Choosing the correct battery capacity is not a one-size-fits-all decision. The right answer depends on what you want the battery to achieve and how electricity moves through your home.
A battery should solve a defined problem: increasing solar self-consumption, shifting energy to a cheaper tariff, providing resilience or supporting a highly electrified property. Four considerations shape the design.
1. Using more of your own solar energy
Exporting unused solar electricity can provide additional income, particularly during the brighter months when a large array may generate more than the home needs during daylight hours.
However, exported electricity is often worth less than the electricity later purchased from the grid. Storing surplus solar energy for evening use can therefore be more valuable than exporting it immediately, depending on the household’s tariff and consumption.
The calculation should compare likely daytime surplus with evening demand. A battery that is too small may fill early and export the remaining generation. One that is too large may regularly remain partly empty and provide poor value.
2. Resilience and power cuts
Homes in areas that experience frequent outages may place greater value on stored energy. Off-grid properties have an even stronger requirement because the battery forms part of the essential supply strategy.
Battery capacity determines how long essential loads can be supported, but capacity alone does not create backup. The system must include compatible inverter equipment, a gateway or changeover arrangement, appropriate circuits and a design that can operate safely when the grid is unavailable.
Resilience also requires choices. Keeping lighting, refrigeration, communications and heating controls operational demands far less energy than attempting to run every high-power appliance without restriction.
3. Winter load shifting
Between late autumn and early spring, solar generation is lower and may not refill a large battery consistently. During this period, a time-of-use tariff can become an important part of the strategy.
Load shifting—sometimes called energy arbitrage—means charging the battery when grid electricity is cheaper and using that stored energy during more expensive periods. Tariffs and off-peak windows change, so the design should use the customer’s current tariff while remaining flexible enough to adapt.
Daily electricity consumption and the level of grid independence desired determine how much storage is worthwhile. A fully electric home with heating, cooking and vehicles may justify much more capacity than a gas-heated property with modest evening use. OmniOrigin’s founder, for example, uses 47kWh of storage in a fully electric home; that is a personal design choice, not a standard recommendation.
4. Future demand
An extension, heat pump, air conditioning system, hot tub, home office or growing family can change a property’s energy profile. Electric vehicles add substantial energy demand but are normally charged directly from the grid or solar-aware charger rather than deliberately emptied from the home battery.
Modular storage can be valuable where future growth is likely, provided the inverter, battery family and manufacturer’s expansion rules support the proposed upgrade.
Capacity and power are different
Battery capacity is measured in kilowatt-hours (kWh). It describes how much usable energy the battery can hold. Inverter output is measured in kilowatts (kW). It describes how much power can be delivered at one moment.
A home may have 20kWh of stored energy but still import from the grid when several appliances run together if the inverter can discharge only 3.68kW. Conversely, a powerful inverter attached to a small battery may support a high load briefly but exhaust the stored energy quickly.
Peak demand, continuous output, surge capability and the property’s supply phase must therefore be considered alongside storage capacity. Larger inverter arrangements may also require a G99 application and DNO approval.
A battery should be calculated, not guessed
The best design begins with annual bills, half-hourly data where available, expected solar generation, tariff periods and a discussion about resilience. These inputs let the installer model how often the battery may cycle and how much of its capacity is likely to be useful.