Sizing guide
What Size Home Battery Do I Need?
Choosing a battery size is a balancing act. Too small and you leave savings on the table; too large and you pay for capacity you never use. This guide explains how to size a battery from your own electricity use, and the factors that matter most in Scottish homes.
Last updated October 2026
How do I work out what size battery I need?
Start with how much electricity you use in the evening and overnight, outside the cheapest hours. A battery should roughly cover that, not your whole day. Your annual usage in kilowatt hours, shown on your bill, is a good starting point, and half-hourly smart meter data is even better.
- Under about 3,500 kWh a year: around 5 kWh is often enough
- About 3,500 to 5,000 kWh a year: around 9 to 10 kWh
- Over about 5,000 kWh a year, or with an EV or heat pump: around 13.5 kWh or a modular system
These are rough starting points, and your survey confirms the right size. If your supplier app shows usage by the half hour, look at a typical weekday from late afternoon until bedtime. That is the energy a battery most often replaces. Allow some headroom for colder months, when lighting and heating loads rise.
Remember that usable capacity is what counts. A battery sold at a certain size may let you use slightly less than that, so compare usable figures between models.
Does having solar change the battery size?
Yes. With solar, the battery also stores daytime surplus for the evening, so size it around what you typically export on a good day as well as your evening use. In Scotland that surplus is large in summer and small in winter, so avoid sizing purely for the sunniest months.
Sizing for a June afternoon can leave a large battery half-empty for much of the year. Many households find that a middle-sized battery, topped up from cheap overnight electricity in winter, gives a better balance than the biggest option on offer.
Should I size for an electric car or heat pump?
If you already have one, or plan to within a few years, it is worth allowing for it. Both add significant electricity use, and much of it can be shifted to cheaper times. A modular battery that can be expanded later is a sensible middle ground if your plans are not yet fixed.
Electric cars are usually charged directly on a cheap overnight rate rather than from the battery, but a car tariff can make the whole house cheaper overnight, which changes how the battery is best used. Heat pumps run through the day in winter, so a battery can help cover their use during peak hours.
Many rural Scottish homes are off the gas grid and already rely on electricity for heating, which tends to point towards a larger battery.
Is there such a thing as too big a battery?
Yes. Every extra kilowatt hour costs money, and if it sits unused most days it lengthens your payback. The right size is the one you can fill and empty on most days of the year. Bigger is only better if your usage, solar or tariff will actually make use of it.
There are practical limits too. The power rating of your inverter controls how fast the battery can charge and discharge, and the network operator may need to approve larger systems before they are connected. In the north of Scotland that is SSEN, and in central and southern Scotland it is SP Energy Networks. Your installer handles that paperwork.
What else do people ask about battery sizing?
What is the difference between capacity and power?
Capacity, measured in kilowatt hours, is how much energy the battery stores. Power, measured in kilowatts, is how quickly it can deliver that energy. A large battery with low power may struggle to run an oven and kettle at once, so both figures matter when comparing systems.
Can a battery power my whole house?
It can cover most of a typical household’s evening use, but not unlimited high-power appliances at the same time. Whether it can run the whole house in a power cut depends on the backup hardware fitted. Many systems back up only selected essential circuits.
Does a flat need a smaller battery?
Often, because flats tend to use less electricity, but space and access are usually the deciding factors. In tenements and flats the installer needs a safe location that meets current fire-safety guidance, and that can limit the size and type of battery that fits.
Can I start small and add more later?
With a modular system, yes. You add units to the same inverter as your needs grow. Not every battery works this way, so if you might add an electric car or heat pump later, mention it at the survey so the system can be chosen with expansion in mind.
Who decides the final size?
You do, with advice from the surveyor. They look at your usage, any solar, your tariff and where the battery can go, then recommend a size and explain why. You can ask for alternative sizes to be quoted so you can compare cost against expected benefit.