A solar battery can make a well-designed solar PV system work harder for your household. Rather than sending every unused unit of daytime generation to the grid, it stores electricity for the evening, early morning or periods of higher demand. But what size solar battery is right for your property? The answer is rarely a single number. It depends on how much electricity you use, when you use it and what you want the system to achieve.
For many UK homes, a battery between 5kWh and 10kWh is a sensible starting point. That does not mean it is automatically the best choice. A smaller battery may offer better value for a low-use household, while a larger unit can be worthwhile for homes with high evening demand, an electric vehicle, a heat pump or a time-of-use electricity tariff.
What size solar battery should you choose?
Battery capacity is measured in kilowatt-hours, or kWh. This tells you how much energy the battery can store. A 5kWh battery can theoretically hold 5kWh of electricity, while a 10kWh battery stores twice as much.
The right capacity should usually be based on your usable surplus solar generation and your electricity consumption after the sun goes down. Installing the largest battery available is not always the most cost-effective route. If a battery is regularly left half full because there is not enough spare solar energy to charge it, you have paid for capacity that is doing little work. Equally, if a small battery fills before lunchtime and your household uses significant electricity in the evening, it may limit the savings your solar system could deliver.
A good design considers the whole picture: your annual consumption, half-hourly usage pattern where available, roof size and orientation, solar array output, existing electrical equipment and future plans. A proper survey matters because two households with the same annual electricity use can have very different daily demand.
Start with your daily electricity use
Your electricity bill gives a useful first indication. Divide your annual electricity use in kWh by 365 to find an average daily figure. For example, a household using 3,650kWh per year consumes around 10kWh per day on average.
That is only a starting point. A battery is most valuable when it covers electricity you would otherwise buy from the grid. If your family is out during the day and returns to cook, wash clothes and watch television in the evening, a larger share of your consumption falls after solar generation has reduced. In that case, battery storage can make a noticeable difference.
A household that uses around 6kWh to 10kWh each day may find 5kWh to 8kWh of usable storage appropriate. Homes using 12kWh to 20kWh per day, particularly those with higher evening use, often consider 8kWh to 13kWh. Larger homes or properties with electric heating, frequent EV charging or substantial overnight consumption may need a larger or expandable battery system.
These figures are guides, not promises. Winter solar generation is much lower than summer generation in Yorkshire and across the UK, so a battery cannot be expected to fill from solar every day of the year. The battery can still provide value through intelligent tariff charging when this suits your electricity plan.
Match the battery to your solar PV system
Solar panel size is another key factor. A typical 4kWp domestic solar array may produce a healthy surplus during bright spring and summer days, but much less on short winter days. Pairing it with an oversized battery can mean much of the capacity is only fully used for part of the year.
As a broad guide, a 4kWp to 6kWp solar system is often paired with around 5kWh to 10kWh of battery storage. A larger array may justify 10kWh or more, especially where daytime export is high and evening use is consistent.
However, solar array size should not be treated as a fixed battery-sizing rule. A south-facing roof may generate most of its power around midday, whereas east-west panels spread production over a longer part of the day. A family working from home may use more solar directly, leaving less surplus to store. A household that is empty all day may export more and benefit from additional storage.
The aim is to use as much of your own lower-cost solar electricity as reasonably possible, without buying capacity that will sit unused.
Usable capacity matters more than the label
When comparing batteries, check the usable capacity rather than looking only at the headline figure. Manufacturers may protect part of a battery’s total capacity to support its performance and lifespan. A battery advertised as 10kWh may therefore have a slightly lower usable amount.
It is also worth considering warranty terms, expected cycles, efficiency and whether the system can be expanded later. An expandable system can be a sensible choice if you expect to add an EV, heat pump or more solar panels in the future. It allows you to start with a capacity that suits today’s usage rather than over-specifying from day one.
Capacity is not the same as battery power
Capacity tells you how long a battery can supply electricity. Power, measured in kilowatts or kW, tells you how much it can deliver at one time.
This distinction becomes important when several appliances run together. A battery with sufficient capacity but a modest power output may not cover the full demand from an oven, kettle, shower, tumble dryer and EV charger at the same moment. The grid will supply any additional power required, which is normal, but it affects how much imported electricity you avoid.
For many homes, a battery with around 3kW to 5kW of output is suitable. Higher-demand properties may need more. Your installer should assess your incoming supply, consumer unit, solar inverter arrangement and typical loads to make sure the design is safe, compliant and practical.
Consider off-peak tariffs and backup requirements
Solar batteries are no longer only charged by solar. With a suitable tariff and compatible system, you may be able to charge during cheaper overnight periods and use stored electricity at more expensive times. This can make battery storage useful in winter, when solar output is lower, but it changes the sizing calculation.
A household using an off-peak tariff may choose a battery large enough to cover a meaningful portion of the morning and evening peak. The financial benefit depends on tariff rates, export payments, daily usage and how reliably the system is controlled. Honest projections should account for these variables rather than relying on a best-case summer day.
If resilience during a power cut is a priority, say so early in the design process. Not every solar battery automatically provides backup power when the grid fails. Backup capability requires the correct inverter, protection equipment and circuit design. You may choose to support essential circuits such as lighting, refrigeration, broadband and selected sockets rather than attempting to run every appliance in the property.
A whole-home backup arrangement can require more battery power and capacity, and may not be necessary for every household. It is a useful feature where continuity is important, but it should be specified around realistic loads and expected outage duration.
Examples of sensible solar battery sizes
A couple in a modest home using 2,500kWh to 3,000kWh per year, with a 3kWp to 4kWp solar array and low evening demand, may be well served by around 5kWh of usable storage. They can retain more daytime generation without paying for an unnecessarily large system.
A family using 4,000kWh to 6,000kWh per year with a 5kWp to 6kWp array, regular evening cooking and laundry use may benefit from 8kWh to 10kWh. This gives more room to carry solar energy into the evening and can also work well with an off-peak tariff.
A larger household with a 7kWp-plus solar system, an EV, electric heating or a heat pump may look at 10kWh to 15kWh or a modular system that can grow over time. EV charging should be assessed separately, though. Using a home battery to charge an EV at high speed can empty it quickly, so smart charging directly from solar surplus or low-cost tariff periods is often the better approach.
Get a design based on real data
The most reliable way to size a battery is to review actual consumption and generation data. Smart-meter information can reveal when electricity is used, while a site survey confirms the right solar array size, cable routes, consumer unit condition and equipment location. This avoids a one-size-fits-all recommendation and helps set realistic expectations for savings.
At Evolve Electrical Contractors, solar PV and battery storage systems are designed around the property, the household’s goals and the electrical installation already in place. The process covers survey, system design, installation, testing and certification, with clear advice at each stage.
The best battery is not necessarily the biggest one. It is the one that is used regularly, works safely with your solar system and tariff, and supports lower bills for years to come.