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Knowledge Centre
Battery Storage: Usable Capacity vs Power vs Energy Explained
Capacity tells you how much electricity a battery can hold; power tells you how fast it can deliver or absorb that electricity — and usable capacity is usually less than the headline number.
Energy (kWh) is not the same as power (kW)
Think of a battery like a water tank. Its capacity in kilowatt-hours (kWh) is the size of the tank — how much energy it can hold. Its power in kilowatts (kW) is the size of the tap — how fast it can fill or empty. A large tank with a small tap can store plenty but cannot release it quickly; a small tank with a large tap empties fast but runs out sooner.
Usable capacity vs nameplate capacity
The headline capacity printed on a battery is usually the total stored energy. Manufacturers keep a reserve of that capacity to protect the cells from deep discharge and to maintain longevity over thousands of cycles. The figure that matters for daily use is the usable capacity — the amount the system will actually let you draw.
Depth of discharge
Depth of discharge is how much of the battery's stored energy is used in a single cycle. Regularly using a high proportion of the battery stresses the cells and shortens life. Limiting the depth of each cycle is one reason batteries reserve part of their total capacity.
Why bigger is not always better
A battery only saves money if it is charged and discharged regularly. An oversized battery that sits partly empty is expensive capacity you are not using. We size storage against your actual consumption pattern — when you use power, when you generate it, and what tariff you are on — rather than simply fitting the largest unit possible.
AC-coupled and DC-coupled systems
AC-coupled batteries connect to your existing wiring on the AC side, which makes them easy to retrofit to an existing solar array. DC-coupled batteries share the solar inverter on the DC side, which is usually more efficient for new installations where the battery and panels are designed together.
Related service
If you are considering a project, read more about our Battery Storage design and installation service.
Last reviewed
20 August 2026
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