Sizing a battery around an unrealistic worst-case outage is a common way to overspend. Here's a more grounded way to pick the number.
Check your actual outage history first
Most areas have a fairly predictable pattern of outage frequency and typical duration — utilities often publish this data, or your own experience over the past several years is a reasonable guide. Sizing around your area's typical outage, with some margin, is usually more cost-effective than sizing around a rare multi-week worst case that a generator backup could handle instead if it ever happened.
We've broken out sizing by duration
To make the math concrete across different scenarios, we've built dedicated sizing pages from a 12-hour outage up through a full week. Find the duration closest to what's realistic for your area, then refine with your actual essential-circuit load in the full battery size calculator.
Average load determines energy; surge determines whether things start at all
Total capacity needed scales with average continuous load over the outage duration — but don't stop there. Any appliance with a compressor or motor (fridge, well pump, AC) briefly draws several times its running wattage at startup, which is a separate check against your inverter's surge rating, not your battery's total capacity. See the surge watts calculator for that piece.
Consider a hybrid approach for rare extreme outages
Sizing a battery for your typical outage, with a generator kept in reserve for the rare multi-day extreme event, often costs less in total than sizing a battery alone for the worst case that might happen once a decade. See the battery vs generator cost comparison for how that trade-off plays out over 10 years.
Frequently asked questions
Should I size my battery for the worst outage I've ever had?
Not necessarily — sizing for your area's typical outage with reasonable margin is usually more cost-effective than sizing for a rare extreme event, which a generator kept in reserve can handle instead if it ever occurs.
Does solar recharging really extend battery backup meaningfully?
Yes, for daytime hours during a multi-day outage — a battery paired with solar can partially recharge each day rather than simply draining continuously, which can meaningfully extend how long essential circuits stay powered.