When is the best time to charge a home battery?

Question

Accepted Answer

The best time to charge a home battery is when storing energy now costs less than obtaining it when you will need it, after losses and any export income you give up. That may be a cheap overnight window, a lower-priced daytime period or surplus solar generation. On a changing tariff it can change daily, and sometimes the best decision is to leave the battery partly empty.

Start with the next use for the energy

Ask when the household will need electricity that solar cannot supply directly. The purpose of charging is to meet that need at a lower net cost, or to support a deliberate backup reserve. A kilowatt-hour, written kWh, measures energy. Battery capacity tells you how much energy can be stored; charging power tells you how quickly it can be added.

A tariff is your supplier's pricing agreement. On a time-of-use tariff, the price depends on when electricity is used. If you pay the same import price throughout the day, moving grid purchases between hours does not create a price advantage by itself. Because storage loses energy, charging from the grid merely to use it later at the same price can increase energy costs.

When overnight charging makes sense

A predictable cheap overnight window can provide a useful starting schedule. Charge enough to cover expected demand before useful solar arrives, and later expensive demand that solar is unlikely to cover. Check whether the available window and charging power can actually deliver the target. The battery's nominal capacity alone cannot tell you that.

Avoid treating a full battery as the goal every night. If tomorrow's solar surplus would otherwise charge it, buying a full charge may use up valuable space. If tomorrow is dull or demand is expected to be high, a larger overnight charge may be sensible. Seasonal changes in sunlight and heating can make a previously good fixed target inappropriate.

Why the afternoon can beat the night

On a variable tariff, compare the published periods instead of assuming an overnight minimum. Half-hourly pricing means the unit price can differ for each half-hour. In the Octopus public API for London, region C, on 31 August 2026, Agile cost −1.62p per unit at 13:00 and 42.84p at 17:00. Those are dated UK observations, not a universal schedule.

The arithmetic is 42.84 − (−1.62) = 44.46p per unit, roughly a 44p swing. That gap explains the opportunity to move a purchase through time, but it is not the saving for each unit of battery capacity. Charging losses, available space and the amount of later expensive consumption determine how much value can actually be captured.

Decide what surplus solar is worth

Surplus solar is generation left after the home has used what it needs at that moment. Storing it can avoid buying electricity later. But export, sending electricity to the grid, may earn a payment under a separate agreement. That forgone payment is part of the cost of solar charging.

Compare the export value now with the import cost you expect to avoid later, allowing for losses. If selling solar and buying later is cheaper than storing it, maximising self-consumption is not the same as minimising the bill. If export is restricted or unpaid, the comparison changes. Use the terms that apply to your installation rather than a generic assumed export rate.

Charge enough, with room for uncertainty

Work backwards from expected demand to a target charge level. Subtract energy already stored and solar energy you reasonably expect to have available. Preserve any intentional backup reserve and check the equipment's limits. You do not need false precision: a forecast is an estimate, and a plan should be revisited when the weather or household routine changes.

There is a practical trade-off between charging early for certainty and waiting for a potentially better opportunity. Waiting may be cheaper, but a forecast error or unavailable charger could leave you buying at an expensive time. Charging early may avoid that risk but occupy space unnecessarily. The useful target balances those possibilities rather than always filling or always waiting.

Check that the timer is doing what you intended

Read back the schedule after setting it. Confirm the tariff's time periods, the device clock, grid-charging permission and charge target. Check whether another app or control mode can override the plan. A charging window entered correctly in one interface may still conflict with a schedule elsewhere.

Then review actual behaviour: battery level before the window, energy charged, charge remaining before expensive demand and any grid imports during that demand. If the battery cannot cover a brief demand spike, the issue may be discharge power rather than timing. If it empties too early, consider both the target and the household forecast before changing the window.

Let the plan change when the inputs change

PredBat re-plans every 5 minutes over a 48-hour horizon, considering changing prices, demand and solar forecasts. The service costs £9.99/month. Its purpose is to maintain a useful schedule as those inputs change; it cannot make every charging cycle profitable or guarantee that a forecast will be right. A stable tariff and an already effective timer may leave less extra value for automation.

Whether you use a timer or an optimiser, compare net electricity costs under similar conditions. Include charging energy and export income, not just the reduction in imported units during an expensive window. Keep a fallback schedule for unavailable data or control. The best charging time is the one that serves the next real need at the lowest overall cost, within the limits and reserve you have chosen.

When is the best time to charge a home battery? | PredBat