Here's where the scales tip toward gas generators for many homeowners. Since batteries can provide power only for so long before needing a recharge, they're most appropriate for short-term backup (anywhere from a few hours up to a couple of days, depending on the system and your usage). Gas generators will run as long as you need them to, provided you keep filling them with gas, and barring any mechanical breakdowns. Portable gas generators may need refills every few hours, while large stationary units can run for days on large propane or diesel tanks or a natural gas hookup. The limited supply of battery backup power has prompted some homeowners to keep a gas generator on hand to recharge the battery system during long outages -- a backup for the backup.