Home batteries explained: kWh, kW and backup

A home battery stores electricity for use later. For most solar homes, that means keeping surplus daytime solar for the evening and overnight. Here is what the key terms mean and what to consider.

Capacity is measured in kWh

Capacity, in kilowatt-hours (kWh), is the amount of energy a battery can store. Check the usable capacity, which can be lower than the total figure, because some energy is reserved to protect the battery.

Power is measured in kW

Power, in kilowatts (kW), is how much electricity the battery can supply at any moment. It determines how many appliances it can run at once. As a simple illustration, a battery with 10 kWh of usable capacity could in theory supply 2 kW for about five hours, or 5 kW for about two hours. Real-world results are a little lower because of conversion losses.

Backup during outages is not automatic

Many people assume a battery will keep the lights on during a blackout. That only happens when the system is designed for it. Backup depends on the battery and inverter, how circuits are arranged in the switchboard and how the system is installed. Some systems back up selected circuits only. If backup matters to you, say so at the start.

Adding a battery to existing solar

There are two common approaches. AC-coupled batteries have their own inverter and can often be added alongside an existing solar inverter. DC-coupled batteries share a hybrid inverter with the solar panels. Which suits you depends on your existing equipment, switchboard and plans.