Whether a battery pays depends less on the battery than on your bill: when you use your units, what you pay for them and what you’re paid for the ones you send to the grid. This guide shows how to read your own bill for those three things, with the example estimate as a worked case.
Why the timing of your use decides it
Panels make most at midday. Most homes use most in the evening. Whatever the house isn’t using when the roof makes it goes out to the grid, and after dark the house buys from the grid again.
A battery sits between the two. It stores what’s spare in the middle of the day and gives it back after dark, so fewer units are exported and fewer are bought.
A battery doesn’t make more power. It moves some of it to the evening.
So the question isn’t whether a battery is good. It’s whether the units it moves are worth enough, on your tariff and with your pattern of use, to cover what it costs.
Finding your use by the hour
Your bill gives a total for the period, in kWh. That’s the starting point, but a total doesn’t say when the units were used, and for a battery, when matters more than how many.
If you have a smart meter, your supplier’s app or online account may show your use by the hour or the half hour. What you can see depends on the supplier and your settings, so it’s worth looking. When you do, look for:
- how much you use between late afternoon and bedtime, compared with the middle of the day
- a normal weekday against a weekend
- a winter week against a summer one
- anything large on a timer, such as a dishwasher, a washing machine or an electric car
Without hourly figures, walk through a normal day instead. Who is home, and when? When do the oven, the kettle and the washing machine run? The answers won’t be exact, but they’ll show which way your use leans.
What evening use and daytime use each mean
If someone is home through the day, more of the roof’s output is used as it’s made. A battery has less spare to store, and less to do.
If the house is empty from morning to evening, most of the midday output is exported and most of the evening’s power is bought in. That’s the pattern a battery is built for.
A battery is usually worth it when:
- your evening use is high
- you’re out most of the day
- you’re on a time-of-use tariff
- an electric car charges overnight
It’s often not worth it when:
- someone is home through the day
- the array is small for the house
- your export rate is already strong
Your hourly figures answer a second question too: how big. A battery should be matched to what you use after dark, not the biggest box on the list. One that is larger than your evenings rarely empties, so part of what you paid for does little. In winter, the roof may not make enough spare to fill it at all.
Unit prices, export rates and the standing charge
Four things on your bills shape the answer. Tariffs and export rates vary by supplier, so use your own.
The unit price is what you pay for each unit you buy. Every unit the battery supplies in the evening is a unit you don’t buy at that price.
The export rate is what you’re paid for each unit you send to the grid. Every unit the battery holds back is a unit you don’t export, so what the battery really gains on each one is the gap between the two, not the whole unit price. The stronger your export rate, the less a battery adds.
A time-of-use tariff changes the price through the day, usually cheapest overnight. On one of these, some homes also fill the battery from the grid overnight and use it when prices are higher, which can make the case stronger than the solar alone suggests.
The standing charge is the fixed daily part of the bill. It stays the same whatever the roof or the battery does.
The same roof, priced both ways
The example estimate is for a semi-detached house in Exeter: 6.2 kWp on a roof facing S–SW, making about 5,400 kWh a year. It assumes a £1,500 yearly bill, about 4,300 kWh, with 30p a unit bought, 16p a unit exported and a £210 standing charge.
Without a battery:
- 28 per cent of what the roof makes is used at home
- the saving is about £1,080 a year: £460 off the bill and £620 in export payments
- installed, it’s £7,200–£8,900, and the payback is about seven years
With a 10 kWh battery:
- 48 per cent of what the roof makes is used at home
- the saving is about £1,230 a year: £780 off the bill and £450 in export payments
- installed, it’s £11,700–£13,400, and the payback is about ten years
Read it as two columns. The battery moves units from the export column to the bill column, where each is worth 30p instead of 16p. The yearly saving rises. But the price rises by £4,500, so the payback stretches from about seven years to about ten. Taken on its own, the battery adds about £150 a year, so it would take about 30 years to repay by itself.
To test it against your own bill, swap in your own figures: your yearly use, your unit price and your export rate. The shape of the sum stays the same. The wider the gap between what you pay for a unit and what you’re paid to export one, and the more of your use that falls after dark, the more the battery’s column gains.
On these assumptions, the system without a battery pays back sooner. Change the assumptions and the answer can change: more use after dark, a time-of-use tariff or a weaker export rate all tilt it towards the battery. Some people choose the battery for reasons that don’t sit on a payback line. The choice stays yours.
What we do
We price every estimate with and without a battery, side by side, and show the payback both ways. What each warranty covers, and who to call when something isn’t right, are set out on our warranties and aftercare page. If you’d rather wait, a battery can be added later, and telling us now can change the inverter we choose.
