What Size Solar Battery Do You Need?

✔ A three-bedroom home will need an 8 kilowatt storage battery

✔ The average cost of a storage battery is £4,500

✔ Storage battery capacity is between 1 and 13 kW

If you want to make the most of your solar panels, and break even on the cost of your solar panels quicker, you’ll want a solar battery.

It can be tricky knowing what size battery you need though, because you don’t want to have one too small and find yourself relying on the grid more than you’d planned. However, you also don’t want to spend too much on an oversized battery.

We’ve created this guide to help you work out what size solar battery you’ll need, looking at the differences between large and small solar batteries, if you can have multiple batteries, and what to consider before you buy.

You could also start searching for solar panels-plus-storage by filling in our easy-to-navigate form. Just enter a few simple details and we’ll put you in touch with our trusted suppliers, who’ll get back to you with bespoke quotes for you to compare.

How are solar battery sizes measured?

Battery sizes are measured by their capacity to store electricity, but it’s important to consider usable capacity rather than just what the total capacity is.

That’s because you don’t want to actually use a battery’s entire capacity, as this can damage it. The usable capacity is called depth of discharge (DoD), and most modern batteries have a DoD of between 90 and 95%.

Most storage battery capacities range from 1–13 kilowatt hours (kWh) and you’ll typically spend more money for larger capacity.

You also need to consider power output, because size isn’t everything. If you have a battery with a large capacity but low power output, it’ll last for a long time but you won’t be able to power all the appliances in your home.

What size solar battery do you need for your home?

Each home’s individual energy needs are different, but the table below will give you a rough idea of what size battery you might need, depending on the size of your property:

The average cost of a storage battery for a three-bedroom home is £4,500 — if you pair your battery with solar panels, you’ll need to purchase two within the lifespan of your solar panel system.

4 kW solar system with a battery

If your home has a 4 kilowatt peak (kWp) solar panel system, you would need a battery with a capacity of roughly 8–9 kW for your solar panels to efficiently charge it. This is because a 4 kW solar panel system will generate roughly 8 kW per day — enough to fill the battery’s capacity.

It’s important to note that this is assuming you don’t use the electricity during the day, as this will limit how much you can charge your battery by.

5 kW solar system with a battery

A 5 kWp solar panel system will pair perfectly with a solar battery with a capacity of 9.5–10 kW. Having a smaller solar panel system is unlikely to be able to charge this size battery properly, which could lead to your battery undercharging and suffering damage as a result.

It’s usually better if the electricity your 5 kWp solar panel system generates during the day goes straight to the battery. Otherwise, it might not be able to fully charge your solar battery.

6 kW solar system with a battery

A 12 kW solar battery is best complemented by a 6 kWp solar panel system. This’ll ensure your battery is charged optimally during the day, and not undercharged by an underpowered solar panel system because this can lead to battery damage.

8 kW solar system with a battery

Own an 8 kWp solar panel system and wondering what size battery you’ll need? Go for a solar battery with a capacity of 16 kW if you want your solar panel system to efficiently charge it during the day.

You’ll find if you have a smaller solar panel system that you won’t be able to fully charge your solar battery, and this can lead to a cycle of chronically undercharging it.

10 kW solar system with a battery

The ideal size solar battery for a 10 kWp solar panel system is 20–21 kW, as it’ll be able to make sure the battery is properly charged throughout the day. If you want to avoid undercharging your solar battery and causing damage to it, don’t get a smaller solar panel system as it won’t be able to fully charge it.

Large solar batteries vs small solar batteries

Larger storage batteries are generally a better option than smaller batteries, but you’ll want to have a solar panel system powerful enough to charge it.

Otherwise you’ll want a smaller storage battery, because there’s little point paying more for a large battery you’ll barely be able to charge.

Smaller batteries work better for homes with lower energy needs, and typically are well suited to homes with limited space for larger batteries.

You can still have home battery storage without solar panels, but you’ll be paying your energy supplier’s electricity rates to charge your battery.

Bear in mind that a solar battery might not be worth it if you get the wrong size, and could cost you more in the long run.

Should you buy an oversized battery for your property?

Choosing to buy an oversized battery for your property depends a lot on what you expect your energy usage to increase by.

You should get a larger capacity battery if you’re planning on getting technology such as a heat pump or an electric car with a home charger.

You can also get an oversized battery just in case, but if you’re getting a storage battery to save on energy bills, this won’t make sense financially.

There are some interesting options if you do want a battery you can scale up as and when needed. Acceleron, a company offering scalable battery modules, lets you increase your battery’s capacity simply by slotting in an extra battery unit.

Can you use a solar battery to charge an electric car?

You can charge an electric car with a storage battery, but it’s typically not worth it because you’ll almost certainly need to tap into the grid to finish charging.

You’ll need either a battery with a very large capacity, or multiple batteries, as the typical capacity of an electric car is around 40 kWh.

It’s much better instead to use your solar panel system or the grid to charge your electric car directly, and save your storage battery for other uses.

Can you have multiple storage batteries?

You can stack multiple batteries to increase your capacity, but there are caveats. Firstly, if you’re hoping to charge your battery using solar power alone, you will struggle to fill multiple batteries without a large solar panel system.

There’s also the risk of one or several batteries failing in a multi-battery system, which can reduce the overall effectiveness and how much power you can access.

You’re typically better off sticking to one or two larger batteries, as these last longer and are more reliable in general.

Things to consider before buying a solar battery

Work out what your power usage is

There’s little point buying a battery with a capacity much larger than your power usage (both current and future), so taking a moment to figure out what you use each month is a good idea.

Having a smart meter makes this much easier, because it’ll tell you exactly how much you’re using.

If you don’t have a smart meter, you’ll need to work out roughly what you use each day and multiply it by the days in the month. Look at your monthly bill and it’ll tell you how much electricity you’re using each month.

You can then work out what size battery you’ll need based on your rough electricity consumption. It’s a good idea to go a little over what you consume, but not so much that you spend more than you need.

How powerful is your solar panel system?

You’ll need a solar panel system capable of providing enough power to charge your storage battery during the day.

If not, you’ll spend multiple days charging your battery and eliminating the benefit of accessing clean, solar powered electricity at night.

Don’t forget battery cycles

A storage battery’s cycles means how many times it can be charged and discharged — a greater number of cycles is better because you can use your battery more before it starts to degrade.

Your battery’s warranty is typically connected to how many cycles it has, though many newer batteries offer time-specific warranties with unlimited cycles, such as the Tesla Powerwall 2.

Check out our list of the best storage batteries for expert advice on home battery storage.

Are you planning on going off-grid?

If your aim is to stop or drastically reduce your grid reliance, consider spending extra to get a battery with enough capacity and power output to meet your needs.

Summary

Working out what size battery you need is important to getting the most out of your storage battery, and being able to power all the things in your home you’ll need.

Think about your future energy usage too — get a larger battery if you expect to add something like a heat pump to your home.

It’s also important to recognise that batteries are expensive. You’ll spend an average of £4,500 for a storage battery and if you have a solar panel system, you’ll end up buying two batteries within its lifetime.

If you’re ready to find out what solar-plus-storage could cost you, why not fill in our simple form? We’ll connect you to our expert installers, who’ll get in touch with bespoke quotes for you to compare.

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