How big should a battery for plug-in solar be?

Big enough to absorb a typical sunny day’s surplus, not more. For an 800 W kit, that is often one or two kilowatt-hours, but your own surplus and evening use decide. A bigger battery stays partly empty for much of the year.

Does a battery give me backup power?

Only if it has a separate off-grid output and you plug devices directly into it. The grid-tied output switches off in a power cut, like any plug-in inverter.

Can I add a battery to my existing kit?

Often yes. Some batteries sit between the panels and your existing microinverter, others replace the inverter with their own. Check compatibility with your panels and the rules in your country.

Is a battery worth it with a dynamic tariff?

Possibly, if the battery can also charge from the grid when prices are low. Whether that is allowed and supported depends on the product and your country. Our tariffs section explains the basics.

Plug-in solar with a battery: is it worth it?

A battery makes plug-in solar pay off faster only when the solar power it saves from export, valued at your electricity price, repays the battery’s cost well within its lifetime. For many households with a modest surplus that is not the case: the battery adds more cost than it saves. For households that are out all day and export most of their midday output, the numbers can work. The way to find out is a short calculation with your own figures, set out below.

Step 1: find your surplus

Start with the basic yield and self-consumption estimate from our yield and payback calculator. The surplus is what is left over:

surplus = annual yield × (1 − share used at home)

With the calculator’s example values, 680 kWh a year at 40 % self-consumption leaves 408 kWh of surplus. If your share used at home is already high, for example because someone is home all day, the surplus is small, and a battery has little to work with.

Step 2: estimate what the battery can capture

A battery cannot store the whole surplus:

On sunny summer days the surplus can exceed the battery’s capacity; the rest is still exported.
In winter there is often no surplus at all, so the battery sits empty.
Conversion losses occur when power is stored and released; manufacturers state a round-trip efficiency on the data sheet.
Your evening use must be large enough to empty the battery each night, or it starts the next day partly full.

As an illustration, assume the battery captures 60 % of the surplus and loses 10 % on the round trip. These are assumptions, not measured values:

Step	Calculation	Result
Surplus	680 kWh × (1 − 40 %)	408 kWh
Captured	408 kWh × 60 %	245 kWh
After losses	245 kWh × 90 %	220 kWh
Saving per year	220 kWh × 0.30 per kWh	66 per year

Step 3: compare with the price

Divide the battery’s price by the yearly saving. If the battery in this example cost 1,000, the payback would be about 15 years, which is long compared with the warranties many manufacturers give. Check the warranty period and the guaranteed cycles or remaining capacity on the data sheet, and compare them with your result.

Change one assumption at a time: a higher electricity price, a larger surplus or a cheaper battery shortens the payback; a small surplus lengthens it sharply.

How the systems connect

Batteries for plug-in solar come in a few designs:

Design	How it works
DC-coupled before the inverter	the battery sits between panels and microinverter and passes power on when needed
All-in-one with built-in inverter	panels connect to the battery, which feeds the socket through its own inverter
AC-coupled	the battery charges and discharges through a socket, separately from the solar kit

The designs differ in efficiency, compatibility and in how they deal with legal limits. Our data-sheet comparison of battery systems covers capacity, output, expansion and backup sockets.

Matching output to demand

A battery only saves money if it releases power when you use it. Some systems release a fixed amount per hour, others follow your actual consumption using a meter reader. Following consumption avoids exporting stored power by accident. How that works is explained in zero export with a smart meter reader.

Rules to check first

Great Britain. Reporting on the 2026 plug-in rules says that battery systems importing and exporting through a socket are not covered by the first set of rules. Check the current position in the regulations themselves (SI 2026/848) and on our UK and Ireland page before buying.
Germany. The national product standard for plug-in solar devices does not cover storage. Batteries for plug-in systems are sold and used, but registration and the 800 VA feed-in limit still apply to the whole system.
Elsewhere. Check the rules for your country on the rules hub.

Cheaper ways to raise self-consumption

Before spending on a battery, try what costs little or nothing:

Run the dishwasher, washing machine and tumble dryer around midday.
Charge laptops, e-bikes and tools when the sun is up.
Use timers on appliances such as water heaters where that is safe.
Measure your base load and switch off standby consumers you do not need.

If you are on a time-of-use or dynamic tariff, the value of stored power changes over the day; see dynamic electricity tariffs.