The kilowatt (kW) rating of a generator determines its output. You can choose between portable models at 3 to 15 kW or stationary models at 7 to 45 kW. The average home uses from 10 to 12 kW without an air conditioner. But your home may be different, so you will need to calculate in terms of how much equipment you will use at the same time.

Make a List

For an emergency generator to be used only in power outages, you need to decide what you want to protect. If the generator is to be used as a primary power source, you will need to inventory everything. Air conditioners, heaters, stoves and dryers use the most power, but list only the things you will likely run with your generator. Don't forget small appliances like light bulbs, cordless phone, TV, computer and battery chargers.

Include the Wattage

Every electric appliance has a tag showing how many watts or kilowatts of power it uses. But some only show voltage and amperage. To determine wattage, simply multiply voltage times amperage. For light bulbs, just add up the wattage. Ten 100 watt bulbs is 1000 watts or 1kW. Add the wattage to everything on your list. Don't confuse kW with watts.

Maximum Calculated Load

Everything on your list will not all run at the same time. To calculate your maximum power load, add up the power of the most equipment you will likely use at the same time. Since a generator works harder at full load, select a size about 10% to 20% larger than your maximum calculated load, just to be on the safe side. If you have any questions, consult an electrician.

Source:

Sizing Residential Generators

Sizing: Determine Your Power Needs