Why battery sizing is harder than it sounds****
There is a version of this question that seems simple: how many kilowatt-hours do I use overnight, and can a battery cover it? The problem is that most households have no idea how their consumption is distributed across the day, and the conversation does not always start there.
A battery that is too small leaves you drawing from the grid at 11pm. A battery that is too large costs significantly more than it needs to and takes longer to pay back through savings. Getting this right matters.
Four things drive the answer: how much electricity you actually use overnight, how deep you can safely discharge your battery, how much solar generation you have to charge it during the day, and whether you need true backup power or just overnight self-sufficiency.
How much electricity does a UK home use overnight?****
The average UK household consumes around 8 to 10 kWh between sunset and the following morning. That covers the fridge running constantly, the heating controls, a few lights, phone charging, and the odd kettle boil before bed. Not a tumble dryer. Not an electric shower. Definitely not an EV charger.
Household type Typical overnight use Notes 1 to 2 person flat or small semi 4 to 6 kWh Low-load appliances, no EV Family home, 3 to 4 people 8 to 12 kWh Typical UK average range Larger home or high-appliance use 12 to 18 kWh Includes underfloor heating, older appliances Home with overnight EV charging 20 to 40 kWh EV adds 7 to 20 kWh depending on range needed
Your actual figure is on your smart meter or in your energy supplier’s app. If you have a smart meter, look for the hourly breakdown and add up everything from roughly 9pm to 7am on a typical weekday and a weekend. Average those figures. That number is your starting point.
Usable capacity vs nameplate capacity****
Battery storage products are sold in kilowatt-hours, but the number on the label is not what you get to use. Lithium iron phosphate (LFP) batteries, the most common type in UK home storage, typically allow discharge down to about 10 to 20% of their total capacity before the battery management system steps in to protect the cells.
In plain terms: a 10 kWh battery gives you roughly 8 to 9 kWh of usable power. A 13.5 kWh model gives you around 11 to 12 kWh that you can reliably draw on.
This is the number to size against, not the headline figure.
How to estimate the battery size you need****
The calculation below gives you a starting figure. It is a guide, not a final specification. A proper site survey by a qualified installer will account for your actual usage patterns, your existing electrical setup, and your solar system’s real-world output.
Estimating your nameplate battery capacityStep 1: Find your overnight kWh use from your smart meter app Step 2: Divide that number by 0.85 (standard usable depth for LFP batteries) Step 3: The result is the nameplate capacity that covers your overnight use
**Example:**9 kWh overnight ÷ 0.85 = 10.6 kWh nameplate capacity needed
How solar panels change the calculation****
Most homeowners buying battery storage are also buying solar panels, or adding storage to an existing system. This is where things get genuinely interesting, and where the sizing question gets a second layer.
A solar array charges your battery during the day. The more surplus your panels generate above your daytime consumption, the more energy is available to fill the battery before the sun goes down. If the battery is full by early afternoon on a summer day, that is all the overnight cover you can get from that system. Going larger does not help unless the solar array is also larger.
In the UK, a 4 kWp solar array generates roughly 3,400 kWh per year. That works out to around 9 kWh on a good summer day and as little as 1 to 2 kWh on a dark December afternoon. The average daily generation across the year sits somewhere between those figures, but averages hide the seasonal variation that matters most when you are trying to run through the night.
The winter problem****
Here is the part that some sales conversations gloss over: in November through January, a UK solar system simply does not generate enough to fill a battery every day. Days are short, the sun sits low, and cloud cover is frequent. A 6 kWp system, generously sized for a UK home, might only generate 2 to 3 kWh on a dull January day.
That does not make battery storage a bad investment. It means expectations need to be realistic. Through spring, summer, and much of autumn, a well-matched solar-and-battery system can cover your overnight use with no grid draw. In the depths of winter, you will likely draw some electricity from the grid. A time-of-use tariff that allows your battery to top up at cheap overnight rates can help offset this.
Solar array size Recommended battery size Best for 2 to 3 kWp 5 to 7 kWh usable Small homes, mainly daytime self-use 4 to 5 kWp 8 to 12 kWh usable Typical family home, spring to autumn overnight cover 6 kWp or more 12 to 20 kWh usable Larger homes, near-year-round independence Any size with EV Add 10 to 20 kWh For overnight EV charging from solar surplus
Backup power or overnight self-sufficiency?****
These sound like the same thing. They are not.
Overnight self-sufficiency means your battery powers your home from stored solar energy during the hours when your panels are not generating. The grid stays connected in the background and simply is not called on. If there is a power cut, however, your battery typically shuts down too, because the inverter detects the grid has dropped and disconnects to protect engineers working on the network. This is a legal safety requirement for grid-connected systems in the UK.
True backup power requires an inverter with islanding or emergency backup capability, one that can automatically disconnect from the grid and continue running your home from the battery alone. Not all home storage systems include this. It often costs more, requires specific circuit configuration, and needs to be planned at the design stage rather than retrofitted later.
If backup power matters to you, whether because of frequent outages in your area or because of medical equipment that cannot lose power, make this a clear requirement when you discuss your system. It changes which equipment goes on the specification.
Getting your installation right****
The quality of the installation is as important as the quality of the equipment. The steps below outline what a thorough process looks like when working with a reputable installer.
Conduct a proper site survey. Sizing a battery storage system correctly requires understanding your actual energy use, your roof orientation, any shading, your existing electrical setup, and whether your consumer unit needs upgrading. A phone estimate is not a site survey.
Provide a written quotation with clear pricing. Itemised quotes let you see exactly what you are paying for. If a quote arrives as a single lump sum with no breakdown, ask for the detail before you proceed.
Issue a written contract before work begins. This should cover equipment specification, installation timeline, warranty terms, and the process for resolving any issues.
Register the installation with the relevant network operator. Battery systems and solar arrays above 3.68 kW must be notified to your Distribution Network Operator. Your installer handles this. If they suggest this step is not necessary, treat that as a concern.
Issue an Electrical Installation Certificate on completion. This confirms the work meets UK wiring regulations and is required for insurance purposes and any future property sale. Always ask for it.