This is a tricky one, and there is no single right answer. It comes down to your budget, how much electricity you use, and what you are trying to achieve. The good news is you do not have to get it perfect on day one. Battery systems are modular, so you can start small and add more as you learn how your home really behaves. In this post I will explain how to size storage, how time of use tariffs change the maths, and how my own system has grown from a small battery to a large stack.
What a solar battery actually does
A solar battery stores electricity so you can use it later. It captures the surplus your panels make during the day, then releases it in the evening when the panels stop and your demand goes up.
It does a second job too. On a time of use tariff you can charge the battery overnight when grid electricity is cheap, then run the house off that stored power during the day. More on that shortly, because it is the part most people overlook.
The three things that decide your battery size
When I size a battery for a customer, three factors lead the decision:
- Cost. This is usually the big one. Quite often the budget runs out before the ideal battery size is reached, and that is fine.
- How much electricity you use. A home with electric heating and two cars needs far more storage than a low use household.
- What you are trying to achieve. Some people want to shave the evening peak. Others want to ride right through a winter day without buying any expensive electricity. Those are very different goals and very different battery sizes.
There is no point fitting a huge battery you will never fill, and no point fitting a tiny one if your goal is full self sufficiency. The right size is the one that matches your usage and your budget.
Why modular storage takes the pressure off
The best feature of modern battery systems is that they are modular. That means they are built in blocks, so you can start with one battery and add more later.
This matters because the use case for a house changes over time. You might fit solar today, then a year later get an electric car, switch the heating to electric, or have the family grow. Your electricity demand shifts, and your storage can shift with it.
So if the budget is tight, start with what you can afford. Live with it. Watch your usage through an app. Then add capacity when you understand exactly how much you need. You are not locked into a guess made on day one.
My own journey from 5 to 40 kilowatt hours
I will use my own home as an example, because it shows how this plays out in real life.
When I first put solar in, I fitted a 5 kilowatt hour battery. At the time that was plenty, because we did not use much electricity. The house ran on gas for heating and our demand was modest.
Then things changed. We moved our heating over to electric. We added other electrical loads. Our demand crept up and up, and the small battery was no longer enough.
So I added to it over time, and I now run a large battery stack of 40 kilowatt hours. That is a big amount of storage, and it does a specific job for me, which brings me to tariffs.
How a cheap overnight tariff changes everything
A time of use tariff charges you different prices at different times of day. Overnight power is cheap because demand on the grid is low. Daytime power is far more expensive.
My overnight rate is currently around 7 pence per unit. My day rate is far higher. So my battery does something clever in winter.
On a short, rainy December day the panels make very little. But the night before, I fill my 40 kilowatt hours of battery on the cheap 7 pence rate. The next day I run the whole house off that stored power. That covers:
- The electric heating
- The lighting and the appliances
- Even the hot tub
The result is that I should never really need any expensive daytime electricity, even in the depths of winter. I bought it all at 7 pence the night before.
That has a considerable impact on the payback period. A battery is a serious investment, and the way you justify it is by using it to dodge the expensive rate every single day. A big battery on a smart tariff is not just storage, it is a tool for buying all your power at the cheapest possible price.
So how many do you need?
To pull it together:
- If you simply want to cover the evening and store your daytime solar, a modest battery does the job.
- If you want winter self sufficiency on a cheap overnight tariff, you need a much larger stack like mine.
- If the budget is tight, start with one and add more later. The system is built to grow.
The honest answer is that the right number depends on your home, your habits and your goals. That is exactly the conversation we have on a survey.
The bottom line
How many batteries you need depends on cost, usage and what you want to achieve. Start with what suits you now, then add capacity as your needs change, because the systems are modular. Pair a decent battery with a cheap overnight tariff and you can run your home on power you bought at the lowest rate.
We design and fit battery storage across Cambridgeshire, Bedfordshire and Northamptonshire, and we will size it honestly around your real usage. Call the office on 01480 400607 or request a survey through our website.
Jason Pope
Owner, Selec Group