Battery Storage Basics

A home battery stores electrical energy chemically and releases it when the home needs it, typically in the evening after solar production has fallen. Two figures describe it: capacity, measured in kilowatt-hours, which is how much energy it can hold, and power, measured in kilowatts, which is how fast it can deliver that energy. A large-capacity battery with low power can run lights and a fridge for hours but cannot start a large appliance, while a high-power battery with small capacity can deliver a burst but empties quickly.

Usable capacity is smaller than nameplate capacity, because batteries reserve a margin to protect their lifespan, and that margin varies by chemistry and maker. Depth of discharge, cycle count and warranty terms together describe how much of the battery can be used regularly and for how many years.

In a solar home, the battery charges from surplus panels during the day and discharges in the evening, which increases self-consumption and reduces evening grid imports. Some systems also charge from the grid overnight on a cheap tariff, then discharge at peak price times, which is a different use pattern that depends entirely on tariff structure and may not suit every household.

Backup power is a separate feature, not automatic. Many batteries shut down during a grid outage for safety unless they include islanding equipment and a dedicated backup circuit, and even then backup usually covers selected circuits, not the whole home. Location matters too: batteries need ventilation, temperature limits, fire separation and access, and some makers restrict garage, loft or outdoor siting.

Measure evening demand honestly before sizing: note which appliances run after sunset, whether an electric shower, oven or heat pump runs in the evening, and how that pattern changes in winter when solar surplus is small. Ask for usable capacity, continuous and peak power, round-trip efficiency, expected cycle life and the warranty in both years and throughput.

Also check the physical space, weight, ventilation and fire guidance for the proposed location, and whether the existing consumer unit and inverter suit battery addition or need upgrading. A battery added later to an incompatible inverter can cost far more than one planned from the start.

In the EU, battery installation, safety and recycling rules are set nationally and through EU product and waste rules, with member states differing on fire separation, location restrictions and whether batteries qualify for support schemes alongside solar. Recycling and end-of-life duties for lithium batteries are an EU-level concern, but collection arrangements are national.

In the UK, MCS covers battery storage in certified solar installations, building regulations and fire guidance govern siting, and electrical safety rules govern connection. Some local fire services and insurers have their own expectations for battery location, so those should be checked for the specific property and insurer.

A common mistake is buying capacity for the largest imaginable evening, which then sits half-used most nights and degrades from calendar ageing rather than useful cycling. Another is assuming backup power is included, then discovering during an outage that the battery is designed to shut down with the grid.

A third mistake is ignoring round-trip losses: charging and discharging lose energy as heat, so a kilowatt-hour stored is worth less than a kilowatt-hour used directly. The battery case should use usable, after-loss figures, not nameplate capacity. Another battery question concerns end-of-life handling, including who removes and recycles the battery, what that service costs and whether the maker offers a take-back route, since a battery bought without an exit plan can become an awkward liability long after its useful cycling life has ended.

Keep the battery data sheet, the usable capacity and warranty terms, the siting and ventilation specification, the inverter compatibility note and the commissioning record. Those papers matter for insurance, for a later battery replacement and for any future buyer.

Ask a qualified installer: what usable capacity and power does this battery offer, what circuits get backup if any, where will it sit and why is that location suitable, and what does the warranty cover after ten years. Specific answers show a sized battery, not a generic add-on.