How a solar battery works: following the energy through a day
Behind the marketing, a home battery follows a short list of rules about where each unit of energy goes. Once you know them, most of the questions about size, winter and backup answer themselves.
In short
Solar power goes to whatever the house is using first. Only the surplus charges the battery, and only until it is full; anything beyond that is exported. When the panels make less than the house needs, the battery supplies the difference until it reaches the reserve it keeps back, then the grid takes over.
The order every kWh follows
A home with panels and a battery has three places to send solar electricity, and the system works through them in a fixed order. Picture the house as the first customer, the battery as the second and the grid as the last.
| Step | Without a battery | With a battery |
|---|---|---|
| 1. The house is using power | Solar supplies it | Solar supplies it |
| 2. Solar exceeds what the house uses | The extra is exported | The extra charges the battery |
| 3. The battery is full | (no battery) | The extra is exported |
| 4. Solar falls below what the house uses | The grid fills the gap | The battery fills the gap until its reserve |
| 5. The battery reaches its reserve | (no battery) | The grid fills the gap |
Two things follow straight away. A battery never takes solar away from the house, because the house always comes first. And a battery only changes what happens to surplus: on a day when the house uses everything the panels make, a battery has nothing to store and does nothing.
What decides when it charges and empties
The inverter and the battery's own management system watch the flow of power at your meter continuously. If power would be flowing out to the grid, they send it into the battery instead. If power would be flowing in from the grid, they draw from the battery to cover it. The aim is to keep the meter close to zero for as long as the battery has room, or charge, to do so.
Most systems let you change that behaviour. The common settings are:
- Self-use, the default described above: store surplus, cover the shortfall.
- A reserve: a share of the battery kept back and not used day to day, either to protect the cells or so there is something left for backup in a power cut.
- Timed charging: on a time-of-use tariff, the battery fills from the grid during the cheaper hours, then runs the house later. See a battery without panels for how that works in kWh.
The settings change when energy moves, not how much a battery can hold. A larger reserve means less usable energy each day; a smaller one means more, with less left for a cut.
Why a battery gives back less than it takes in
Every time energy goes into a battery and comes back out, a small part is lost as heat in the cells and the electronics. The share that comes back is the round-trip efficiency, which manufacturers state on the specification sheet. It matters more for a battery charged from the grid every night than for one that only stores spare solar, because grid charging is bought energy. Our round-trip efficiency guide explains where the losses happen and how to read the figure.
Usable and nominal: two sizes for one battery
The size printed on a battery is its nominal capacity. Most batteries hold back part of it to protect the cells, so the energy you can actually draw out, the usable capacity, is usually a little lower. MCS records nominal capacity for every registered battery, and DESNZ's statistics use it. When you size a battery for your home, use the usable figure from the specification sheet; what size battery works through it.
Power as well as energy
A battery has two limits. Its capacity in kWh says how much energy it can hold. Its power rating in kW says how fast it can deliver it. Switch on an oven, a kettle and a shower pump together and the house can briefly need more power than the battery can supply, even when it is full; the grid makes up the difference. The kW and kWh guide explains why both numbers matter.
What happens on different kinds of day
A long summer day
The battery may fill before lunch. From then on, surplus is exported just as it would be without storage, and in the evening the battery covers the house until it reaches its reserve. Summer is when a battery is busiest, and also when the most solar is exported regardless.
A grey winter day
The house uses nearly everything the panels make as they make it. There may be no surplus at all, and the battery sits at its reserve unless it is charged from the grid on a cheaper tariff. PVGIS figures for Leicester show about three and a half times as much solar on a June day as on a December day, which is why winter and summer deserve separate thought.
A day away from home
With the house using little, the battery fills quickly and the rest is exported. A battery does not make an empty house use more of its own solar; it only moves energy to a time when the house is using it.
A power cut
Unless the system has a backup function, the battery disconnects with the grid, as the law on sources running alongside the network requires. Backup in a power cut explains what backup adds.
Seeing it happen
Most systems come with a monitoring app that shows the four flows in real time: solar in, house use, battery charge and grid import or export. A few weeks of watching it is the best way to understand how storage behaves in your home, and to spot settings that don't suit you, such as a reserve set higher than you need. Ask at handover how to read the app and how to change the settings; living with a home battery lists what the handover should include.
If you are weighing up storage for panels you already have, go next to storage for existing panels; if panels and battery would go in together, to designing both at once.
What a battery doesn't change
It helps to be clear about what storage leaves alone. It doesn't make the panels produce more: generation depends on the roof, the weather and the season. It doesn't reduce what the house uses: the kettle takes the same energy whether it comes from the panels, the battery or the grid. And it doesn't change the network's role: on most winter days, and most nights, the grid still supplies much of the home. What a battery changes is the timing, moving part of the day's surplus to the evening, and that is the lens to use when reading any claim about what it will do.
What this page is based on
- DESNZ statistics on home batteries registered with MCS in existing homes (workbook to March 2026) (read 3 Nov 2026)
- PVGIS, the EU Joint Research Centre solar estimator (annual and monthly yields) (read 1 Nov 2026)
- Electricity Safety, Quality and Continuity Regulations 2002, regulation 22 (sources running in parallel with the network) (read 3 Nov 2026)
- Which? guide to solar panel battery storage (secondary, updated 30 Oct 2026) (read 3 Nov 2026)
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