Most UK homes with 4-5 kW solar systems need 10-20 kWh battery storage to cover evening energy needs and maximise self-consumption without paying for unused capacity.
Solar panels cover only part of the job. Daytime power often slips away or gets sold off cheaply, then evenings rely on costly grid energy. Battery size shapes the whole system. A well-matched battery lowers bills and gives real control. A poor choice drains cash, adds clutter, and leaves the grid calling the shots.
Key Takeaways
- Modern lithium batteries offer 90-95% depth of discharge, so a 10 kWh nominal battery provides roughly 9-9.5 kWh usable capacity for powering your home.
- UK homes typically require 10-20 kWh batteries to cover evening energy needs for most households with 4-5 kW solar installations
- Planning for EV charging (adds 6-10 kWh daily) or heat pumps (adds 8-15 kWh daily) requires 30-50% more battery capacity than current consumption alone suggests.
What Does Battery Size Actually Mean?
Battery size is simply how much energy your battery can hold for later. It is measured in kilowatt hours (kWh) and gives a rough idea of how long your home can run on stored power before pulling from the grid again.
Not all of that stored energy is usable, though.
Depth of discharge (DoD) explains how much energy you can safely use. Modern lithium batteries allow around 90–95% DoD, so a 10 kWh battery provides roughly 9–9.5 kWh of usable energy. Older battery technologies allowed far less, which is why the headline capacity figure has never told the full story.
This is why usable capacity matters more than nominal size. A smaller battery with higher usable energy can outperform a larger one on paper.
When comparing batteries, always calculate usable kWh (nominal capacity × DoD percentage). A 10 kWh battery with 95% DoD delivers more usable energy than a 12 kWh battery with 70% DoD.
In the UK, home batteries typically range from 5 to 20 kWh. The right size depends on how much electricity your household uses each day. This approach also reflects how battery storage supports both individual homes and the wider energy system, as outlined in UK Parliament research on battery energy storage and grid stability.
How Much Energy Does Your Home Actually Use?
Establishing your baseline consumption is the foundation of any sizing calculation. An average UK household uses around eight to ten kilowatt hours a day, but that figure moves a lot depending on the home and the people in it.
Down below in the following table, you will see how different household sizes link to daily use, evening demand, and the battery sizes that usually make sense.
| Household Size | Typical Daily Consumption | Evening / Night Usage (60–70% of total) | Recommended Battery Size (Usable) |
|---|---|---|---|
| 1–2 people | 5–8 kWh | 3–5.6 kWh | 5–8 kWh |
| 2–3 people | 8–12 kWh | 4.8–8.4 kWh | 8–10 kWh |
| 3–4 people | 12–16 kWh | 7.2–11.2 kWh | 10–13 kWh |
| 4–5 people | 16–22 kWh | 9.6–15.4 kWh | 13–18 kWh |
These figures represent typical consumption without EVs or heat pumps.
Your own electricity bills are the best place to begin. Breaking monthly usage into daily figures gives a reliable baseline, while smart meters add clarity by showing when energy is actually used, usually mornings and evenings. Looking at UK solar generation patterns alongside these habits shows how much energy you can use as it is produced and how much needs storing for later.
Energy use also rises sharply in winter, often double summer levels, so it is important to think year-round. Solar produces power at midday, when homes are quieter, which explains the need for storage.Â
Looking ahead matters too, because changes like electric cars, heat pumps, and more home working all raise daily electricity use and shift when power is needed, which directly affects how your battery should be sized and used.
Matching Battery Size to Your Solar PV System
Battery size needs to match what your solar system can realistically produce; otherwise, energy is wasted, or money is spent on storage that rarely gets used.
UK homes usually install solar systems between 3 and 5 kW. A 4 kW system can produce around 12 to 15 kWh per day in summer and only 2 to 5 kWh per day in winter, with a yearly average of roughly 8 to 10 kWh per day.
This large seasonal difference is why correct sizing matters.
A battery that is far too large, such as a 25 kWh unit paired with a 4 kW system, will only fully charge on occasional summer days, leaving much of its capacity unused. A battery that is too small, around 5 kWh, creates the opposite problem, with excess summer solar exported at low rates while grid electricity is still needed in the evening and through winter.
For that reason, battery sizing should always be considered alongside the solar system itself. A well-sized battery should then cover evening and overnight demand, which usually accounts for around 60 to 70 percent of daily electricity use.
As electricity prices remain high and the grid relies more heavily on variable renewable generation, government energy data shows growing uptake of home solar and battery systems designed to maximise on site use rather than export. Correctly matching battery size to solar output plays a key role in making that approach work in practice.
The Step-by-Step Battery Sizing Method for UK Homes
Follow this framework to calculate your specific battery needs accurately rather than relying on generic recommendations.
Step 1: Calculate average daily electricity consumption.Â
Review 12 months of bills, total the kWh consumed, divide by 365. This accounts for seasonal variations and gives a realistic baseline.
Step 2: Determine your solar PV system’s daily generation.Â
For existing systems, check monitoring data.
For planned systems, use 850-1,100 kWh annual generation per installed kWp in the UK (so a 4 kW system generates roughly 3,400-4,400 kWh yearly, or 9-12 kWh daily average).
Step 3: Identify evening/overnight energy needs.Â
Most households use 60-70% of daily consumption outside solar generation hours. A home using 12 kWh daily typically needs 7-8.5 kWh stored for evening through to morning.
Step 4: Account for depth of discharge.Â
Multiply your evening energy needs by 1.05-1.1 to determine required nominal battery capacity. If you need 8 kWh usable, you need roughly 8.5-9 kWh nominal capacity (assuming 90-95% DoD).
Step 5: Add buffer for future needs.Â
EV charging adds 6-10 kWh daily for typical commuting. Heat pumps add 8-15 kWh daily, depending on property insulation and climate. Add 30-50% capacity if either is planned within five years.
To put that into a real-world example, take a four-person household with a 4.5 kW solar PV system using about 14 kWh of electricity a day. Roughly 9.5 kWh of that is used in the evening and overnight, when the panels aren’t doing much. With a 95% usable battery, that works out at roughly a 10 kWh system.
If an electric car is likely in the next couple of years, adding around 7 kWh a day, the sensible option would be to size up to an 18–20 kWh battery to avoid outgrowing it too quickly.
UK Policy and Financial Factors That Affect Battery Sizing
Battery sizing decisions happen within a financial and regulatory landscape that dramatically affects returns on investment.
- Smart Export Guarantee rates influence whether you prioritise storage or export. With SEG paying 3-15p per kWh whilst grid imports cost 25-35p, every kWh you store and use later saves 10-30p compared to exporting then reimporting. This makes self-consumption through battery storage financially superior to exporting under current rate structures.
- Zero percent VAT on residential battery installations (introduced recently) reduces upfront costs by 20%, improving payback periods substantially. This policy change has accelerated UK residential battery adoption significantly.
- Time-of-use tariffs make stored energy even more valuable. Tariffs charging 7-10p per kWh overnight but 35-45p during peak hours reward storing cheap overnight grid electricity or daytime solar for peak-time use, adding another dimension to battery value beyond just solar storage.
- Grid connection standards and safety requirements follow PAS installation standards, ensuring batteries integrate safely with both solar systems and grid connections. Professional installation by certified contractors ensures compliance whilst protecting warranties.
The UK’s battery storage targets of 23-27 GW by 2030 signal long-term grid changes favouring distributed storage. As the grid becomes increasingly reliant on weather-dependent generation, home batteries provide both personal benefits and system-level grid balancing value.
Conclusion
Battery size shapes solar performance. When matched to real usage, power flows into evening and bills ease. A professional assessment aligns usage, plans and design, avoiding guesswork. We tailor sizing to your household so your system works, keeps delivering value.
Get your free battery sizing assessment today, and we’ll look at how your home actually uses energy, factor in future plans, and recommend a setup that fits properly.
FAQs
What is the most common battery size for UK homes?
UK homes typically install batteries between 10 and 13 kWh. This works well with a 4 to 5 kW solar system and supports evening use for households of three to four people.
Can I add more battery capacity later if I need it?
Yes, many systems allow expansion. It usually costs more later though, as extra labour and upgrades are often needed, so getting the size right early makes sense.
How long will a fully charged solar battery power my home?
It depends on use. A 10 kWh battery will usually cover normal evening demand from sunset through to bedtime.
Does battery size affect how much I can save on electricity bills?
Yes, to a point. The right size avoids wasted capacity and can save around four to eight hundred pounds a year.
Will I need a bigger battery if I buy an electric vehicle?
Yes. Regular charging adds heavy demand, so extra capacity is usually needed.