When you go camping in the Peaks or the Lakes, you might take a water container with you. You fill this up at the tap and use it throughout the day. Depending on the extent to which you cook, clean and drink, you might use a little… or a lot. So, the question is, how do you know what size of container to take with you? We’ll leave you to think that one over.
In the meantime, solar storage batteries work in a similar way. Your panels feed it and you drain it through the day (and night). You don’t want to get anything too big or too small, because you’ll waste your investment. The right size depends on how much you need to store when the sun isn’t shining. This article walks you through the process.
Understanding what you use and produce
The first step is to work out how much electricity your home uses in a typical day. You’ll find this on your energy bill as kilowatt-hours (kWh). For example, the average UK household uses between 8- 10kWh daily, but yours may differ. Smart meters or monitoring apps give you your exact figures over a few weeks.
Next, think about what your solar panels supply. A typical a 4kW system on a south-facing roof in Derby might produce an average of about 10 kWh each day throughout the year, making it technically a perfectly sized array. However, you’ll get much more than that in summer and less in winter. Factor in these seasonal differences when selecting your solar storage battery.
If your panels make more than you use at the time, the extra goes into your battery for later. But if that battery is too small, you’ll send the surplus back to the grid, and essentially waste it.
If it’s too big, part of its capacity may sit unused, meaning there was no need to pay for such a large model.
Matching storage to your household’s electrical needs
You usually don’t need solar battery storage for peak daylight hours, because your solar panels power everything directly. So, you need to decide how much of your electricity you want to power from your battery storage system. As a result, some households want only enough for the evening and early morning. Others want storage for multiple days in case of poor weather or power cuts.
In short, don’t look for generic answers on Google. Sit down and work out what you need. That’s the best way to size your solar battery storage system.
A simple method is to multiply your daily use by the number of days you want stored. If your home uses 8kWh per day and you want two days of backup, that’s 16kWh of storage. In practice, you’ll adjust this for seasonal differences. In summer, your panels might refill the battery daily. In winter, you might rely on stored energy more often.
These days, battery storage systems come in all shapes and sizes. The battery type affects how much of its stated capacity you’ll actually use. For example, lithium-ion batteries allow 80% to 90% depth of discharge (DoD). Lead-acid batteries are closer to 50% DoD to protect their lifespan. So, if you buy a 10kWh lithium-ion battery at 90% DoD, you have 9kWh usable. For a lead-acid battery of the same size, you’d get around 5kWh usable.
How to balance the cost and benefit – what’s your ROI and payback period?
Battery systems range widely in cost. But if you look after them, they should last at least ten or fifteen years. The right size depends on how you value self-sufficiency against payback time. A larger battery lets you store more and buy less from the grid, but it also costs more and may take longer to pay off through savings. In general, here’s what we might recommend for different properties:
- Flats or small houses: 3-5kWh usable capacity
- Semi-detached homes: 5-10kWh usable capacity
- Detached homes: 10-15kWh usable capacity
The return on investment depends on how well the solar storage battery matches your generation and use. Oversizing adds cost without benefit. Undersizing leaves you buying more electricity from the grid. And don’t forget seasonal variation. There’s no point in getting a 30kW battery for most of us, because a typical 4kW set of solar panels will very rarely generate that much electricity in a day, especially in winter. Make sure you don’t make your battery redundant in the darker months.
If you’re in any doubt here, a professional assessment is the way to go. It’s a small investment and may even come as part of your battery installation costs. These assessments measure your actual energy usage over time, double-check your roof’s solar potential, and model the best battery size for your goals. This also accounts for technical limits, such as inverter capacity and grid connection agreements.
How can Electrical Innovations (Derby) Ltd help?
Getting the right match will turn your battery from an expensive box on the wall into a working part of your energy plan. Speak to us at Electrical Innovations today about designing and installing a system that stores exactly what you need, when you need it. We look forward to hearing from you.
