Adding a battery to your solar panel system means you can store electricity generated during the day and use it later, rather than immediately exporting it to the grid. But deciding how much storage you need isn’t as simple as matching a battery to the size of your solar array.

Your household electricity consumption, when you use that electricity and whether you have a smart time-of-use tariff can all affect the overall calculation. What’s more, you may also need to think about future changes, such as installing a heat pump or buying an EV.

Phil Steele, future technologies evangelist at Octopus Energy, says the way homeowners should think about battery sizing is changing.

“With time-of-use tariffs, it’s becoming more [about] sizing it to your home use rather than the size of the solar system,” he says.

What does solar battery size mean?

Home battery capacity is normally measured in kilowatt-hours (kWh). Put simply, the higher the figure, the more electricity the battery can store.

That’s different from its power output, measured in kilowatts (kW), which determines how much electricity the battery can deliver at once. A battery may have enough stored energy to run several appliances over a period of time, for example, without necessarily being able to supply all of them simultaneously.

There isn’t one battery size that’s right for every home. The Energy Saving Trust says a typical home battery system might have around 10kWh of capacity, but stresses that there is no exact formula and an installer should assess your individual needs.

How to work out what size solar battery you need

Your electricity bills or smart meter data are a good place to start because they can show how much power your household actually consumes.

Ofgem’s latest Typical Domestic Consumption Values, introduced in July 2026, put annual electricity use for a standard single-rate household at 1,600kWh for low consumption, 2,500kWh for medium consumption and 3,800kWh for high consumption. That’s equivalent to roughly 4.4kWh, 6.8kWh and 10.4kWh a day respectively.

As a rough starting point, that gives you the following comparison:

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| Low | 1,600kWh | 4.4kWh | Around 5kWh |
| Medium | 2,500kWh | 6.8kWh | Around 5-10kWh |
| High | 3,800kWh | 10.4kWh | Around 10kWh or more |

*These battery sizes are indicative starting points rather than Ofgem recommendations. The right capacity depends on your individual consumption, solar generation and tariff.

Octopus’s current range broadly reflects those capacities. Its 5kWh systems are aimed at small to medium-sized homes, while it offers 10kWh batteries for larger homes and higher electricity demand. It also offers the 13.5kWh Tesla Powerwall 3 for households with greater storage requirements.

However, your daily consumption shouldn’t simply be converted directly into battery capacity. The more important question is how much electricity you realistically want the battery to store and supply.

Why the size of your solar system isn’t the only consideration

Traditionally, battery sizing focused heavily on solar generation. The goal was to store the electricity your panels produced during daylight hours so you could use it later in the evening. But Steele says smart tariffs have changed that calculation.

A few years ago, he explains, households were more likely to size batteries according to the amount of solar they could generate because there were fewer opportunities to charge cheaply from the grid.

Today, a time-of-use tariff can allow the battery to charge when grid electricity is inexpensive as well.

“There’s much more opportunity to charge the battery from cheap off-peak time-of-use tariffs,” Steele says. “So therefore, the battery size depends more on what you’re capable of consuming and what your solar system is doing. That’s a message that has changed in the last year or two.”

Energy Saving Trust gives similar guidance, stating that home batteries can work with solar panels, a smart time-of-use tariff or both. That means a battery can store surplus solar electricity during the day but also charge from the grid during cheaper periods and discharge when electricity costs more.

This all means that when you consume your energy is now particularly important.

A home that’s empty during the day but uses lots of power in the evening could benefit from storing more solar generation for later. Someone who works from home, meanwhile, may consume a larger proportion of their solar electricity directly as it’s generated and potentially need less battery capacity.

Should you get a bigger battery if you’re installing a heat pump or buying an electric car?

It’s also worth considering how your electricity use may change in the future. A heat pump will increase the amount of electricity your home consumes, so Steele says that’s something he would factor into battery sizing.

A battery can also potentially work particularly well alongside a heat pump and a smart tariff. It could, for example, charge during a cheaper electricity period before supplying some of the heat pump’s demand when prices are higher.

“I would size it around a heat pump and not a car,” he says. “There’s no need to oversize your home battery if you’re thinking of using it to charge your car.”

The reason, he explains, is the difference in scale. A home battery might store 10kWh or 20kWh, while an EV battery can be 70kWh, 80kWh or even 100kWh. Using 10kWh of stored household electricity to charge an EV would therefore only provide a relatively modest top-up.

“So generally the advice is use the home battery for your own home rather than as a way of storing energy to then charge the car,” Steele says.

Bi-directional charging could eventually alter that relationship by allowing compatible EVs to supply electricity back to a house or the grid, although this technology is not yet widely available.

Is a bigger battery always better?

Not necessarily. A larger battery gives you more storage, but you’re also paying for that extra capacity. If you rarely fill or empty it, some of your investment may not be working particularly hard.

Instead of starting with the biggest battery available, look at how much electricity you consume, when you consume it, how much surplus solar power you’re likely to generate and whether your tariff lets you charge cheaply from the grid.

Then think ahead. If a heat pump is part of your future plans, allowing for that increased electricity consumption could make sense.

The biggest change, though, is that choosing a battery is no longer necessarily about asking, “what size battery matches my solar panels?”

As Steele explains, household energy use is increasingly the more important part of the equation.