Every installer in the country is selling batteries right now, and for a good number of households the numbers do not support the purchase. Saying so out loud is most of the reason this site exists, so here is the honest version.
What a home battery actually earns
A battery does not generate anything. It moves electricity through time, storing surplus solar generation in the afternoon so you can use it in the evening instead of buying from the grid.
So its earnings are not "the value of the electricity". They are the gap between two prices: what you would have been paid for exporting that unit, and what you would have paid to buy it back later. A battery captures that difference, minus round-trip losses, on every unit it successfully shifts.
That framing does a lot of work. It immediately explains why a battery can be excellent for one household and pointless for the one next door, and it gives you the three questions that decide the answer.
The three cases where it usually fails
1. Low annual consumption
A battery can only earn on electricity you actually use. If your annual consumption is low, there is simply not much evening demand to shift into, and the battery spends much of the year partially cycled.
The saving stays small no matter how large the battery is, because the constraint is your demand rather than the storage. Payback then runs well past the warranty, and a battery that has not paid for itself by the time its guarantee expires has not really paid for itself at all.
2. A battery sized for a much larger array
Storage capacity only earns when it cycles. On a modest array, a large battery never fills in winter, because there is not enough surplus generation to charge it, and never empties in summer, because the house cannot use that much stored energy overnight.
The capacity you paid for sits idle. You are not buying more savings, you are buying more kilowatt-hours of shelf that never gets used. Undersizing a battery slightly is usually the cheaper error.
3. A high installed cost per kWh
Divide the total installed price by the usable capacity in kWh. That single number tells you more about whether a quote is competitive than the brand on the box.
Above a certain price per kWh, the depreciation on the hardware simply outruns the annual saving, and no amount of clever tariff optimisation closes the gap. Get more than one quote, and compare on price per usable kWh rather than headline capacity, because usable and nominal capacity are not the same figure.
When a battery genuinely does pay
The case can be strong, and we say so when it is. It usually needs several of these together:
- High annual consumption, so there is real evening demand to shift into.
- A usage pattern that does not overlap with generation. An empty house during the day is the classic profile.
- A poor export rate, which widens the gap the battery is capturing on every unit.
- A sensible installed price per usable kWh.
- A tariff with cheap off-peak import, which lets the battery earn on grid arbitrage as well as solar surplus.
That last point matters more than it used to. A battery on a time-of-use tariff can charge cheaply overnight and displace expensive daytime units regardless of what the sun is doing, which changes the arithmetic for households whose roofs are mediocre.
How to check your own case
Ask for the assumptions behind any payback figure you are quoted. Specifically: what export rate, what import price, what self-consumption rate, and what battery cycle count per year. If a quote cannot produce those four numbers, the payback period it prints is decoration.