The easiest way to size backup battery capacity is to start with the loads you actually want to run, not with the battery first. Make a list of each device, its running watts, and how many hours you expect to use it during an outage. Then multiply watts by hours to get watt-hours. Add those numbers together, and that gives you a rough daily energy need.
For example, a typical fridge might average 80 to 150 watts over time, even though the compressor starts with a much higher surge. If you want to cover a fridge that averages 100 watts for 10 hours, that alone is about 1,000 watt-hours. Add four LED lights at 10 watts total for 6 hours, another 60 watt-hours. Add phone charging and a small fan, maybe 100 to 200 watt-hours more. In that simple case, you may already be near 1,200 to 1,300 watt-hours for a single day of backup.
A useful rule is to size the battery bigger than your exact math suggests. Real-world systems lose some energy through inverter conversion, wiring, and battery limits. If you use lithium batteries, you can usually use most of the rated capacity, but it is still smart to leave a buffer. If you think you need 1,200 watt-hours, a battery around 1,500 to 2,000 watt-hours is often a more comfortable choice. That gives you breathing room for startup surges, cloudy weather if you are recharging with solar, and the fact that some appliances use more power than the label suggests.
If you only need short-term backup for phones, internet gear, and lights, a smaller unit in the 500 to 1,000 watt-hour range can be enough. If you want to keep a fridge running and still have power left for other essentials, 1,500 to 3,000 watt-hours is a much more realistic range for many households. If you want to run a freezer, CPAP, or multiple appliances for a full day, you may need even more.
Also pay attention to inverter size, not just battery size. A battery with enough energy can still fail to run your fridge if the inverter cannot handle the compressor startup surge. For backup use, I would check both the continuous watt rating and the surge rating.
If you want a practical answer, figure out your must-run loads, total the watt-hours for the longest outage you want to cover, then add 25 to 50 percent headroom. That is usually a much safer way to buy than guessing by battery size alone. People who have sized their own systems often say the same thing: the backup battery you regret is the one that is too small when the power goes out.
