Nameplate capacity and usable capacity are two different numbers

Two numbers on the same box

Every home battery has at least two capacity figures, and the larger one is the one on the advertisement. Total or nominal capacity describes the energy the cells contain when fully charged. Usable capacity describes the portion the management system will actually let you take out, and it is always the smaller of the two.

The gap between them is not a trick, though it is presented inconsistently enough across the market that comparing two products honestly takes some care. Some manufacturers quote usable capacity prominently and mention nominal in a footnote. Others do the reverse. When comparing, insist on comparing usable against usable, because that is the number that corresponds to anything happening in your house.

The difference is rarely trivial. A reserve of a tenth or more of nominal capacity is ordinary, and on some chemistries it is larger.

The reserve exists for good reasons

Lithium cells degrade faster at the extremes of their state of charge. Sitting at full charge accelerates chemical ageing, and discharging deeply stresses the electrodes in ways that shorten cycle life. Reserving both ends is the cheapest available method of making the battery last longer, and it costs the manufacturer nothing except a number on a specification sheet.

There is a second reason that has nothing to do with ageing. A battery pack is many cells in series, and they do not have identical capacities, so a management system spends part of its life balancing them. It needs headroom at the top to do that, and it needs a margin at the bottom so that no individual cell is driven below its safe floor while the pack as a whole still reads as having charge left.

A third reason is that the battery must keep its own electronics alive. Communications, sensing and thermal management still draw power when the pack reports empty, and a genuinely flat lithium pack can be difficult or impossible to revive.

Energy and power are separate specifications

Capacity in kilowatt-hours says how much energy the battery holds. It says nothing about how quickly that energy can come out, and that is a completely independent limit expressed in kilowatts.

This catches people out on the day the oven, the kettle and an electric shower coincide. A battery with generous capacity but a modest power rating cannot cover that spike, so the house imports from the grid regardless of how full the battery is. The stored energy is there; the door it comes through is too narrow.

Charge power is limited too, often to a lower figure than discharge, and often reduced further when the pack is cold or nearly full. On a bright day with a large array, that can mean the battery cannot absorb everything the roof is producing, and the surplus goes to export whether you wanted it to or not. Any sizing exercise that considers only kilowatt-hours has ignored half the specification.

Standing losses run whether you use it or not

A battery system is not passive when it is idle. The inverter or hybrid unit, the battery management electronics, the communications link and any cooling all consume power continuously, and that consumption is drawn from somewhere — from the battery, or from the grid, but always from you.

On a system that cycles daily, this parasitic draw is a small tax on a large flow and barely registers. On a system that sits mostly full because the household load is small, or on one installed principally for backup and rarely cycled, the same draw is a constant cost against very little benefit. It is one of the reasons an oversized battery performs worse than its specification suggests, not merely no better.

Cold weather compounds it. Lithium cells lose usable capacity and accept charge more slowly at low temperatures, and packs installed in an unheated garage or outdoors in a cold climate may spend real energy warming themselves before they can work properly.

Size the battery to the evening, not to the array

The instinct is to match battery capacity to what the roof produces. That is the wrong anchor. A battery earns its keep by shifting energy from when you have it to when you need it, so the quantity that matters is the load between sunset and sunrise, not the surplus at noon.

Work out roughly what the house consumes overnight, on an ordinary day rather than the worst one, and you have the honest upper bound on what a battery can usefully store. Capacity beyond that is capacity that will sit unused most of the year, paying its standing losses and ageing on the calendar while it waits.

Undersizing is the less costly mistake. A battery that empties every night is a battery being fully used, and every cycle it completes is a cycle earning whatever the local tariff difference makes it worth. A battery that never falls below half is telling you plainly that you bought more than the house had a use for.

A note on comparing quotes

When two systems are quoted side by side, the comparison worth making has four columns: usable capacity, continuous power out, continuous power in, and warranted throughput. Nominal capacity belongs nowhere in that table.

Ask too whether the quoted power rating is continuous or peak. A short-duration peak figure describes a few seconds of motor starting current, not a heating element running for an hour. Both are legitimate; only one is relevant to whether your evening runs off the battery.