The fastest way to overspend on a battery is to size it to your total daily usage instead of your overnight critical-load usage, which is usually a third of the total.
The method
List what you actually want powered during an outage: refrigerator, a few lights, internet, and phone charging is typical. Add up the wattage, multiply by the hours of outage you’re planning for, and that’s your target capacity in watt-hours — not your whole home’s daily draw.
A worked example
A refrigerator (150W average), router (15W), and five LED lights (50W total) run about 215W continuous. For an 8-hour overnight outage, that’s roughly 1.7 kWh, well under a single standard battery module’s capacity in most residential systems.
When to size bigger
If you have a well pump, medical equipment, or live somewhere with multi-day outages, size to that specific load rather than the general critical-load estimate above.