Tom Melhuish 5 min read

What size solar storage battery do I need for my business?

Solar panel batteries are an expensive but valuable addition to a commercial solar panel installation. Choosing the suitable battery storage capacity will help you avoid unnecessarily adding thousands to your commercial solar panel installation costs and improve return on investment.

In this blog, we’ll look into how commercial solar batteries are used and how to calculate the appropriate battery storage size in each case.

Jump straight to the reason you’re considering a solar battery:

Solar battery size: Minimising expensive grid electricity
Solar battery size: Back-up power supply
Solar battery size: Off-grid electricity supply

What are the different uses for commercial solar batteries?

PV solar batteries are designed to store electricity generated from your commercial solar array, giving you control over the usage of the electricity you’ve generated.

Without a PV battery system, the electricity produced from your commercial solar panels must be consumed, exported to the grid or simply assumed lost if unused. On the other hand, batteries allows your business to:

Minimise the use of expensive grid electricity

Business electricity prices are significantly higher than the revenue recieved from a smart export guarantee scheme. A solar battery system can store excess electricity generated during daylight and use this as a cheaper alternative to grid electricity at night.

Prevent electricity outages

For some types of business, an electricity outage is disastrous. A sufficiently large solar battery can provide a backup power supply and be a viable alternative to a permanent backup diesel generator.

Off-grid electricity system

Off-grid businesses are completely disconnected from the local electricity distribution network. In this case, a sufficiently large solar battery can provide power when generation from solar and other micro-renewables is lower than demand.

Size, State of Charge & Power Output

The size of solar battery storage required in each of these cases differs. The storage size of the battery, known as its capacity, is measured in kilowatt-hours (kWh). A 20kWh battery, at maximum charge, can theoretically provide 2kW of power for ten hours.

However, it’s important to note that fully discharging solar batteries should be avoided to prevent significantly shortening the battery’s lifespan. The maximum capacity recommended by the battery manufacturer is typically about 90% of the overall capacity for a lithium-ion battery. This figure is referred to as ‘usable capacity’.

💡When choosing a solar PV battery for your business, consider the power output. Power output, measured in kilowatts (kW), determines how quickly the battery can deliver its stored electricity. The peak power output must exceed the power required in the intended use of the battery.

💡Undercharging your battery is also detrimental to the health of your solar battery. Avoid choosing excessive battery storage capacity, as this can also reduce the battery lifespan.

Solar battery size: Minimising expensive grid electricity

To minimise the use of mains electricity, your solar battery’s useable capacity should exceed:

The amount of electricity your company typically uses between sunset and sunrise when your solar array won’t generate electricity.
The excess electricity generation you expect from your solar panels each day.

This calculated capacity will also be sufficient to reduce peak demand charges.

Solar battery size: Back-up power supply

To provide a backup power supply for your business, your battery’s usable capacity should exceed the result of multiplying:

The quantity of electricity your commercial property requires each hour during the working day and
The number of working hours you would need a backup electricity supply before sourcing an emergency generator.

Solar battery size: Off-grid electricity supply

The size of a solar battery in an entirely off-grid system, without a backup generator, should exceed the multiplication of:

The typical daily electricity usage of your business
The maximum number of continuous days you would expect without direct sunlight

Typically, it’s more cost-efficient to opt for a significantly lower battery storage capacity than this calculation suggests and instead relies on a backup generator during poor weather spells.

How to choose your commercial solar battery size

You’ll notice that in this article, we’ve avoided giving any specific recommendation for battery storage size. The capacity of your solar battery needs to work in the context of the rest of your photovoltaic system, specifically:

Solar panel output under standard test conditions
The number of solar panels
Angle and direction of your solar panels
Location within the UK (hours of sunlight and expected weather)
Ambient temperature of battery location
Inverter max input and output power
Peak and off-peak business energy consumption

We, therefore, recommend consulting with an expert commercial solar installer who will consider all these factors and provide the best recommendation for battery power output and storage capacity.