Capacity tells you how much heat a battery can hold. Power tells you how quickly energy can move. Confusing kW with kWh can leave a system that is large enough on paper but too slow at the moment of demand.
The short answer
Divide the energy needed for a full charge, in kWh, by charging power, in kW, to get the ideal minimum charging time. Real charging may slow near the end or be limited by controls, supply capacity and the heat source.
Charging power
A store needing 12 kWh with a 3 kW input needs at least four hours in ideal conditions. That matters when the cheap tariff window is short or split into several periods.
Discharge power
Hot-water demand can be brief and intense. A shower or bath requires heat to transfer quickly, even if daily energy use is modest. Space heating may need a steadier output for several hours.
Match three clocks
- Tariff clock: when electricity is cheap.
- Charge clock: how long the store takes to become ready.
- Demand clock: when the home needs heat and how quickly.
The system works when those clocks overlap in a usable way. A large store that cannot finish charging is not large in practice. A full store with insufficient output power may not sustain the desired flow.
Watch the units in brochures
kW is a rate. kWh is an amount. Litres per minute describes water flow and still needs a temperature rise. Cylinder-equivalent litres need test assumptions.
Your next step
Ask the installer to show one busiest-day timeline with charge power, cheap hours, stored energy and peak demand on the same page.
Sources
Primary and independent references checked by our editorial team.
- Energy storage options explained — Energy Saving Trust Checked 10 Nov 2026