What Size Solar Battery Do I Need? 5kWh vs 10kWh vs 13.5kWh

Visual suggestion: a “small home, family home or high-use home” decision graphic.

What does solar battery capacity mean?

Solar battery capacity is measured in kilowatt-hours, or kWh. It describes how much energy a battery can store, not how much power it can deliver at one time.

A 10kWh battery, for example, does not automatically supply a 10kW electrical load.

Usable capacity

Always compare usable capacity, because this may be lower than the headline or nominal capacity.

Some battery systems reserve a proportion of their energy to protect the cells. Energy is also lost when electricity is converted, stored and discharged. Which? advises buyers to distinguish the stated capacity from the amount of energy that can actually be used.

Battery capacity versus inverter power

You must also check the inverter’s continuous power output.

A battery could contain enough energy to cover your evening consumption but still import electricity from the grid when an electric oven, kettle and induction hob are operating together.

Capacity determines approximately how long the stored energy may last. Inverter output determines how much electrical demand the system can support at the same time. Inverter limitations are also a recurring concern among UK solar battery owners.

Is a 5kWh solar battery enough?

A 5kWh solar battery will often suit a smaller property or a household with relatively limited electricity demand after sunset.

Depending on usable capacity and appliance use, it may cover:

• Lighting
• Refrigeration
• Broadband equipment
• Televisions and laptops
• Moderate cooking
• A washing machine or dishwasher cycle
• General overnight background consumption

Best for

A 5kWh battery is generally best for:

• One- or two-person households
• Smaller homes
• Properties without electric heating
• Smaller solar panel systems
• Buyers prioritising a lower initial cost
• Households with predictable evening demand

Things to consider

A 5kWh battery can drain quickly when several high-demand appliances operate together. It is also unlikely to provide significant EV charging without leaving less stored energy for the home.

Choosing a modular system can reduce this risk because additional battery capacity may be installed later, subject to product compatibility and manufacturer requirements.

Example UK use case

A couple generates surplus solar electricity during the day but is usually at work while the panels are producing. They want enough storage to reduce peak-rate grid imports between late afternoon and bedtime.

Expert recommendation

Choose 5kWh when your measured evening and overnight demand normally fits within the battery’s usable capacity—not simply because it is the cheapest quotation.

Is a 10kWh battery the best all-round size?

For many family homes, approximately 10kWh is the practical middle ground.

It provides more capacity for evening cooking, laundry, entertainment and home working without immediately moving into the premium large-battery category.

Best for

A 10kWh battery is often suitable for:

• Families
• Three- or four-bedroom properties
• Higher evening electricity users
• People working from home
• Homes with approximately 4–6kWp of solar panels
• Households planning an EV or other electrical upgrades

Key advantage

The main advantage is flexibility.

A 10kWh battery is more likely to retain useful energy into the following morning. It also creates greater scope for charging from the grid at a cheaper off-peak rate during winter, when the solar panels may not produce enough electricity to refill it.

Things to consider

A bigger battery is not automatically a better investment.

When a battery regularly ends the day partly charged and there is no valuable reason to retain or export the remaining electricity, some of the capacity is not producing a financial return.

UK battery owners frequently debate whether to size storage around average consumption or occasional winter peaks. In most cases, paying for capacity that will be used regularly is easier to justify than designing the entire system around a few exceptional days.

Expert recommendation

A 10kWh battery is the strongest starting point for many family homes, but the final decision should be checked against at least 12 months of electricity consumption data.

When does a 13.5kWh battery make sense?

A 13.5kWh battery is designed for homes with consistently higher electricity demand, larger solar generation, electric heating, frequent EV charging or a significant backup-power requirement.

Tesla’s Powerwall 3 is one well-known example. It provides 13.5kWh of storage and includes an integrated solar inverter.

Best for

A 13.5kWh battery may be suitable for:

• Larger households
• Homes with heat pumps
• Properties with significant overnight demand
• EV owners
• Larger solar panel installations
• Households using time-of-use tariffs
• Buyers prioritising power-cut protection

Key advantage

More electricity can be moved from sunny or inexpensive periods into expensive peak periods.

The larger capacity can also provide a greater energy reserve during a power cut. However, backup capability must be included in the system design and is not automatically provided by every solar battery.

Things to consider

A large battery needs enough solar surplus, low-cost grid-charging opportunities or regular household demand to justify its higher price.

It does not produce additional solar electricity. During winter, it may depend heavily on off-peak grid charging.

Expert recommendation

Choose 13.5kWh when consumption data shows that a 10kWh battery would be routinely exhausted, or when backup power and future electrification are clear priorities.

How to calculate the right solar battery size

1. Find your actual daily electricity use

Divide your annual electricity consumption by 365 to calculate a rough daily average.

Do not stop there. Use smart-meter or supplier data to identify when the electricity is being consumed.

2. Measure the period you want the battery to cover

For most homes, this will be late afternoon until the following morning—not the full 24-hour consumption figure.

3. Subtract direct daytime solar use

Electricity consumed while the solar panels are generating may be supplied directly by the panels and may not need to pass through the battery.

4. Compare usable capacity

Check how much of the advertised capacity is available for regular use. Do not compare systems using product names or nominal capacity alone.

5. Check inverter output

Confirm that the battery inverter can support the appliances you expect to run simultaneously.

6. Include future changes

Tell the installer about planned additions such as:

• An electric vehicle
• A heat pump
• An induction hob
• Electric underfloor heating
• Air conditioning
• A home office
• A property extension

7. Avoid designing around one exceptional day

A sensible growth buffer can be valuable, but permanently unused capacity normally weakens the financial return.

8. Request a monthly performance simulation

A professional quotation should explain anticipated summer charging, winter grid imports, battery cycling and electricity exports.

A useful rule of thumb is to select usable storage close to the electricity you regularly consume outside solar-generating hours, then add a modest allowance for future growth.

Visual suggestion: a flowchart moving from annual consumption to evening demand, usable capacity, inverter output and recommended battery size.

Cost guide: 5kWh, 10kWh and 13.5kWh

Energy Saving Trust says battery systems can range from approximately £1,500 to £10,000 and estimates a typical 5kWh system at around £4,600.

As a current market reference, Octopus Energy listed battery-only starting prices of £3,947 for 5kWh, £5,336 for 10kWh and £7,999 for 13.5kWh, based on April 2026 prices. These figures are useful benchmarks, but an accurate quotation requires a property survey.