely.
2. What the daytime rate is, not just the overnight one. The peak side is where whole-home structures claw money back.
3. Whether your charger or your car is on the supplier's compatibility list, since several of the best rates require it.
4. Whether the window is long enough to put the charge you need into the car, or the battery, in one night.
5. Whether you need a smart meter with half-hourly consent, which most of these tariffs require before the rate applies.
6. Whether there is an exit fee, and how the tariff interacts with any export rate you are on if you also have solar. Both are set out per tariff in the comparison.

Standing charges sit outside all of this and are not usually where the difference lies, but they are real: the capped average is 57.19p a day. Our tariff guidance keeps the current picture dated, and county pages carry local calculators you can reach from the coverage map.

Clearline gives independent guidance and does not sell energy. Suppliers are named for factual comparison only. Rates are region dependent and change frequently, so always confirm directly with the supplier before switching.