What Size Solar Battery Do I Need in the UK?

Several factors need to be considered to determine the right size for a solar battery in the UK, including your household’s energy consumption patterns, the power output of your solar panels, and your specific energy goals.

Use this helpful guide to pick the correct size.

How Are Solar Battery Sizes Measured?

Solar batteries come in various types and sizes, and it can be confusing to determine which size is the best for your system. Solar battery storage capacities are typically measured in kilowatt-hours (kWh), which tells you the maximum amount of power the solar battery can store at any given time. The higher the kWh rating, the more electricity can be stored.

However, one thing to note is that while you may have a certain amount of total storage capacity available, you won’t want to use all of it. Using the entire capacity can cause damage, so instead, we recommend considering the usable capacity, called depth of discharge (DoD), which is typically around 90-95%.

Sizes can range anywhere from 1 up to 13 kWh. Many plug-and-play solutions, like EcoFlow’s DELTA series, offer substantial expandability. For example, DELTA Pro’s storage capacity is expandable to 25kWh. 

The higher the storage capacity, the more you’ll spend, but the more appliances you’ll be able to power for longer.

What Size Solar Battery Do I Need in the UK If I’m On-Grid?

A solar battery is optional if you have an on-grid solar power system. If your solar panel array doesn’t generate sufficient electricity to meet your consumption needs, your balance of system will automatically switch to utility power. 

Solar energy is intermittent by nature. Solar panels don’t produce electricity at night and at a reduced rate in cloudy weather. That’s why solar batteries aren’t optional in an off-grid solar system.

But just because you don’t have to add a solar battery to your grid-tied solar power system doesn’t mean you shouldn’t. Due to insufficient supply, extreme weather, and ageing infrastructure, on-grid electricity is becoming less secure. Without a solar battery for backup, you’ll be left without power during a blackout.  

You’ll want to optimise your system for use during solar peak times, which varies from country to country. While you may be able to sell excess electricity back to the grid via the Smart Export Guarantee (SEG), you’ll typically benefit more from using it yourself.  

With a big enough solar battery, you can store the excess electricity generated during peak hours and use it later when the sun’s not out.

So, think of it this way: At a minimum, your solar battery should be large enough to store the electricity you over-generate daily.

What Size Solar Battery Do I Need in the UK If I’m Off-Grid?

If you’re going completely off-grid, your calculations will be slightly more complicated.

For household solar power solutions, you need to start by accurately estimating your total electricity consumption.

Here’s how…

 Identify the wattage requirements of your appliances. Survey the starting and running wattage requirements of the appliances and devices you plan to plug into the generator. You can usually find the wattage requirements labelled on the appliance, but we’ve also compiled the starting and running watts of typical household appliances in the table below. 

1. Convert volts/amps to watts. If your appliance’s power requirements are in volts or amps, you can calculate an appliance’s running watts with this equation: 

Volts (V) x Amps (A) = ​​Watts (W)  

2. Count the running watts of your appliances. Add up the running watts of the appliances you plan to use — does the total exceed the running watts listed on your generator? If so, consider buying a generator with more output capacity.
3. Factor in starting watt requirements. Identify the appliance with the highest starting wattage. Add that appliance’s starting wattage to the running wattage total.
4. Calculate the sum. That final number is the total starting watts you need from your generator.  As discussed above, to avoid overloading your generator, do not exceed its starting watts rating.
   

Starting and Running Watts of Typical Household Appliances 

The calculations below may be more appropriate if you want to use solar to power appliances on your boat or motorhome.

Many appliances for mobile off-grid applications measure power in amp-hours and milli-amp hours.

If you need to calculate your total consumption using Ah or mAh, convert amp hours to kilowatt hours. 

Multiply the solar panel battery voltage by amps and divide it by 1,000.

The calculation looks like this:

Voltage x Amp hours / 1000 = kWh

In the case of a 12V, 200 Ah, here’s your calculation:

12 V x 200 Ah / 1000 = 2.4 kWh

As a general rule for solar panel systems, whether on vehicles, boats, or even homes, aim for a solar battery size at least twice your daily usage.

If you use 5 kWh of electricity daily, aim for a battery size of around 10 kWh so you’ll have more than enough for each day and plenty left over to store for a rainy or dark day.

EcoFlow’s DELTA Series offers portable power stations expandable up to 25kWh of electricity storage that can power an entire home.

What to Consider Before Choosing a Solar Battery

Aside from determining your electricity consumption needs, there are a few more things to consider before selecting your solar battery.

Here are the most crucial:

How Powerful Is Your Solar System?

Remember, your solar panel array needs to capture enough solar energy to fully charge your battery during daylight hours. Otherwise, you won’t have the benefit of using solar energy at night or when peak sun hours are limited.

Battery Cycle Life

Cycle life measures how many full discharges and recharges it takes before battery performance diminishes. The more cycles, the longer you can use a solar battery before storage capacity and efficiency decreases. Unlike lead acid and other solar battery chemistries, lithium-ion solar batteries and subsets like LiFePO4 don’t degrade over time but with use.

For example, the Delta Pro offers 3,500 cycles before storage capacity diminishes to 80%. That’s up to a decade of regular use. Keep in mind, reduced storage capacity doesn’t mean the solar battery won’t work, just that it will take longer to charge. 

On vs Off-Grid

Lastly, consider whether you currently have an on-grid or off-grid system and your future needs. If you’re on-grid now but are interested in going off-grid soon, consider an expandable system that allows you to go fully solar without starting from scratch.

Frequently Asked Questions

Final Thoughts

When selecting a solar battery size, consider your electricity consumption, the energy generation potential from your solar panel array, and whether you’re looking for an on-grid or off-grid solution. 

Additionally, consider battery chemistry and cycle life. The cheapest option is rarely the best in the long term. 

Whether you’re going on or off-grid, EcoFlow’s industry-leading rigid, flexible, and portable solar panels offer +/- 23% efficiency.

If you want to take your home off-grid, consider a Smart Home Ecosystem.