Solar Battery Size Calculator (kWh)

How the calculation works

Solar Battery Size: How Many kWh Do You Need?

Solar panels produce most of their electricity in daylight, and none at night. A battery fills that gap. It stores surplus generation from the day and releases it when the panels are not producing, in the evening, overnight or during a dull spell. Sizing it correctly avoids two opposite errors. A battery that is too small leaves surplus energy unused, while one that is too large represents capital that never earns back its cost.

How the sizing formula works

The formula starts from two figures: the household's daily electricity consumption in kWh, and the number of days of autonomy. Autonomy is how many consecutive days without sun the battery should cover on its own. Multiplying the two gives the energy the battery must deliver.

The battery cannot deliver all of its rated capacity, for two reasons. The first is depth of discharge (DoD), the share of capacity that can be used without shortening the battery's life. The second is round-trip efficiency (η), the share of the energy put into the battery that comes back out after a full charge and discharge cycle, with the rest lost as heat and in conversion. Dividing the energy requirement by DoD and efficiency gives the nominal capacity needed.

The final term, (1 + m), adds a margin for degradation. The calculator uses 15% by default. This is a planning allowance rather than a measured figure, and it keeps the system able to cover winter nights after the battery has aged.

Worked example: a home that uses 8 kWh a day, with one day of autonomy and LiFePO4 chemistry, has an energy requirement of 8 × 1 = 8 kWh. Dividing by 0.90 × 0.95 (0.855) gives 9.4 kWh of nominal capacity. Multiplying by 1.15 gives a recommended capacity of 10.8 kWh.

Battery chemistries on the UK market

Three chemistries account for most home battery systems. Each one has different defaults in the calculator, and those defaults change the size needed for the same household.

LiFePO4 (lithium iron phosphate) is a common choice for new home installations. The calculator assumes a DoD of 0.90 and an efficiency of 0.95 for it, the most favourable values of the three. It is generally regarded as having a longer cycle life and better thermal stability than other lithium chemistries, which is why it is often preferred where safety and longevity matter. Its main drawback is a higher upfront cost.

NMC (nickel manganese cobalt) offers high energy density, which allows compact units. The calculator assumes a DoD of 0.80 and an efficiency of 0.93, so the same daily use needs a somewhat larger nominal capacity than with LiFePO4. NMC suits installations where space is the main constraint.

Lead-acid (VRLA or AGM) is the oldest of the three. It is usually the cheapest option per kWh of nameplate capacity, but the calculator assumes only a DoD of 0.50 and an efficiency of 0.85. For the same usable energy, a lead-acid system needs about twice the nameplate capacity of a LiFePO4 system, since the combined DoD and efficiency factor is 0.425 against 0.855. It is mainly considered for off-grid sites where the upfront cost matters most.

Autonomy: one day or more?

For grid-connected systems, one day of autonomy is a common planning choice. The battery's job is to store the day's surplus and use it in the evening and overnight, because the grid covers longer periods without sun.

Off-grid systems, and homes in rural areas with frequent power cuts, are normally designed for two to three days. Homes in cloudier regions, where generation drops sharply in winter, may need to plan for more days than the same home in a sunnier location. Some designers add a fraction of a day for the darkest months rather than design for several full days.

Common sizing mistakes

The most frequent mistake is to size the battery for total daily consumption without considering the energy used directly while the sun is shining. The calculator uses total consumption, so for a home with a lot of daytime use it gives an upper estimate. Suppose a home uses 8 kWh a day, but 4 kWh of that is used directly during daylight by a washing machine, a cooker or a computer. Only the remaining 4 kWh needs storing for night use. A 9.4 kWh nominal battery in that case would be only about half used.

The second mistake is to ignore degradation. Lithium batteries lose capacity gradually with cycling and age, so a system sized exactly for today's needs may fall short in later years, particularly in winter. The 15% margin is there to absorb that gradual loss, but it is an allowance, not a guarantee. Check the warranty terms for guaranteed capacity retention, and compare them with the number of cycles you expect each year.

In practice, installers round the calculated figure up to the nearest module size they can supply. If the calculation gives 10.8 kWh, a 10 kWh system made of two 5 kWh modules falls just short, so a larger configuration would be chosen instead. Compatibility with the inverter often decides the final choice, because not every inverter works with every battery voltage and chemistry.

A third mistake is to treat the result as a guarantee of backup power. The calculator sizes a battery for a planned pattern of use. A battery cannot be expected to power every appliance through a long outage unless the system is designed for that load.

Installation rules in the UK

A battery is part of an electrical installation, so the work falls under the Building Regulations and the wiring standard BS 7671. In England and Wales, Part P of the Building Regulations covers electrical work in dwellings. In Scotland, Building Standards apply instead. The battery's own safety should be assessed against BS EN IEC 62619, the safety standard for lithium secondary batteries used in industrial and stationary applications.

Connecting a battery to a solar system and to the grid requires notification or approval under the Energy Networks Association's engineering recommendations: G98 for smaller systems and G99 for larger ones. The distribution network operator for your area must be informed before the system is connected. Installers certified under the Microgeneration Certification Scheme (MCS) work to these requirements as part of their normal process.

When to use this estimate, and when not to

Use the calculator to compare options and to understand what drives the size of a battery. Do not treat its output as a final design. It does not model the hourly profile of your household, the seasonal output of your panels, your tariff structure, or the charge and discharge limits of your inverter.

For a final design, an installer should measure consumption hour by hour rather than relying on a daily total, check the battery's compatibility with the inverter and inspect the existing electrical installation, including the consumer unit. Off-grid systems, which need a full energy balance over a whole year and often a backup generator, go beyond what this calculator is designed to do.