How much a battery saves by charging on cheap night rates

The spread, in one number

Charge a battery from the grid at an 8p off peak rate, use that electricity in the evening instead of buying at the capped average of 26.11p per kWh, and you keep roughly 17p for every usable kWh that comes back out. The gross spread is 18.11p. Round trip losses take about a penny of it. Everything else on this page is either where that penny goes or how many times a year you can repeat the trade.

Where the losses happen

Round trip efficiency is the share of energy that survives the journey in and out of the battery. Three things take a cut: converting AC from the grid into DC for the cells, converting DC back to AC for the house, and the standing draw of the battery management system, cooling and inverter idle. Together they are worth 6 to 10 per cent, and the biggest variable is how the battery is wired. A DC coupled battery shares a hybrid inverter with the solar array, so grid charging passes through fewer conversions. An AC coupled battery, which is what most retrofits use because it bolts onto working solar without replacing the existing inverter, runs one more conversion each way. The gap is about two percentage points, worth roughly £25 a year on a 10 kWh battery cycled daily. That is a tiebreaker between two new quotes, not a reason to rip out a working inverter.

Loss ranges come from published UK AC and DC coupling comparisons. The cost and saving columns are our own arithmetic on an 8p off peak rate against the Ofgem capped average unit rate of 26.11p per kWh for July to September 2026. Standing charges are excluded because you pay them either way.

What a full year actually looks like

Annual saving is usable capacity multiplied by how many times you fill and empty it, multiplied by the spread. The mistake in most quotes is the middle term. A battery that also takes free solar in summer, and that sometimes reaches morning with charge still in it, does not manage 365 full cycles. Three hundred full cycle equivalents a year is a fair working assumption.

Our arithmetic, at the AC coupled net saving of 17.2p per usable kWh. The 365 rows are a ceiling rather than a forecast: they assume a full charge and a full discharge every single day of the year, which almost no household achieves.

Put that against the money. Installed battery cost sits at roughly £400 to £900 per kWh, so a 10 kWh system lands somewhere between £4,200 and £6,500, and standalone batteries fitted without solar are quoted at around £4,000 to £5,000 for that capacity. Battery storage also carries 0 per cent VAT until 31 March 2027, reverting to 5 per cent on 1 April 2027, and that relief covers retrofits and standalone batteries as well as new solar and battery installations. A 10 kWh battery saving £516 a year on off peak charging alone pays for itself in eight to thirteen years, which matches the published payback ranges. Our battery storage guide covers the capacity and backup side of that decision.

The catch: qualifying for 8p in the first place

The cheap rates that make this arithmetic work are attached to electric vehicles, not to batteries. Intelligent Octopus Go quotes from 8p per kWh with a guaranteed window of roughly 23:30 to 05:30, and the cheap rate applies to the whole house during that window, batteries included. E.ON Next Drive Smart quotes 8p per kWh from midnight to 6am, also whole home. Both require a compatible electric vehicle or charger and a smart meter with half hourly readings.

• No smart meter, no tariff. Time of use rates cannot be applied without half hourly metering, so this is the first thing to fix.
• No EV, fewer options. A battery on its own rarely unlocks the 8p tier. If getting a car charger fitted is what opens it up, our EV charging guide covers what that involves.
• Economy 7 is the fallback, and a weaker one. Night rates averaged 15.43p per kWh in April 2026, with the cheapest around 12.5p. At 15.43p and a 10 per cent round trip loss the net saving falls to about 9p per usable kWh, roughly half the return.
• The window has to be long enough. A six hour window at a 3 kW charge rate puts 18 kWh in. That is ample for a 10 kWh battery and tight for a 20 kWh one.

Which windows exist, which appliance unlocks each one, and the solar and battery tariffs built around them, are set out in off peak tariffs and battery charging. Tariff conditions move faster than anything else in this market, which is why our tariff guidance carries the date it was last checked, and why the county pages under our coverage area carry local calculators that run this arithmetic on regional rates rather than a national average.

How to check your own numbers

What breaks the maths

• Exporting what you bought. Electricity charged from the grid and pushed back out is not covered by the Smart Export Guarantee obligation. Suppliers may pay for it, but they are not required to, and terms differ by tariff. Confirm before you plan a charge low, export high strategy.
• Throughput clauses. Some warranties cap total energy through the battery as well as the number of years. Heavy daily grid cycling is exactly the pattern those clauses are written for, so read the throughput figure, not just the headline term.
• Summer competition. From roughly May to August the battery should be filling from the roof for nothing, so grid charging contributes far less. The off peak case is strongest in winter, which is precisely when the solar case is weakest.
• The cap moves. The 26.11p figure is the July to September 2026 capped average and it is reviewed quarterly. If the standard rate falls while the off peak rate holds, the spread narrows and every figure on this page shrinks with it.

Rates verified against supplier and Ofgem pages on 3 August 2026. The 26.11p figure is the Ofgem capped average unit rate for 1 July to 30 September 2026 and varies by region and payment method. Clearline is an independent guidance service and does not sell or fit equipment; installations are carried out by an MCS-certified installation team for your area.