A solar panel system without a battery generates electricity when the sun shines — which isn't always when you need it. A battery stores what you generate and releases it when you do. But does adding one make financial sense for your home?
Why self-consumption matters so much
The economics of solar hinge on self-consumption — the proportion of your generated electricity you use on-site rather than export. Every unit you use yourself is worth around 24p (the rate you'd otherwise pay to import from the grid). Every unit you export is worth only 4–15p via the Smart Export Guarantee.
Without a battery, a typical household self-consumes around 30–50% of what their panels generate. The rest is exported cheaply. A well-sized battery can push self-consumption to 70–90%, dramatically improving the economics of the solar installation.
What adding a battery actually saves
Take a 4kW solar system generating 3,600 kWh per year. Without a battery at 40% self-consumption, you self-use 1,440 kWh (saving ~£346) and export 2,160 kWh (earning ~£216 at 10p SEG). Total annual benefit: ~£562.
Add a 10kWh battery and self-consumption rises to 75%. You now self-use 2,700 kWh (saving ~£648) and export 900 kWh (earning ~£90). Total annual benefit: ~£738. The battery adds roughly £176/year to the benefit of the solar system alone — against a battery cost of around £5,500–£7,000 installed.
That gives the battery itself a payback of 30+ years on solar self-consumption alone. However, a battery paired with a time-of-use tariff (charging cheaply overnight at 7p/kWh and avoiding expensive daytime grid electricity at 24p+) adds a further £300–£500/year, bringing the combined payback to a much more attractive 8–12 years.
When it makes sense to combine from day one
Combining solar and battery at the same time tends to be cheaper than retrofitting a battery later. Installation costs are shared (scaffolding, labour, commissioning), and you can opt for a DC-coupled system where the battery connects directly to the panels before the inverter — which is slightly more efficient and typically cheaper to install together.
It makes most financial sense to combine from day one if you're on a time-of-use electricity tariff or planning to switch to one, if you want backup power capability, if you have an EV you charge at home, or if your household uses a lot of electricity in the evenings.
When it makes sense to add a battery later
If your budget is tight, starting with solar alone and adding a battery later is a perfectly valid approach. Solar payback is strong on its own, and battery technology and prices continue to improve — waiting 3–5 years may mean a better product at a lower price.
AC-coupled batteries (like the Tesla Powerwall 3 and GivEnergy AIO) retrofit easily to any existing solar installation, working alongside whatever inverter you already have. The installation is more expensive than DC-coupled but still straightforward for a competent MCS installer.
What to look for when combining solar and battery
Not all solar systems are easily expandable with a battery. If you're planning to add one later, ask your solar installer to specify a hybrid inverter from the start — this is battery-ready and avoids the cost of replacing the inverter when you later add storage.
For battery capacity, match it to your evening and overnight electricity usage rather than your generation. A household using 8–10 kWh after sunset doesn't benefit much from a 13.5kWh battery — the extra capacity goes unused most nights. A 9.5–10kWh battery is typically the sweet spot for a 3–4 bedroom UK home.