What size solar battery do I need in the UK?

What is the difference between physical size and solar battery size?

The physical size of the battery is the height, width, and depth of the battery unit, which determines how much space it occupies. This determines where the battery can be installed in your home, with most batteries fitting into basements, garages and cupboards. Today, the average size of a battery is smaller than a washing machine.

What matters more is the capacity of the battery, which, confusingly is also often referred to as the battery size. Which refers to the amount of energy the battery can store and deliver, measured in kilowatt-hours (kWh). The higher the capacity, the longer your house can be powered.

Why does the size of the solar battery matter?

An undersized solar battery will not provide enough energy storage to meet your needs, especially during peak usage times. This limitation can lead to increased reliance on the grid, reducing the potential savings and benefits of having a solar battery system. Additionally, cycling an undersized battery too frequently can shorten its lifespan, as it will undergo more charge-discharge cycles, leading to more frequent replacements and higher long-term costs.

An oversized solar battery will increase the payback period on your investment. The battery will never be fully utilised if the household's energy consumption does not match the battery's storage capacity.

The questions to ask yourself before sizing a battery

Before deciding upon a battery there are several questions you need to ask yourself no matter which of the 3 cases you are sizing your battery for.

How much electricity do you use per year?

This is a simple one to answer by just looking at your electricity bills, however, it is crucial in ensuring that you don’t over- or undersize the battery. For a quick estimate, the below table provides the average electricity usage and average heat pump usage dependent the size of the house.

Are you going to add any high electricity use items?

Oversize your battery for future expansion. If you’re planning on adding a heat pump as part of your continual home improvements, adding an extension or you have an electric car lined up your electricity consumption will go up. With a bigger battery you can utilise more cheap energy to power these high use items.

Are you on a smart tariff like Octopus Flux and do you have a smart meter?

Smart meters and tariffs will allow greater utilisation of your battery. Allowing you to benefit from time-of-use electricity, where prices vary throughout the day and night. You can buy electricity when it is cheaper and sell it when it is more expensive. This increases the benefits your battery can provide you if you are not at home or the sun isn’t shining. Once a battery is installed it is a no-brainer to get on a smart tariff.

Now that you have answered the qualifying questions, you will want to find the right battery size for your specific target. In the following section, we will discuss how you can find the best battery size for the three most common goals:

1. Maximising self-consumption from your solar panels
2. Reducing electricity costs with a standalone battery storage
3. Protecting your household against power outages

1. Sizing your solar battery for maximum self-consumption

Adding batteries to solar panels is always a good option, allowing you to store the excess energy produced by your panels that is not consumed during daylight hours. The stored energy is then available for you to use at night, allowing you to maximise the time you are off grid, increasing savings, self-consumption and ensuring your electricity is green.

When sizing your solar battery, it is crucial to ask yourself about the size of your Solar PV system and when you use your electricity.

What proportion of electricity do you consume during the day / night?

The more electricity you consume during the day, the smaller the battery you need as you will be using the electricity from your solar panels. In the case for a 5 kWp Solar PV system, if you use 50% of your electricity during the day, a solar battery system of 5 kWh would suffice to cover the average usage of a British household.

If you don’t consume much electricity during the day, then it is better to oversize your battery so that you can store and then consume at night. This would usually equate to two times the kWp rating of your solar PVs. So, for a 5 kWp solar array, you would add a 10 kWh solar battery. This is the average daily usage of a British household.

So let the installer know when you consume most of your energy, so that they can size the battery accordingly.

How much electricity do your solar panels produce?

Now that you and the installer know your electricity usage, you can work out how much electricity your panels will produce and how much needs to be stored.

The main component to consider then is the size of your system, a 5 kWp system that produces on average 12.5 kWh per day does not need a 15 kWh battery as you will also be consuming that electricity during the day.

By working out your average daytime solar production which is = Average Peak Solar Hours in the UK (avg. 2.5 hours) * Size of Solar Array Size (kWp). You can then work out how much electricity you use in the daytime hours, then subtract daytime hours from produced hours to give you the size of the solar battery.

Although solar production does stay consistent from one year to the next, please bear in mind that the above is all based on averages and all households are different.

2. Sizing a battery for a stand-alone battery project

For stand-alone battery projects, size the battery at the same capacity or more than the average electricity consumption of your household. The aim of a stand-alone battery is to cover as much of your electricity consumption as possible without overspending on your battery. The underlying principle here is that you charge the battery on a cheap off-peak tariff and use the same electricity when it would be more expensive at peak times. Making the project economically viable is the most important factor when sizing the battery.

In some cases, it can make sense to oversize your system compared to your household’s consumption when participating in multiple revenue streams with your stand-alone home battery.

3. Sizing a battery for power outages

When sizing a battery for power outages, purchase a battery for the expected amount of time of the outage. If your critical items use 10 kWh a day of electricity and you expect the average outage to last for two days, then get a 20 kWh battery. While the duration of an outage varies, you should base this on your own experience with outages in your area.

A trusted MCS-certified installer can also provide you with the relevant information to accurately size a battery for power outages based on information from your area.

So now you have been provided with the information to size a battery for your house, it is time to hand over to the experts. If you would like to learn more about a battery installation in your home, get started by answering the short survey below: