What size solar battery do I need?
There is no single solar battery size that suits every UK home. The right starting point depends on how much electricity your home uses, when you use it, how much surplus solar you generate and what you want the system to achieve.
A battery can store electricity for later use. For a solar home, that may mean moving some daytime surplus into the evening. For a home without solar, it can also mean charging on a suitable off-peak tariff and using that stored electricity later. However, capacity is only one part of the decision.
The Energy Saving Trust explains that there is no exact formula for every household. A survey-led design is more useful than choosing a battery only because a particular kWh number sounds large.
Start with your electricity use, not a standard battery size
Your smart-meter data, bills and daily routine provide the best starting point. In particular, look at how much electricity you use after the sun has gone down, because that is often when stored solar electricity could be useful.
A household with modest evening demand may not benefit from the same capacity as a busy home with electric cooking, home working, an electric vehicle or planned changes to its heating. Equally, a larger battery is not automatically better if much of its capacity is rarely used.
The goal is to choose a system that reflects the way you use electricity now, while allowing for realistic future plans. HEG’s battery storage guide explains the different routes available for homes with solar, without solar and with an existing system.
What affects the right solar battery size?
Your solar generation and surplus
A battery cannot create electricity. It can only store electricity that is available from solar generation or the grid. Therefore, a system with limited daytime surplus may need a different approach from a system that regularly exports electricity.
Your roof layout, shading, panel output and daytime demand all affect how much solar energy is left over to store. A proper solar design should consider those factors together rather than sizing the battery in isolation.
If you are planning solar and storage at the same time, a combined design can consider panel capacity, inverter choice and battery capacity as one system. You can explore HEG solar panels before deciding whether a combined package is suitable.
When you use electricity
Evening use matters because solar generation is lower after daylight hours. A battery may help move some available energy into the period when a household normally needs it. However, its usefulness depends on the pattern of that demand, not only the home’s annual electricity total.
For example, an EV that is usually away during the day, a heat pump, electric cooking, a home office or a growing family can all change the profile. That is why it is sensible to tell the designer about planned changes, not just today’s bills.
Capacity and power are different
Battery capacity is measured in kilowatt-hours (kWh). It describes how much energy can be stored. Power, measured in kilowatts (kW), describes how quickly the system can charge or deliver electricity.
Both matter. A battery may have enough stored energy for a period of use but still need an appropriate power rating, inverter design and electrical arrangement for the loads in the home. The MCS battery storage guidance highlights the need to consider both storage capacity and how much energy a system can deliver at once.
Tariffs and charging strategy
A battery can work with solar, with a suitable time-of-use tariff, or with both. Time-of-use tariffs have different prices at different times. They can suit households that can move some electricity use or charging into lower-priced periods.
Tariffs are not identical. Their rates, time windows, requirements and terms can change. Therefore, check the whole tariff and your likely usage pattern instead of relying on a headline off-peak rate. The Energy Saving Trust’s guide to time-of-use tariffs sets out practical questions to ask before switching.
For Hanchu system and tariff context, see HEG’s Octopus Energy guide. Compatibility and final controls still need to be confirmed for the individual system and property.
Should I add a battery to existing solar panels?
Often, yes — but the route depends on the equipment already installed and the home’s objectives. Some homes may be suited to a battery added alongside the existing system. Others may benefit from an inverter upgrade as part of a wider redesign.
That choice can affect the available battery options, controls, installation work and the way the system is monitored. HEG’s solar inverter upgrade guide outlines the survey-led approach for existing solar owners.
If you are considering a battery for whole-home backup, that is a separate design question. Backup capability depends on compatible equipment, protected circuits and site-specific electrical arrangements. It should never be assumed from the battery capacity alone.
Questions to answer before choosing a size
Before a final recommendation, it helps to gather:
- a year of electricity bills or smart-meter data;
- details of existing solar panels, inverter and monitoring, if you have them;
- your typical daytime and evening use;
- any planned EV, heat pump or electric-heating changes;
- the tariff you use now and any tariff you are considering;
- whether you want to prioritise solar self-use, tariff flexibility, backup capability or a combination of these.
Get a survey-led battery recommendation
The best solar battery size is the one that fits the property, the household’s priorities and the wider electrical design. It should not rely on a generic capacity rule or a savings promise.
If you would like to explore battery storage with or without solar, use HEG’s Battery Storage Tool for an indicative starting point, or call 01535 210050 to discuss the options. A final recommendation, compatibility check and price are confirmed through the survey process.