A solar battery stores electricity your panels make during the day, so you can use it in the evening instead of buying it from the grid. It is optional. Many systems work well without one, and a battery can often be added later. Whether you need one comes down to three questions: how much you use after dark, how big a battery that calls for, and whether you also want power during a power cut.
How a battery works with solar panels
During the day the panels supply the house first. When they produce more than the house uses, the surplus charges the battery instead of going straight to the grid. When the battery is full, the rest goes to the grid. In the evening, when the panels stop, the house draws on the battery until it reaches its lower limit, and then on the grid again.
This is managed by the inverter. A hybrid inverter handles both the panels and the battery in one unit. Alternatively a battery can have its own inverter, connected on the house side of the system, which is a common way to add a battery to an existing installation. Either way the system switches between panels, battery and grid by itself.
When a battery makes sense
- You use a fair amount of power in the evening and at night: cooking, air conditioning, a pool pump that has to run late, an electric car charged overnight.
- Your panels regularly produce more than you use during the day.
- You live in the house most of the year, so the battery is used most days.
- You want back-up during power cuts, and the system is designed for it (see below).
When it adds little
- You use most of your power in daylight already.
- Your overall use is low, or the house is empty for long periods.
- Your panels rarely produce a surplus.
In those cases a battery would spend much of its time full or empty and do little work. For a holiday home in particular, see solar panels for holiday homes.
Capacity: what the number means
Battery capacity is given in kWh. Two things are worth checking:
- Usable capacity. A battery is not normally run from completely full to completely empty. The datasheet gives the usable figure, which is what matters for your evening.
- Power. How many kW the battery can deliver at once. A large battery with low power may not run an oven and air conditioning at the same time.
Size the battery to your evening and night use, not to the size of your panels. A battery larger than your evening use sits partly unused; one much smaller is empty early in the evening. Your hourly consumption data shows how much you use after sunset.
Back-up is a separate feature
This is the most common misunderstanding. A battery on its own does not keep the lights on during a power cut. For safety, a grid-connected system switches off when the grid fails, battery included, unless it has a back-up function: an inverter that can supply the house on its own, and a switch that separates the house from the grid while it does. Back-up usually covers selected circuits, not everything, and only as long as the battery lasts.
One customer in San Miguel de Salinas had exactly that fitted:
“A special switch box that makes sure the power keeps coming from the batteries during a power cut. Very important. Without this switch you have nothing during a power cut!”
Jan W., San Miguel de Salinas, Google review, translated from Dutch
More on this in do solar panels work during a power cut.
A battery versus a “virtual battery”
Some electricity companies sell a so-called virtual battery: exported power is kept as a credit to use against later bills. That is a commercial contract, not a physical battery, and it does not give back-up. How exported power is credited is explained in selling excess solar electricity and how self-consumption works.
Adding one later
If you are not sure yet, choose an inverter that can take a battery later, or plan for a separate battery inverter. Some owners prefer to start without one and decide after a year of seeing their own production and use in the app.
Is it worth the money?
That depends on the price of the battery, how much evening power it replaces, and the value of the exported power it replaces. We do not give a general figure; we work it out from your consumption. For the wider sums, see are solar panels worth it in Spain. If you want us to look at your figures, ask for a quote and tell us about your evenings.
