How Many Solar Batteries Do You Need? A Battery Sizing Guide for UK Homes

There's a question that comes up almost as often as "how much will this save me?"—and it's "how big a battery do I actually need?" Get it right and you're using far more of your own solar power, cutting your evening grid costs and getting genuine backup when the power goes out. Get it wrong and you're either paying for storage capacity that sits half-empty most nights, or watching your battery run dry by 9pm and buying electricity back at peak rates anyway.

There's no single right answer—it depends on how much electricity your household actually uses, how big your solar array is, and what you want the battery to do for you. Here's how we work it out with customers.

Start With Your Household Usage, Not the Battery Spec Sheet

Before looking at any battery, you need a rough idea of your daily electricity consumption. Most UK homes use somewhere between 8kWh and 12kWh a day, though this varies a lot depending on household size, whether you have an EV, and how electric-heavy your heating and cooking setup is. Your energy supplier's app or a recent bill will usually show your average daily usage—that's your starting point.

The next question is how much of that usage happens after the sun goes down. For most households, evenings and early mornings account for the bulk of demand—dinner, lighting, TV, washing machines running after work. A battery only helps with the portion of your usage it can realistically cover, so if you're using 10kWh a day and 6kWh of that falls outside daylight hours, that's roughly the target you're sizing towards.

Matching Battery Size to Your Solar System

Your battery also needs to be sized against your solar array, not just your usage. A battery is only as useful as the surplus electricity available to fill it. On a 4kW system in the UK, you might generate an excess of 4–8kWh on a decent day in spring or summer—but far less in December. Oversizing a battery on a small system means it rarely reaches full charge in winter, so you're paying for capacity you can't use for months of the year.

As a rough guide, we usually see:

• 3.6kW solar system → 5–8kWh battery

• 4–5kW solar system → 8–13kWh battery

• 6kW+ solar system → 13–20kWh battery

These aren't fixed rules—they're a starting point for the conversation, and every property is different once you factor in roof orientation, shading and household routine.

Depth of Discharge and Usable Capacity

One detail that catches people out is the difference between a battery's advertised capacity and what you can actually use. Most lithium batteries have a depth of discharge (DoD) of around 90–100%, but older or budget models can be lower. A 10kWh battery with 90% DoD gives you 9kWh of usable storage, not 10kWh. Always ask for the usable figure when comparing quotes, not just the headline number on the spec sheet.

Do You Want Backup Power, or Just Bill Savings?

This is often the deciding factor. If you simply want to shift your own solar generation into the evening and reduce what you buy from the grid, a mid-sized battery paired sensibly to your array is usually enough. Systems like the Sigenergy SigenStack and SigenStack home platforms we install are built around this kind of day-to-day self-consumption.

If you also want the property to keep running during a power cut, that changes the sizing conversation. A whole-home backup setup—like the Sigenergy HomeMax Gateway we fitted on a recent 8.16kW installation in Northamptonshire—needs enough spare capacity held back to cover essential circuits, not just your evening usage. Homeowners going down this route often size up a battery tier or add a second unit, rather than relying on a single small battery stretched across both jobs.

A Few Real Examples

To put rough numbers into context, on recent installations we've sized (with their case studies attached):

• A 10.12kW solar system with 16kWh of Sigenergy battery storage for a family home in March, Cambridgeshire, alongside a consumer unit upgrade

• An 8.16kW solar system with 10kWh of storage and a HomeMax Gateway for whole-home backup on a property in Northamptonshire

• A 32.25kW commercial system paired with Sigenergy's commercial-scale platform for a business with high daytime consumption

Each was sized around the specific usage pattern of the building, not a generic formula—which is really the point. Battery sizing works best as a conversation, not a calculator exercise.

Don't Forget Future Demand

It's worth thinking a step ahead too. If you're planning to get an EV, install a heat pump, or simply expect your household's electricity use to grow, it's often more cost-effective to size slightly larger now than to add a second battery later. Not every battery platform allows easy expansion, so this is worth raising directly with your installer at the design stage.

Getting the Sizing Right

The honest answer to "what size battery do I need?" is: it depends on your usage, your solar system, and what you want the battery to achieve. A proper site survey—looking at your actual consumption data, roof space and goals—will always beat a rule-of-thumb calculation.

Thinking about a battery for your home? Get in touch for a free, no-obligation site survey. 📧 enquiries@optimumelectrics.co.uk | 📞 01733 601698

Optimum Electrics is MCS, NICEIC and HIES accredited and an Octopus Energy Trusted Partner. We design and install solar, battery storage and EV charging systems across Cambridgeshire, Peterborough, Cambridge, Bedford, Northampton and Leicester.