What Size Solar Battery Do I Need?

Are you considering switching to solar power and wondering about the size of the solar battery you need? With the rising popularity of renewable energy, many homeowners are opting for solar panels as a means to save on energy costs. However, determining the right size of a solar battery can be slightly tricky!

In this article, we will look at everything you need to consider when choosing the appropriate size battery for your solar installation.

Factors to Consider

Before you determine the size of the solar battery you need, there are a few factors to take into consideration:

Power Consumption

Start by assessing your average daily power consumption. This can be determined by calculating the watts used by your appliances and electronics.

A great idea is to record the wattage rating of each electrical device or appliance you utilise throughout the day, accounting for both their power requirements during operation and when dormant. 

Multiply the wattage rating of each device by the number of hours it is used within a given day to obtain the cumulative watt hours consumed. 

For example, let's say you have a refrigerator that consumes 100 watts and runs for 8 hours a day, a laptop that consumes 50 watts running for 4 hours a day, and a TV that consumes 150 watts running for 3 hours a day.

The total watt hours would be (100 watts x 8 hours) + (50 watts x 4 hours) + (150 watts x 3 hours) = 800 watt hours + 200 watt hours + 450 watt hours = 1450 watt hours. 

This calculation will give you invaluable insights into your daily electricity usage and empower yourself to develop strategies for conservation and efficiency.

Sunlight Hours

The number of sunlight hours in your area is crucial in determining how much solar energy your panels can generate. The UK is famous for its’ erratic weather, although, the maximum sunlight hours tend to follow a similar pattern every year with May, June and July producing the highest number of sun days. 

In 2022, an average of 4.9 daily sun hours was recorded across the UK, with Eastbourne topping the chart for the most amount of sun per day on average. It’s always worth checking the number of sun hours in your location, the Met Office has a useful tool for analysing the average sunshine hours in UK locations. 

Battery Efficiency

Different solar batteries have varying degrees of efficiency. It is essential to consider the efficiency rating of the battery you are considering. A ‘C-rating’ measures the rate at which a battery is discharged relative to its maximum capacity.

Lithium batteries, like the ones sold here at Fogstar, have a very high efficiency rating of around 1C.

How can I calculate the size of solar battery I need?

Step 1: Determine your daily energy consumption: Multiply the power consumption of your appliances and electronics by the number of hours you use them daily. This will give you the total watt-hours consumed.

Step 2: Adjust for sunlight hours: Divide the total watt-hours consumed by the number of sunlight hours in your area. This will give you an estimate of the average wattage your solar panels need to generate per hour.

Step 3: Factor in battery efficiency: To determine the size of your solar battery, divide the average wattage your panels need to generate per hour by the efficiency rating of the battery. This will give you an estimate of the wattage the battery needs to store per hour.

Step 4: Calculate usable capacity: Multiply the wattage the battery needs to store per hour by the number of hours of backup power you want your battery to provide. This will give you the total usable watt-hours your battery should have.

Example Calculation

Let's illustrate this with an example. Suppose your daily energy consumption is 5,000 watt hours, and you have an average of 5 hours of sunlight a day in your area. You are considering a solar battery with an efficiency rating of 90% and you want it to provide backup power for 10 hours.

Step 1: 5,000 Watt hours / 5 hours = 1,000 Watts

Step 2: 1,000 Watts / 0.9 (90% efficiency) = 1,111 Watts

Step 3: 1,111 Watts x 10 hours = 11,110 Watt-hours

Based on this calculation, you would require a solar battery with a capacity of at least 11,110 watt-hours. 

To work out the kWh of the required battery, use this calculation; total watt hours ÷ 1,000 , in this instance it will be 11.11kWh of total battery capacity required.

The cost of installing a solar battery

The cost of a solar battery in the UK can vary depending on various factors such as the brand, capacity, functionality, and the chemistry of the system used.

On average, a solar battery can range anywhere from £1,100 to £9,000+. Lithium battery prices have fallen dramatically over the last few years, so now is the ideal time to add storage to your existing solar set-up. 

Our Fogstar Energy Rack is a 48v 100A 5.12kWh system that offers an incredibly high specification battery for just £245 per kWh. It also contains heating, TFT LCD touch screen, 16 Grade A EVE Cells, a DC Rated Breaker, an 8 year warranty and several pre-loaded inverter protocols for brands such as Growatt, SunSynk and Victron - all for just £1,249.00. 

We also offer a budget Topband Solar Rack battery, specifically designed for residential applications. This unit is a 48V 100Ah unit with Grade A cells and BMS, RS485/CAN - it serves as an excellent storage solution for your solar setup and costs just £1099.99.

The newest addition to the Fogstar Solar family is the Fogstar Energy 15.5Kwh 48V Battery. This pre-assembled battery is the best priced 15kWh system on the UK-market, and provides an unparalleled combination of cutting-edge technology and exceptional durability. This brand new solar battery is due to arrive in early 2024.

Can I build my own solar battery?

Yes, you can! DIY solar batteries are becoming a popular option, but it requires expertise, knowledge of electrical systems, and most importantly time! This approach, whilst incredibly cost effective, is certainly not recommended for beginners. 

Those feeling confident enough to build their own system can do so from just £1,999.00 (for a 15kWh battery). The popular Seplos Mason kits, stocked here at Fogstar, come complete with everything required to build a DIY 15.2kWh battery (including the cells). 

DIY Solar battery kits are becoming increasingly popular as homeowners look to save costs when installing batteries - especially as the storage can be expanded by up to 15 units! There is a comprehensive blog on the Seplos Mason kits which goes into a lot more depth, here.

Are solar batteries worth the investment?

With proper maintenance and usage, modern solar batteries can last anywhere between 10 to 15 years. This longevity ensures that you can benefit from their energy storage capabilities for a considerable period. Additionally, the recovery time for your investment depends on various factors such as sun hours, your energy consumption and the cost of grid electricity. 

Generally, it takes an average of 5 to 10 years to recoup the initial investment. However, it's crucial to mention that this recovery period can be shorter or longer depending on your specific circumstances. Ultimately, with the ability to reduce electricity bills, coupled with their extended lifespan, solar batteries can indeed be a worthwhile investment for those seeking long-term sustainability and financial gains.

In conclusion
Using a battery with your solar panels offers several advantages that make it a smart and efficient choice. Firstly, it allows you to store the excess energy generated by your solar panels during the day, enabling you to utilise it when the sun isn't shining, such as during night time or cloudy days. This ensures a consistent and uninterrupted power supply, reducing your reliance on the grid and saving you money in the long run. 

Moreover, solar batteries like the ones offered by Fogstar are specifically designed for solar systems, ensuring compatibility and optimal performance.

By investing in a solar battery, you are not only embracing a more sustainable future, but also enhancing the efficiency and reliability of your solar system.