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A battery does not generate anything. It moves electricity from a moment when it is cheap or free to a moment when it would have been expensive. Everything about whether one pays follows from that single sentence.

This hub covers what storage costs on a comparable basis, how to size one to the way your household actually uses electricity, and the cases where a battery makes sense even with no panels on the roof.

Compare cost per usable kilowatt hour

Battery quotes are hard to compare because the headline capacity is not the capacity you get. Two figures matter and only one is usually advertised:

• Nominal capacity is the total the cells hold.
• Usable capacity is what the system will actually let you take out, after the reserve it keeps to protect cell life.

Divide the installed price by the usable figure. That is the number to compare between quotes, and it is the number that goes into any calculation of whether the battery pays.

If a quote does not state usable capacity, ask. A system quoted on nominal capacity can look materially cheaper than one quoted honestly.

Size it to your evening, not to your roof

The common instinct is to size the battery against the solar array. That is the wrong reference. The battery’s job is to cover the demand that happens when the panels are not producing, so the figure to size against is how much electricity you use between sunset and sunrise.

You can measure this rather than guess:

1. Get your half hourly data. If you have a smart meter, your supplier can provide it, and several offer it through an app or a data portal.
2. Add up the units consumed between roughly 4pm and 8am across a few typical days.
3. That total is the most a battery could usefully discharge on those days.

Buying much beyond that figure means paying for capacity that will rarely be cycled. A battery that sits half empty still cost full price.

Degradation is a cost, not a footnote

Cells lose capacity with use and with age. Warranties are normally written as a guaranteed remaining percentage after a stated number of years or a stated number of cycles, whichever comes first.

That structure tells you what the manufacturer expects. It also means a battery that is cycled hard every day may reach its cycle limit before its year limit. Any honest projection of what a battery saves has to account for the capacity falling over the period, not hold it flat.

A battery can pay without solar

This surprises people, but it follows directly from what a battery does. If your tariff has a cheap overnight rate and an expensive daytime rate, a battery can charge when electricity is cheap and discharge when it is not. No generation is involved at all.

Whether it pays depends on the spread between the two rates and how much of your usage you can shift. The wider the spread, the better it works. This is arbitrage, and it is the reason batteries are being installed on houses with no panels.

The same logic explains why a battery often improves a solar system’s return by more than people expect: it raises the share of your own generation you consume rather than export, and that gap between import and export rates is usually large.

What to ask before buying

• What is the usable capacity, and what is the price per usable kilowatt hour?
• What does the warranty guarantee, after how many years, and after how many cycles?
• Can the inverter charge the battery from the grid, or only from solar? This decides whether tariff arbitrage is available to you at all.
• What happens in a power cut? Most systems do not provide backup unless they were specified to.
• How much of my evening usage would this actually cover, using my own data?

Work it out on your own usage

The value of a battery is almost entirely determined by your household’s pattern, which is why generic answers are so unreliable. The calculator on this site models storage alongside generation, treats import and export separately, and applies degradation over the period rather than assuming the battery performs on day four thousand as it did on day one.