Home Battery Storage

What a home battery is

A cabinet or wall unit containing lithium cells, a battery management system, and the electronics that connect it to an inverter. The management system is not an accessory — it monitors every cell’s voltage and temperature, balances them, and refuses to charge or discharge outside safe limits. It is the reason a domestic battery is a product rather than a pile of cells.

How storage changes a solar system

Without storage, generation you cannot use at the moment it is produced goes to the grid. Export is paid, but at a rate well below what you pay to import. A battery moves that surplus a few hours forward, from midday when nobody is home to the evening when the oven, the kettle and the television are all on.

That is the solar case. There is a second, separate case that has nothing to do with solar: on a tariff with a cheap overnight window, you can fill the battery from the grid at the low rate and run the house from it through the expensive part of the day. In a UK winter, when generation is small, this is often where most of the saving comes from.

A well-controlled battery does both, switching between them according to forecast and tariff.

Capacity, power and the difference between them

Two numbers describe a battery, and confusing them causes most of the disappointment we are asked to investigate.

Capacity (kWh) is how much energy it holds. It determines how long the battery can cover demand.

Power (kW) is how fast it can charge or discharge. It determines what it can cover. A battery with a modest power rating may hold ample energy but be unable to supply an electric shower and an oven simultaneously, so the grid tops up the difference.

Note also that usable capacity is lower than nominal capacity, because the management system holds back a reserve at both ends of the range to protect cell life. Compare quotations on usable figures.

Coupling

DC-coupled through a hybrid inverter, where solar charges the battery with one conversion. More efficient, and the normal approach on a new installation.

AC-coupled through a separate battery inverter, which converts AC back to DC to charge. A few per cent less efficient, but the right answer when you already have a working solar inverter and replacing it would be wasteful.

Suitability

A battery suits a household that exports a lot, pays a high import rate, or has access to a time-of-use tariff. It suits homes with an EV or a heat pump, where consumption is large enough to make the arbitrage worth having.

It suits poorly a household that already uses nearly everything it generates during the day, or one on a flat tariff with a good export rate and low consumption. In that case the honest advice is to spend the money elsewhere, and we give it.

Installation implications

Weight and space come first — these are heavy units needing a sound wall or solid floor and stated clearances. Temperature range matters: an uninsulated garage that drops close to freezing will restrict charging on some products. Cable routes to the inverter and to the consumer unit need planning, and adding storage may move the connection from a G98 notification to a G99 application requiring DNO approval before work starts.

Warranty and cost

Battery warranties are not simple periods. They usually combine a term, an energy throughput or cycle limit, and a retained-capacity percentage at the end. Read all three — a long term with a low throughput limit may expire on usage rather than on time.

Cost scales with capacity but not proportionally, since installation and inverter costs are largely fixed. That is why adding a second module is usually cheaper per kWh than the first. Your written quotation states usable capacity, power rating and the full warranty terms with the datasheet attached.