Usable capacity: energy for later. Measured in kilowatt-hours (kWh), this is the energy available to use from a charged battery. Compare it with your evening and overnight use, rather than selecting from the advertised total capacity alone. Keeping energy reserved for an outage leaves less available for everyday use.
Power output: appliances running together. Measured in kilowatts (kW), this limits how much power the battery can supply at once. Cooking and heating together can exceed an output that comfortably runs a fridge and lights. More stored energy does not automatically raise that power limit.
Backup: a separate electrical design. Ask for a written list of connected circuits, appliance restrictions and the equipment that separates your home from the grid during an outage. Confirm whether your solar can recharge the battery while the grid is off. How long backup lasts depends on the charge available and what you run.