Do I need a battery with my solar panels?

This is the upgrade everyone asks about, and the one where the answer genuinely splits by household. Treat it as a separate decision from the panels, because it has its own price and its own payback. Done for the right home, a battery lifts your savings. Done for the wrong one, it sits in the garage barely earning its keep. Here's the maths that tells them apart.

Where the savings actually come from

A battery doesn't make electricity. It moves it in time. Panels generate most at midday; you use most in the evening. Without storage, the midday surplus is exported for the Smart Export Guarantee rate, which runs about 12p to 20p per kWh depending on your supplier. With storage, you keep that surplus and burn it at night instead of buying from the grid at around 25p.

So every kWh a battery time-shifts is worth the gap between those two numbers, roughly 10p, rather than the 5p or so it might otherwise earn. That sounds small. Over a year a 5 kWh battery cycled most days moves somewhere near 1,500 kWh, which turns that 10p gap into £150 or so a year from solar alone. It's real money, but it's not enormous, and that's the honest starting point.

The lever that changes the answer: your tariff

The bigger win often isn't solar at all. On a time-of-use tariff such as Intelligent Octopus Go, off-peak electricity can drop to around 7p per kWh overnight. Charge the battery then, run the house off it during the 25p peak, and you arbitrage roughly 18p per kWh whether or not the sun shone. Stack that on top of the solar gap and a battery that looked marginal can pay back years sooner. If you have, or can switch to, a cheap night rate, run the numbers again with it.

When a battery makes sense

• You're out during the day, so little solar is used live. A battery rescues that exported surplus for the evening instead of selling it cheap.
• You have high evening and overnight demand, an EV, or a heat pump. More load to soak up means a bigger pack cycles harder and pays back better.
• You're on a time-of-use tariff with a cheap off-peak window the battery can charge from.

When it might not, yet

• Someone's home all day already using most of the solar live. There's less surplus left to store, so the battery has less to do.
• Your budget is tight. Panels alone usually have the stronger standalone payback (see is solar worth it), and a battery can be retrofitted later when prices have fallen further.
• You're on a flat-rate tariff with a high export rate. If you already earn 20p for export, the gap a battery captures is smaller.

Sizing it right

Bigger is not automatically better, and oversizing is the most common mistake. The sweet spot roughly matches the evening and overnight slice of your daily use that the panels can actually refill the next day. A typical home lands around 5 to 10 kWh of usable capacity; a household with an EV or heat pump can justify more. Go too large and the top of the battery rarely cycles, so you've paid for capacity that never earns. The cost runs about £350 to £600 per usable kWh, so a 5 kWh pack is near £2,000 and a 10 kWh one £4,000 or more (the cost guide has the full picture).

The caveats installers gloss over

Two things worth knowing before you sign. First, lifespan: most home batteries use lithium iron phosphate cells warrantied for around 10 years or a set throughput, often near 6,000 cycles, so the pack may need replacing once within the panels' 25-year life. Second, backup: a standard solar battery does not keep your lights on in a power cut unless it's specified with backup capability and a separate gateway. If outage protection matters to you, ask for it by name.

The bottom line

Model the panels first; they're almost always worth it. Then model the battery on its own, with your real tariff plugged in. It earns its place when your usage pattern, or a cheap night rate, lets it cycle hard most days. The numbers decide it, not the sales pitch.