Battery: when yes, and when not yet

What does a home battery actually do?

A battery generates nothing: it shifts. It stores midday energy, when your panels produce more than you use, so you can spend it at night, when they produce zero. Its value is the gap between what you are paid for a surplus kilowatt hour and what buying that same kilowatt hour costs you after sunset.

In our area a south-facing roof produces a lot, concentrated in the middle of the day. The more your consumption looks like that profile, the less you need a battery; the more nocturnal it is, the more work the battery has.

When does a battery pay off?

The profiles where a battery genuinely works, day after day:

• Homes lived in during the evening and night: the empty house at midday fills the battery, and coming home from work empties it.
• Fixed night-time loads: bedroom cooling in summer, a pool filtering at night, a car charging in the small hours.
• Anyone who wants backup in a power cut: only batteries with a backup function keep the house running when the grid drops, and that is decided at design time, not after.

When is it worth waiting?

And the profiles where we usually recommend starting without one:

• Daytime-heavy consumption: working from home, washing and cooking at midday. The panels already cover that use directly, with no charge-discharge losses.
• Small bills: if you buy little energy at night, the saving a battery can shift is small and takes a long time to pay back its cost.
• A tight budget: it almost always pays more to size the panels properly today and leave the installation ready for a battery tomorrow.

How we decide it in the study

We do not eyeball it. We cross your roof's estimated production, with its real orientation and pitch, against your hour-by-hour consumption, straight from your meter. That crossing tells us how much energy you would buy at night, and therefore how much a battery could shift in your specific case. With that number on the table, the decision takes itself.

And if the answer today is no, the installation is left ready: the day your consumption changes, the battery goes in without redoing anything.

What sizing mistakes come up most often?

The first is buying a battery by round number rather than by real night-time consumption. If your house uses little between dinner and breakfast, a big battery spends its life half full, ages just the same and never delivers what the brochure promised. The capacity should match what you genuinely consume at night, and that comes from your bills and your hourly curve, not from a catalogue.

The second is sizing against the July surplus. In summer there is more than enough sun to fill almost any battery, but in winter, with short days and cloudy weeks, on many days it does not fill at all. A typical installation in Spain averages 5.5 kW at around 1,215 euros per kW according to the latest APPA annual self-consumption report, with payback of 6 to 7 years without a battery. Add a battery that only works at full stretch for half the year and that payback stretches out considerably further than the advertising suggests.

The third is forgetting power and placement. A battery is not just kWh: it also has a discharge limit in kW, and if that limit is low, the house draws from the grid when the oven and the hob start together, even with a full battery. As for placement, lithium suffers in the heat, and a metal shed in the August sun around here is no place for one. Shade, ventilation and a solid wall.

• Battery full at midday while the house buys from the grid at night: not enough discharge power
• Battery that never fills from November to February: it was sized for summer
• Capacity far above night-time consumption: money parked on the wall

Will I have power in a blackout if I have a battery?

In the scattered rural homes of La Costera, some people want a battery mainly as insurance against power cuts. Here comes the surprise almost nobody mentions: when the grid goes down, a grid-connected installation shuts off for safety, so it does not push voltage into a line where crews may be working. And most batteries shut down with it, unless the inverter is a hybrid with an emergency output and the installation is wired to separate the house from the grid at that moment.

Real backup forces you to choose what to protect: fridge, freezer, some lights, perhaps the boiler. Not the whole house, because equipment that carries everything gets expensive fast. If your main reason for a battery is the blackout during the August storm, say so during the visit, because it changes the entire design of the system. And if a salesman promises backup without asking which circuits you want kept alive, be suspicious: either he has not thought it through or he does not intend to deliver it.

Frequently asked questions