By half past four on a winter afternoon in Dover, it’s dark. Your solar panels clocked off an hour ago — and your household is only just getting started. The kettle, the oven, the lights, the telly, maybe an EV plugged in on the drive. This mismatch between when solar generates and when families actually use electricity is precisely the problem a home battery solves.
Here’s how it works in practice, with the real numbers.
The 4pm to 7pm squeeze
Electricity demand across Britain peaks in the early evening. Octopus Energy describes 4.00pm to 7.00pm as the daily peak energy demand period, when wholesale prices are at their highest — which is why time-of-use tariffs charge a premium in exactly that window.
It’s not hard to see why. Everyone gets home, dinner goes on, kettles boil, heating fires up, screens switch on — all at roughly the same time, in millions of homes at once.
For solar households, the timing is awkward. Panels generate most around the middle of the day, when many homes are quiet. Without storage, you export your surplus at lunchtime, then buy electricity back at the most expensive time of day a few hours later. A battery lets you keep that midday generation and spend it on your own evening instead.
How much electricity does an evening actually use?
Ofgem’s typical domestic consumption values put a typical household in England, Scotland and Wales at around 2,700kWh of electricity a year. Spread evenly, that’s roughly 7.4kWh a day.
In reality it isn’t spread evenly — a large slice of it lands between teatime and bedtime, for the reasons above. So the useful question when sizing a battery isn’t “how much do I use in a day?” but “how much do I use between the sun going down and coming back up?”
Where a 5–12kWh battery fits
Home battery capacity is measured in kilowatt hours, and the Energy Saving Trust notes a typical home system might be around 10kWh. Most systems we quote for sit somewhere in the 5–12kWh range. Set those sizes against Ofgem’s typical figures and the picture becomes clear:
- A 5kWh battery holds roughly two-thirds of a typical home’s entire daily electricity use. That’s comfortably enough to carry most typical households through the 4–7pm peak and well into the evening.
- A 9–10kWh battery holds more than a typical home’s full daily usage — for many households, that’s the evening and the overnight hours covered, with the battery recharging from the panels next morning. For scale, GivEnergy’s 9.5kWh unit is a single wall-mounted box backed by a 12-year warranty.
- A 12–13.5kWh battery — the Tesla Powerwall 2, for example, offers 13.5kWh usable — suits higher-usage homes: larger families, home workers, or anyone running a heat pump or charging an EV overnight.
Those are illustrations against typical figures, not promises. A house with electric heating or nightly EV charging uses far more than 7.4kWh a day, and the right size for you comes from your own usage, not the national average.
Two datasheet numbers worth understanding
When you compare batteries, two specifications matter more than the headline capacity.
Depth of discharge
Depth of discharge (DoD) is how much of a battery’s stored energy you can actually take out. Batteries shouldn’t be run flat — as the Energy Saving Trust explains, the management system stops discharge at a set reserve level, perhaps around 20% of capacity, because discharging to zero can significantly shorten a battery’s life.
Modern lithium iron phosphate units handle this differently: the manufacturer builds the protection in and quotes a usable figure. GivEnergy’s 9.5kWh battery lists 9.5kWh usable at 100% depth of discharge; the Powerwall 2 lists 13.5kWh usable on the same basis. The practical rule: always compare batteries on usable kWh, not nominal capacity — two “10kWh” batteries can deliver quite different amounts of electricity.
Round-trip efficiency
No battery gives back every unit you put in — a little energy is lost converting and storing it. Round-trip efficiency measures what survives the journey. Tesla publishes 90% round-trip efficiency for the Powerwall 2: store 10kWh of solar surplus and you get about 9kWh back out.
That 10% or so isn’t a flaw — it’s physics, and it’s already a far better deal than exporting cheaply at noon and importing expensively at 6pm. But it belongs in the sizing sums: a battery that needs to deliver 9kWh of an evening needs slightly more than 9kWh put into it.
Sizing it honestly
The honest way to size a battery is from your own half-hourly smart meter data — how much your household really draws between dusk and dawn, winter and summer — matched against usable capacity, not the number on the brochure. As an MCS-accredited installer based in Shepherdswell, that’s exactly the exercise we work through on every home battery storage survey: your usage pattern first, the hardware second.
