Interactive explainer
Is a home battery worth it?
A battery doesn't make electricity. It takes the surplus your panels produce at noon and hands it back at 8 pm, when power is expensive. Whether that is worth thousands of euros comes down to one number — and you can work it out yourself below.
Illustrative prices: what a kilowatt-hour earns when you export it at midday, and what the same kilowatt-hour saves when a battery gives it back in the evening.
Home batteries come wrapped in big words — independence, resilience, the future. Strip them away and a battery does one simple thing: it moves electricity through time. It soaks up the surplus from your solar panels in the middle of the day, when nobody is home and the house barely uses anything, and releases it in the evening, when the oven is on, the lights are on and the panels have gone quiet.
That trick costs several thousand euros, and the hardware wears out. So the honest question is not “is a battery good?” but “is every kilowatt-hour it moves worth more than it costs to move?” For some homes the answer is a clear yes. For many it is a close call — and it flips with the price of electricity, the size of the battery and how much sun you get.
This guide opens the box, lets you simulate a day in a house like yours, shows how quickly extra capacity stops being useful, and ends with a calculator for the one number that decides it.
01A time machine for electricity
Inside a home battery is a stack of lithium cells — today almost always lithium iron phosphate, LFP. It holds less energy per kilogram than the cells in a phone, but it is far more stable and good for thousands of charge cycles. Around the cells sit a management system that watches every one of them, an inverter that converts between the battery's direct current and the alternating current your home uses, and a casing that keeps it all at a sensible temperature. Select a part to see what it does.
Select a numbered part of the battery to inspect it.
Three numbers on the datasheet matter more than the brand name:
- Usable capacity (kWh). How much energy you can actually take out. It is smaller than the nominal figure, because a battery is never run completely flat or completely full. A “10 kWh” battery that offers 9 kWh usable is a 9 kWh battery.
- Power (kW). How fast it can charge or discharge. A battery rated at 5 kW cannot supply a 9 kW load on its own, however much energy it holds.
- Round-trip efficiency (%). For every 100 kWh you put in, you get back roughly 90 — the rest turns into heat in the cells and the inverter. It sounds small, but the whole economics hinges on it.
You will also meet two ways of connecting a battery. A DC-coupled system plugs it into the solar panels through one hybrid inverter, so energy is converted only once — a few percentage points more efficient. An AC-coupled battery has its own inverter and connects to your home's wiring, which is how you add storage to a solar system that is already on the roof.
02Simulate your day
Below is a typical household on a typical day. Change the size of the solar array, how much electricity the home uses in a year and — the interesting part — how big a battery you add. Drag the clock to see where every kilowatt goes at each hour. Season and sky stand in for the weather.
A simplified hourly model of a typical household — not a forecast for your roof. Assumptions: a typical daily shape of consumption, 25 W of standby draw while a battery is installed, and the round-trip efficiency you choose in section 04. The “year” figures weigh six representative days: three seasons, each clear and overcast.
Try it: set the battery to zero and look at a summer midday — most of the solar output leaves the house. Add a few kWh and the evening is covered from the roof. Now switch to winter, or to an overcast sky: there is hardly any surplus to store, so even a large battery stays mostly empty.
03Why the first kilowatt-hours matter most
Capacity has sharply diminishing returns. The first few kWh are used every day: they fill at noon, they empty at night and they replace the most expensive electricity of the day. Past a point, extra capacity only fills on the sunniest days and sits idle the rest of the year. The chart shows the full-year picture for the home you set above.
04The one number that decides it
Every kilowatt-hour the battery delivers saves you the price you would have paid your supplier. But to fill it you gave up the money you would have earned by exporting that energy, and some of it was lost on the way. What is left is the value of a delivered kilowatt-hour:
Against that stands the cost: the price of the battery divided by every kilowatt-hour it will deliver in its life — cycles per year times years. If the value is higher than the cost, the battery pays back. If not, you are paying for something else, such as backup power or independence. Set your own prices below; the chart shows how much each extra kWh of battery earns.
Simplified: ignores inflation, price changes, subsidies, VAT, financing and the slow fade of capacity (typically around 2% a year). Use real quotes and your own bills for a decision.
05When it makes sense even if the maths is marginal
The formula only counts euros. Some reasons to buy a battery are not about euros — as long as you are honest that they are the reason:
- Backup power. Most grid-connected solar systems switch off when the grid fails — deliberately, to protect line workers. Only a battery with a backup output (often called EPS) and a compatible inverter keeps chosen circuits alive in a blackout. After the large power cut across Spain and Portugal in April 2025 this became the question installers heard most. Ask which circuits it covers, for how many hours, and whether it can restart without sun.
- Time-of-use and dynamic tariffs. If your tariff has a cheap night rate, or prices that follow the market hour by hour, a battery can also charge from the grid when electricity is cheap. The gap between cheap and expensive hours then replaces the export price in the formula — and it has to beat the losses and the wear.
- A heat pump or an electric car. More evening consumption means more for the battery to shift. A home using 6,000 kWh a year gets far more out of the same battery than one using 2,500.
- Low or negative export prices. If your supplier pays almost nothing for surplus — and in some markets prices at noon now go negative — every kWh kept at home is worth more, and the gap in the formula widens.
Before buying storage, also try the free alternative: running the dishwasher, washing machine, water heater or car charger while the sun is out captures a large part of the same benefit with no hardware at all.
06The short version
- A battery moves electricity through time; it does not make any.
- Three numbers matter: usable kWh, kW of power and round-trip efficiency.
- The first few kWh matter most. Beyond your evening consumption, extra capacity mostly sits idle.
- It pays back only if a delivered kWh is worth more than it costs: buy price − export price ÷ efficiency, against price ÷ lifetime kWh.
- Backup power and independence are real reasons to buy one — just don't count them as savings.
More explainers
How a heat pump works
The physics of moving heat instead of making it — the refrigerant loop, step by step.
02Heat pump vs. gas boiler: the real savings
An interactive calculator for whether a heat pump actually costs less to run, using your own prices.
03How solar panels work
The physics of turning sunlight into electricity, plus a calculator for what a system your size could generate.
Common questions
How long does a home battery last?
Most come with a 10-year warranty that also caps total energy throughput or cycles — commonly 6,000 to 10,000 cycles — and guarantees roughly 70–80% of the original capacity at the end. Lithium iron phosphate cells often outlast their warranty, but plan your payback inside it.
Will a battery keep my lights on in a blackout?
Only if the system is built for it. Most grid-tied inverters shut down when the grid fails. You need a battery with a backup (EPS) output and the right wiring, and it normally powers a few chosen circuits rather than the whole house.
How big a battery do I need?
Roughly the electricity you use between sunset and the next morning — often 4 to 10 kWh for a typical home. Beyond that, extra capacity mostly sits idle, as the chart above shows. Size it to your evening, not to your roof.
Can I add a battery to solar panels I already have?
Usually yes. An AC-coupled battery with its own inverter connects to your home's wiring and works with almost any existing system. Some inverters can also be paired with a compatible DC battery. Ask your installer which route fits and whether it affects your warranty or feed-in arrangement.
Is a home battery safe?
Certified lithium iron phosphate batteries are the most stable of the common lithium chemistries, and a management system monitors every cell. Safety mostly comes down to a professional installation, a suitable location — dry, ventilated, away from freezing and direct sun — and certified equipment.
Should I charge the battery from the grid at night?
Only if the price gap pays for it. Charging at a cheap night rate and discharging at the evening peak wins only if the cheap kWh divided by the efficiency, plus the wear on the battery, costs less than the expensive kWh you avoid. Run the numbers with your own tariff before enabling it.