Why storing your solar power changes the maths

Solar panels only generate electricity while the sun is up, but most households use their electricity in the evening. Without a battery, that mismatch means you export the bulk of your midday generation to the grid — often for only a few pence per kilowatt-hour — and then buy it back at 25p or more when the kettle, oven and television come on.

A home battery fixes that timing problem. It captures surplus solar during the day and releases it after sunset, so the electricity you generated powers your evening. For most homes, that single change is worth more than any other upgrade you can make after the panels themselves.

Standard panels alone typically cover around 30 to 40 per cent of a household's annual electricity demand if nobody is home during the day. Add a sensibly sized battery and you can push that self-consumption figure towards 60 or 70 per cent, depending on your habits and how the system is set up.

How much capacity do you actually need?

Battery capacity is measured in kilowatt-hours (kWh), and bigger is not automatically better. An oversized battery sits half-empty for much of the year, and you pay for cells you never cycle.

• 2 to 4 kWh — suits a small flat or a low-use household, or a couple with no electric heating or EV.
• 5 to 7 kWh — the sweet spot for a typical three-bedroom home with a 3 to 4 kW solar array. Covers most evening demand.
• 8 to 12 kWh — sensible if you have a heat pump, an electric vehicle, or you want to charge cheaply overnight and run the house all day.

As a rough guide, look at your daily electricity use in spring and autumn, then aim to cover the evening portion of it. Most usable capacity is slightly less than the headline figure: a 10 kWh battery may only give you 9 kWh once depth-of-discharge limits are respected, and round-trip efficiency typically sits at 85 to 90 per cent.

Also check the power rating, not just the capacity. A battery with plenty of stored energy but a low continuous output may struggle to run a kettle, oven and immersion heater at once.

What it costs, and what you might save

Installed prices in the UK generally land between £3,500 and £6,000 for a 5 kWh system, and £6,000 to £9,000 for 10 kWh, depending on the brand, inverter compatibility and how awkward the installation is. If you are having solar fitted for the first time, adding a battery at the same time usually costs less than retrofitting later, because the scaffolding and electrical work are already happening.

Savings come from three places: avoided import (using your own power instead of buying it), higher export payments if your tariff rewards it, and off-peak arbitrage on a smart tariff. A well-sized battery on a suitable tariff might save £300 to £600 a year for a typical household, though that varies enormously with usage, tariff and system size.

Payback periods commonly quoted sit between 7 and 12 years. Be sceptical of anyone promising three. Battery warranties are usually 10 years, and most modern lithium iron phosphate units are rated for 4,000 to 6,000 cycles — plenty for one cycle a day over the warranty period.

Smart tariffs make a real difference

If your battery is only ever charged by solar, you are leaving value on the table. Time-of-use tariffs offer cheap electricity overnight — sometimes as low as 7p per kWh — and your battery can charge then, ready to run the house through the morning peak when rates are highest.

This works particularly well in winter, when solar generation is thin but your battery still has a role. Charge overnight at the cheap rate, discharge during the expensive 4pm to 7pm window, and you can cut your winter bills meaningfully even without sunshine.

You will need a battery that supports scheduled or smart charging, and an inverter that can be controlled by the manufacturer's app or a third-party energy management system. Check this before you buy — some cheaper units only charge from solar, which limits your options.

Pair this with the Smart Export Guarantee and you have a decent combination: export at 15p per kWh on a tariff that pays well, import at 7p overnight. Just check the terms carefully, as some tariffs have standing charges that offset the gains.

Practical points before you commit

• Space and location — batteries are roughly the size of a small fridge or a wall-mounted boiler. Garages, utility rooms and integrated lofts work well; unheated outbuildings reduce performance in cold weather.
• Inverter compatibility — hybrid inverters handle both solar and battery in one unit. If you already have a solar-only inverter, you may need an AC-coupled battery instead, which is simpler to retrofit but slightly less efficient.
• Noise and ventilation — most units are near-silent, but they do need airflow. Avoid boxing them into a sealed cupboard.
• Future-proofing — if a heat pump or EV is on your five-year plan, size the battery and inverter with that in mind now.
• Certification — use an installer registered with a recognised competent person scheme, and make sure the work is notified so it complies with building regulations.

Deciding whether it stacks up for you

Battery storage is not automatically right for every home. If your electricity use is low, your roof is shaded, or you are rarely in during the evenings, the sums get tighter. If you are at home during the day and already use most of your solar, a battery adds less.

But for the majority of UK households — out at work, home in the evening, heating and hot water increasingly electrified — a battery is the piece that makes a solar array genuinely useful rather than decorative. Ask three installers for quotes, insist on a written estimate of annual savings based on your actual usage figures, and prioritise a system you can control and expand over one that simply looks good on the spec sheet.