What Size Battery Storage System Do I Need?

Choosing the right size battery storage system maximises the benefits of your investment. Buying a battery that ends up being too big or too small can create inefficiencies, which reduces your cost-effectiveness. We want the goldilocks situation with our battery capacity – just right. 

Most households pair battery storage with solar panels, which is a fantastic idea. Essentially, a battery with solar PV allows you to use captured sunlight energy when the sun isn’t shining. It’s most effective to use battery storage with solar, but they can be installed in homes without solar panels too. 

Selecting the appropriate battery size depends on a number of factors, such as energy use, solar generation (if applicable) and export patterns. This article will guide you through the process of determining the right battery storage system size for your specific needs and home circumstances.

How is battery storage size measured?

Battery storage size is measured in kilowatt hours (kWh). The capacity of modern lithium-ion units, the most common type of storage battery, ranges from around 1kWh to 16kWh. 

More and more batteries are becoming modular, meaning you can stack them together over time to increase your overall capacity and spread the costs. As a general rule, prices increase by approximately £900 per kWh of added capacity.

When choosing a battery, it’s also important to distinguish between total capacity (the headline number) and usable capacity (what can actually be used on a regular basis). This relates to the depth of discharge (DoD). 

The DoD indicates the percentage of the battery’s total capacity that can be safely used without damaging it. Most modern batteries now offer a DoD of 90% or higher, which is a major benefit of solar batteries. You’ll also want to consider the power output, which refers to the amount of electricity that can be discharged from the battery at once. To get up to speed, check out our guide on the basics of battery storage. 

What do you want from your battery?

It may sound a little strange, but think about what you want from your battery. You’ll read this elsewhere as your ‘energy goals’.

Do you want to reduce your reliance on energy from the grid? Do you want to save money on your monthly bills? Or are you looking for a reliable backup source of power? It may be a combination of these reasons. 

The average UK home uses between 8kWh and 10kWh of electricity per day. Your consumption may differ slightly but it gives you a rough idea of the corresponding battery size you’d need to cover your daily use. This is especially important at night. Doing this will help you to reduce your reliance on the grid (although not completely) and potentially save money over the long term. 

According to Ofgem, the battery size needed varies based on the number of people in a house. Here are some of the average usage figures for house size and the battery you’d need to cover daily use. With batteries, it’s always better to size up slightly, rather than down:

Calculating what size battery you need

To calculate a good sized battery for your home, you’ll need to get to grips with a few key figures to do with energy use, energy generation and energy export. Let’s take a look. You can find more information on how to calculate battery storage for a solar panel system here.

Understanding your energy usage

The most important aspect when it comes to sizing a storage battery is understanding your energy consumption. This directly influences the battery capacity you’ll need.

There are a few ways to calculate your daily energy usage, some are easier than others. Firstly, you can hand calculate your ‘loads list’ depending on what appliance is running at what time of day. This will be time-consuming. 

An easier way is to refer to your smart metre, which provides real-time information on your energy consumption. You can look back on days and your month’s running total too. 

Alternatively, you can review your energy bills, which often break down your usage over a given period. Some energy providers, such as Octopus, have a user-friendly account area, which allows you to easily track your energy usage. 

These last two simpler methods will give you an overall use, which is what you need, but it’s also crucial to have a rough understanding of energy use by appliance as in the first instance.

Cleaning appliances, such as washing machines, tumble dryers and dishwashers, are generally the most energy-intensive devices. Fridge-freezers are the next biggest consumers, followed by consumer electronics. 

However, if you have an electric vehicle (EV), this will top the list when it comes to energy use to charge. According to EDF, the average EV uses 2,000 kWh of energy per year, or 5.5 kWh per day. 

Breaking this down by car size. A Fiat 500e for example requires 42.9kWh to fully charge the battery, which will give you around 118 miles of driving range. A bigger Kia Niro needs 74kWh to fully charge, providing 240 miles of range. 

Assessing your solar energy generation

For homeowners with solar panels, assessing your solar energy generation is crucial in determining the optimal battery storage size. Understanding how much energy your solar panels produce daily will help you make informed decisions about the battery capacity you need to maximise the benefits of your solar array.

If you already have solar PV installed, this is simple – just take a look at the data from your control box or app. This will tell you exactly how much energy has been generated. 

If you haven’t got solar panels installed yet, you can estimate your energy generation. This depends on the total size of your solar panel system and the amount of daily peak sunlight hours you get. If you haven’t got solar panels installed yet, you can estimate your energy generation. This depends on the total size of your solar panel system and the amount of daily peak sunlight hours you get. 

For example, imagine you have a 4kW solar panel system installed and get an average peak sun time of three hours. From this, you’ll generate around 12kWh of electricity. However, during high sun, long summer days, this generation could be much higher.

Evaluating your energy export

Energy export refers to the excess electricity your solar panels generate that’s not immediately used by your household appliances. Without a battery storage system, your surplus solar energy is automatically sent to the national grid. This is a shame, as self-generated solar energy is the most cost-effective electricity you can get. 

If this is happening to you, evaluating your energy export is essential to determine the optimal battery size for your home.

To see how much you’re exporting, check your solar panel system’s monitoring data. If you’re not home during the day and only basic appliances, such as your fridge-freezer, are running, your solar panels will likely generate more electricity than your home consumes. In this case, most of the generated energy will be exported to the grid.

The amount of energy you export on a regular basis is a key indication of the capacity of your potential battery storage system. For example, if you consistently export 8kWh of energy during the day, a battery with an 8kWh capacity would be able to store and supply that energy for later use. 

Battery storage sizing formula

By knowing how much energy you regularly export, it’s much easier to workout a good battery size for your home. 

To calculate the approximate size for your battery storage, you’ll need three key figures as discussed above: the average daily energy generated by your solar panels, the average daily energy consumed by your home and the average daily energy exported to the grid.

You can then perform a simple calculation:

Battery capacity (kWh) = Daily energy generated – Daily energy used

This will give you a good idea, but it’s always recommended to get the input of a professional before you go ahead with battery storage installation. 

Battery storage size examples

Here are a few scenarios to help you out when sizing your own battery storage.

Case Study 1: Small home without solar panels 

A couple living in a small home without solar panels consumes an average of 5kWh of electricity per day. To reduce their energy costs, they install a 5kWh battery storage system. The battery is charged during off-peak hours when electricity prices are lower and the stored energy is used during peak hours. This helps the couple with significant cost savings on their energy bills.

Case Study 2: Medium-sized home with solar panels 

A family of four living in a medium-sized home with a 4kW solar panel system generates an average of 12kWh of electricity per day. Their daily energy use is around 10kWh and they export an average of 6kWh to the grid. 

To maximise their own solar energy usage, they install an 8kWh battery storage system, allowing them to store excess solar energy during the day and use it during peak evening hours. This minimises their reliance on the grid and reduces costs.

Case Study 3: Large home with high energy demands and solar panels 

A large household with five people and a 6kW solar panel system has a daily energy consumption of 15kWh. Their solar panels generate an average of 20kWh per day, with an average export of 12kWh to the grid. To meet their high energy demands and export rates, they opt for a 15kWh battery storage system. This allows them to store a significant portion of their own excess solar energy, which is the cheapest they’ll get.

Calculating battery storage size for homes without solar panels

For homeowners without solar panels or other renewable energy generation, battery storage can still be a viable option to reduce energy costs and provide back-up power. 

Without solar panels, the energy used to charge the battery will come from the grid. This should be drawn during off-peak hours when electricity prices are lower. Always check your tariff to make sure this is possible. This energy can be greener (depending on your energy supplier) but it’s not as green as your own solar renewable energy. 

To calculate the appropriate battery storage size for a home without solar panels, you need to consider your energy goals and daily energy consumption. If your aim is to cover your entire daily energy usage with stored, off-peak electricity and power your house with batteries, you’ll need a battery capacity that matches or slightly exceeds your average daily consumption.

For example, if your home consumes an average of 10kWh per day, a battery with a capacity of 10-12kWh would be suitable to meet your daily energy needs. However, even in this scenario, it’s still likely that you’d need to draw some energy from the grid to cover all of your use at all times of the day. 

If your goal is to partially offset your energy costs, you can opt for a smaller battery capacity that covers a portion of your daily energy usage. This will still provide some cost savings without requiring a larger investment in battery storage.

You can read more on whether you can have battery storage without solar panels. 

Step-by-step battery storage system sizing guidelines

Now that you have an understanding of what you need to know when it comes to battery storage sizing, here’s a step-by-step rundown of the process

1. Make sure you have space for a battery system and the accompanying hardwear – take a look at this post on how big are battery systems.
2. Goals from a battery system – determine what you want to get out of installing a battery
3. Calculate your daily energy usage
4. Assess your daily solar energy generation (if applicable)
5. Evaluate your energy export (if applicable)
6. Work out roughly what size battery you’ll need – important here to still consult with expert battery installers
7. Weigh up the battery options and costs – depending on the size you go for, battery installations can range from £2,500 to £10,000