Generator wattage calculator
List what you want to run at the same time and its running watts — printed on the appliance's label or in its manual (watts = volts × amps).
Reading the labels
The label usually shows watts, or volts and amps — multiply them (a 120 V, 5 A appliance uses 600 W). Anything with a motor — fridges, freezers, sump pumps, furnace blowers, power tools — needs several times its running watts for a moment when it starts; the label or manual may list these starting watts. Generators list two numbers: running (rated) watts and starting (surge) watts. Your total running watts must fit under the first, and the largest motor's start must fit under the second. Starting motors one at a time helps.
Generator safety
Generators make carbon monoxide, which you cannot see or smell and which kills. The US CDC says: never run a generator inside your home, garage, basement or any enclosed space, even with the doors and windows open, and keep it outdoors more than 20 feet (6 m) from windows, doors and vents. Use a battery or battery-backup carbon monoxide alarm. Never plug a generator into a wall outlet ("backfeeding") — it can kill utility workers; use a transfer switch fitted by an electrician.