Continuous power rating: what the kW figure limits

The continuous power rating is the most the battery is meant to supply for a long stretch. It limits what you can run at the same time. It doesn't say how long. That's the job of capacity.

Power, not storage

Electric power is the rate of transfer of electrical energy¹. A continuous rating in kW is therefore a speed limit. Capacity in kWh, covered in kW versus kWh, is the size of the store.

A big battery with a modest continuous rating can hold plenty of energy yet only run a few things at once. Switch on too many and the battery can't keep up, however full it is.

Continuous versus peak

Spec sheets often list two power figures. The continuous figure is what the battery can hold up for a long time. The peak figure is higher and lasts only briefly.

One example from a grid-scale battery plant: it had a peak output of 1.6 MW and a continuous output of 1.0 MW². Home products can show the same split. One home battery described on Wikipedia was listed with a higher peak power for short bursts than its continuous output³. The sheet for your own product states how long a peak may last.

Peak ratings matter because some devices draw a burst when they start, such as motors in pumps and fridges. A product's own manual says what it can start. I won't quote a figure for a particular device.

Why there's a limit at all

There are usually limits on the rate at which energy can be transferred⁴. As the current rises, the battery, cables and receiving equipment all have more to handle, often in the form of heat and fire risk⁴. A power rating is how the maker draws the line.

Checking your loads

Each appliance label shows its power in watts or kilowatts. One kilowatt is a thousand watts. Add up the items you'd run together and compare the total with the continuous figure.

• If the total is under the continuous rating, the battery is working within its limit.
• If it's over, something has to be switched off, or the battery may shut down or cut output.
• Check the energy side as well. A small total still drains the kWh over time.

Who does the connecting

Connecting a battery to a home's electrical system, including backup circuits and switchgear, is licensed electrician work. Leave it to one. Never feed power back into a house through an ordinary outlet. Local rules vary, so ask your electrical authority or read the maker's installation manual.

Power is only half the story. For the energy side, read about what a battery can really deliver.