Capacity and power are two different numbers
Battery capacity is measured in kilowatt hours and tells you how much energy is stored. Power is measured in kilowatts and tells you how much the battery can deliver at any one moment.
Both matter, and they fail differently. Run out of capacity and the battery empties sooner than you expected. Run out of power and the battery trips when too much switches on at once, even though it is nearly full. Air conditioning, electric water heating, pool pumps and electric ranges are the loads that usually decide this.
Whole home backup and partial backup
Partial backup, sometimes called a critical loads panel, moves a chosen set of circuits onto the battery. Refrigerator, some lighting, internet, a few outlets, sometimes one air conditioning zone.
Whole home backup carries everything, which requires considerably more battery and usually more inverter capacity.
Most homeowners are not certain which of the two they have. It is worth finding out before you need it, and the answer is in your proposal, your electrical drawings, or written on the backup panel itself.
Reserve settings
Many systems let you hold back a percentage of the battery purely for outages, rather than using it every evening. A higher reserve means more protection during a storm and less everyday benefit. A lower reserve is the opposite.
Where supported, this is adjustable, and it is reasonable to raise it when a storm is forecast and lower it again afterwards.
What actually happens during an outage
- The system disconnects from the utility and switches your backed up circuits over. On most equipment this takes a moment and some devices will blink.
- Anything not on the backup panel stays dark. This surprises people more than anything else about owning a battery.
- If the system was configured for it, your solar can recharge the battery during daylight, which is what makes a multi day outage survivable. Not every installation is set up this way.
- Charging from solar during an outage will normally be limited by how much your home is using at the time.
Common alerts and maintenance
Batteries are largely maintenance free, but they are not maintenance immune.
- Keep the area around the unit clear, dry and ventilated. Do not stack storage against it.
- Temperature warnings in a hot garage are worth reporting rather than dismissing.
- A battery that stops reaching full charge, or empties much faster than it used to, is telling you something.
- Communication faults usually mean the reporting has failed, not the battery.
Expansion
Some platforms accept additional modules or a second unit; some are fixed at their original size. Whether yours can grow depends on the battery platform, the inverter it is paired with, and the space and electrical capacity available. Have the model numbers to hand before asking, because the answer is entirely equipment specific.