Power Outage Calculator

Add up the wattage of everything you want to keep running, multiply by the hours of outage you're planning for to get watt-hours (Wh) needed, then divide by your battery/inverter's efficiency (typically 85%) to get the required capacity, since some energy is lost in conversion.

For a generator, size it to your total running wattage (the sum of everything running at once) plus headroom for starting surge — motors and compressors (fridges, well pumps, AC) can draw 2-3x their running wattage for a second or two when starting. Add 20-30% headroom over your calculated total running wattage.

It varies widely: a phone charger might use 10-20W, a refrigerator 100-800W (cycling on and off, not continuous), a window AC unit 500-1500W, and a well pump 750-1500W. Check the device's nameplate or manual for its actual wattage rather than guessing.

Consumer power banks are usually rated in mAh at a nominal 3.7V per cell. To compare a power bank's mAh rating to the watt-hours you need, use: watt-hours = mAh ÷ 1000 × 3.7. This calculator converts your required watt-hours into an equivalent mAh figure for that comparison.