Solar Batteries Explained: Storage, Backup and What They Cost You

A solar battery stores the extra electricity your panels make during the day so you can use it in the evening, when the sun has gone and household demand is usually at its highest. Batteries can also keep essential circuits running during a power cut. But they are the most misunderstood part of a solar system, so this guide explains what the numbers on a battery actually mean before you decide whether one is worth it.

Capacity and power are two different numbers

Two numbers describe a battery, and confusing them is the most common mistake. Capacity, measured in kilowatt-hours (kWh), is how much energy the battery can store — the size of the tank. Power, measured in kilowatts (kW), is how fast it can deliver that energy — the size of the tap. A battery with a large capacity but low power can run small loads for a long time but cannot start a large appliance; a battery with high power but small capacity can run a large load briefly and then runs out.

Usable capacity is smaller than the headline capacity. Batteries are not drained to zero: a limit called depth of discharge protects the battery's lifespan, so the energy you can actually use is the headline figure multiplied by that limit. When comparing batteries, compare usable capacity, not headline capacity, and ask for the figure in writing.

What a battery does on a normal day

On a normal day the pattern is simple. In the middle of the day, when panels produce more than the home uses, the extra energy charges the battery instead of flowing to the grid. In the evening, when the panels stop and the home's demand rises, the battery discharges to cover that demand. Overnight the battery may empty, and the home returns to grid power until the sun rises again.

How much of the evening this covers depends on the battery's usable capacity and the household's evening use. A modest battery may cover lights, the fridge and the television; running air conditioning or an electric cooker from a battery needs a much larger one. A good installer will estimate evening use from your bills and show how much of it a given battery would cover, rather than promising that a battery makes the home independent of the grid.

Backup power during outages

Backup is a separate feature from storage, and it needs to be specified. A standard battery setup without backup capability will shut down with the rest of the system during a grid outage, for the same safety reason as the inverter. Backup-capable systems can keep a set of essential circuits — lights, fridge, internet, a few sockets — running during an outage, but they cannot usually run the whole house, and the circuits that get backup must be chosen in advance.

The length of backup depends on the battery's charge at the time of the outage and how much the backed-up circuits draw. An outage at midday with a full battery lasts far longer than one at midnight with an empty battery. If backup is your main reason for buying a battery, say so explicitly and ask the installer to list which circuits will be backed up and for how long, under stated assumptions.

Lifespan, warranties and the honest trade-off

Most home batteries today use lithium-ion chemistry, the same family as phone and car batteries. Their lifespan is usually described in cycles — one full charge and discharge — and in years, with warranties commonly covering a stated number of cycles or years, whichever comes first, and guaranteeing a minimum remaining capacity at the end. Read the warranty, not just the headline years: the guaranteed capacity at the end of the warranty tells you how much the battery will have degraded.

The honest trade-off is this: a battery adds a large cost to a solar system, and whether it pays for itself depends on local electricity prices, the value of exported power, and how much evening use it actually covers. In places where exported solar power earns little and evening electricity is expensive, a battery can make good sense. Where exported power is well paid, the same money may be better spent on more panels. Ask for the calculation with its assumptions, not a slogan.