How to Calculate Battery Storage Size for Solar Panels

With sunny days somewhat of a rarity in the UK, it’s important to make the most of every ray of sunlight when it comes to generating energy. To achieve this, solar PV panels on their own aren’t enough. You need battery storage too. 

Battery storage allows you to harness your own self-generated electricity at a later date rather than sending it to the national grid for someone else to use, then having to buy it back at a much higher rate. An essential step here in terms of efficiency is making sure you calculate battery storage size properly.

In this article, we’ll guide you through the process of calculating the ideal battery storage size for your solar system to help you make the most of your renewable energy investment.

Importance of solar battery storage

Solar battery storage can be a game-changer for homeowners looking to maximise the use of their PV panels. Although there is a hefty initial investment for a solar storage battery, there’s a good payback pathway to make it highly cost-effective over time. 

And this is the primary benefit of solar battery storage – it makes financial sense. 

By capturing your own unused energy from solar panels during peak day-time hours, you avoid sending it, or even selling it, to the grid. Even if you do sell it, it’s generally at a significantly lower price than what you’d pay for the equivalent. 

The cheapest electricity you can get is your own self-generated electricity. Without a battery, any excess energy generated is essentially wasted for your home, although not wasted overall. 

Benefits to solar battery storage include:

• Maximum efficiency from solar panels
• Reduced energy bills
• More sustainable energy
• Better energy independence and security 
• Future proofing your home
• Increased home saleability 

If you’re new to all of this, take a look at our guide on getting started with the basics of home battery storage. 

Factors to consider when sizing a battery storage system

There are a few core factors at play when it comes to sizing a domestic battery storage system for your solar installation – how much energy you generate, how much your home uses and how much you export to the grid. 

But it’s a balancing act between these factors, plus others such as budget. 

Whilst most people invest in a battery storage system to reduce their reliance on the grid and save money on energy bills, it’s important to have realistic expectations. Unless you have a particularly large battery capacity, you’ll likely still need to purchase some electricity from the grid across the year. Powering your home from battery storage alone is generally not feasible from a financial perspective because of battery size required.

That said, it’s always better to size up rather than down when it comes to your battery storage. A slightly larger battery capacity will offer more flexibility and energy security in the long run. However, there comes a point where it wouldn’t make sense to get a large battery bank if you have a small solar array as the battery won’t be used effectively. 

Solar panel array size

The starting point for calculating battery storage sizing is the energy generating capacity of your existing or planned solar panel array. 

Knowing just how much energy your solar panels can generate (in kilowatt hours) will help you estimate the potential energy that can be stored in your battery. 

It’s also important to address how you intend to use the excess energy from your solar panels. Do you want it primarily as a power back-up or to increase your self-consumption?

Daily energy use (especially evening use)

The most important factor is to know your home’s average daily energy consumption in kWh. This will give you a baseline understanding of how much electricity you typically use and, consequently, how much you need to store. In the UK, the average household electricity use is between 6-10kWh a day.

It’s particularly important to understand your energy use in the evening when the sun’s gone down. This is because the evenings (and the morning after) are peak energy use times and when you’ll be drawing energy from your storage battery. 

If you already have solar panels, how much energy do you have to import when the panels aren’t generating electricity? 

This information will give you a real idea of the type of storage capacity you’d need to make the most of your solar panels and harness your own renewable energy. 

How much energy do you export?

If you have solar panels installed already, how much is being exported to the grid on a regular basis? Look back at your readings and workout your average export in kWh. 

This will give you an idea of available energy that you generally have to send to a battery, rather than sending to the grid. If you’re regularly exporting large amounts of electricity – this can sometimes be 60% and more of what you’re generating – then it makes total sense to invest in battery storage. Your renewable systems have generated the electricity, so why not keep it?

In short, it’s really the data readings of your energy import and export that make a difference. For example, a house with a good solar panel system might regularly export 8-10 kWh during the day and import 8-10 kWh during the night. This data suggests that a 10 kWh battery would be a good choice to cover a lot of energy use without buying it from the grid.

Calculating the ideal battery capacity

Battery capacity determines how much energy your battery can store and as a result, how much of your solar-generated electricity you can use when the sun isn’t around. 

Based on the factors above, you’ll need three figures to calculate an approximate size for your battery storage:

• The average amount of daily energy your solar panels generate
• The average amount of daily energy your home uses
• The average amount of daily energy you export to the grid

With this in mind, a simple calculation would be:

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

This calculation would get you the average amount of excess electricity your solar panels generate in a day. This is likely to be the same as your average export figure. 

Example of battery capacity for solar panels

Let’s take a look at an example with some hypothetical figures. 

Say you have a 5kW solar array in the UK, where the average year-round peak sun time is a modest three hours. On an average day, your solar panels will produce approximately 15kWh of electricity. 

After looking at the figures, it looks like you use 7kWh of energy on a daily basis, leaving you with 8kWh of exported energy to the grid.  

To make the most of this excess energy, you’ll want to consider a battery with a capacity of at least 8kWh. However, it’s generally recommended to oversize your battery slightly to capture more energy during high sun days and give you more leeway to use your own energy.

In this example, a 10kWh battery would be a good choice, as it can supply roughly a full day’s worth of energy for the average household to power all core appliances, such as your lights, fridge, oven, TV and washing machine. 

To power multiple appliances at the same time, you’ll need to make sure the inverter is suitably sized to supply the right amount of power at any one time. 

A note on battery capacity

When looking at your battery options, it’s important to be aware of two key capacity measurements:

• Total capacity – Refers to the maximum amount of electricity a battery can hold.
• Usable capacity –  Is the maximum amount of electricity the battery can discharge based on the recommended depth of discharge (DoD). It’s commonplace for DoD to be around 90%, but top battery models nowadays, such as the Tesla Powerwall, Givenergy All-in-One and Huawei Luna, all have a 100% depth of discharge. This means their total and usable capacities are the same.

Tips to extend solar battery storage life expectancy

Solar batteries typically last between 10 and 15 years. This is roughly half the lifespan of a solar panel system. You can read more on how long solar batteries last here.

There are a few things you can do to maximise your battery lifespan and avoid a premature replacement. 

First and foremost, make sure your solar battery is installed in a suitable location. Ideally, the battery should be placed in a cool, dry area with proper ventilation. To perform at its best, you’ll want to keep your battery away from extreme temperatures and moisture.

For solar storage batteries, it’s important to go through regular charging and discharging cycles as both overuse and underuse can shorten your battery’s lifespan. As a rule of thumb, aim for one charge and discharge cycle per day to keep things running nicely whilst avoiding excessive wear and tear. 

From time to time, make sure you check in on the physical condition of the battery and monitor the data provided by your app as this can help you identify any potential issues or performance problems early on. 

And that’s our guide to calculating battery storage size for a solar panel system. By identifying the figures surrounding your solar energy generation, home usage and the amount you regularly export to the grid, you’ll be able to accurately determine the ideal sized battery for your home.