Battery only – no panels, sunshine optional.

Buy low, use high, daily. On a smart time-of-use tariff the overnight rate can be less than a third of the daytime one, and the battery works that gap every single day, rain or shine – its one great advantage over solar being that it doesn't care about the weather. Without such a tariff, the battery is an ornament with a warranty. If that's your situation, our engine will say so to your face.

The one thing everyone knows about batteries is true: they keep getting cheaper. The cells that cost around $730 a kilowatt-hour in 2013 cost about $115 in 2024 – an 85% fall in eleven years.

A caution before you extrapolate your way to a bargain: that's the cell price. The box on your wall also contains an inverter, an installer, a van and a margin, none of which follow the same curve. UK installed prices drift downwards; they don't plummet. Waiting a year will save you something, not everything.

The heavier your evening use, the better this works – within a limit. The battery can only shift so much in a day, and our engine caps that honestly rather than assuming your household runs a small foundry.

Let's address it rather than whisper past it. Lithium battery fires are rare, but they are real – which is exactly why the rules about where the box goes exist. A properly certified battery, properly installed, lives on an outside wall, in a garage, or somewhere that is neither your escape route nor your loft. A cheap unbranded unit from a marketplace listing lives, eventually, in someone's insurance claim. This is a buy-the-real-thing category. The saving on a knock-off is not worth the conversation with your insurer.

One more thing, because it's coming. An electric car's battery is 60kWh or more – that's six to twelve wall batteries, sitting on the driveway. 'Vehicle-to-grid' technology, where the car powers your house through the evening peak and refills cheaply overnight, is moving from trials to actual tariffs. We don't size it in our engine yet, because the hardware and the tariffs are still settling. But if you drive an EV, the biggest battery you will ever own is already parked outside – which rather changes the question of buying a smaller one for the wall.

EV owners already on a smart tariff – the cheap window is already in your life. High evening users. And homes where solar genuinely can't happen but the tariff maths still can. Renters: you'll need the landlord's consent for the wall mounting.

At today's prices the payback usually outlives the warranty, and quite often the battery. We show you that number rather than rounding it away. If the maths says no, our results page will be the one place on the internet that tells you so.