How Much Juice Do You Need?

The best way to begin comparing battery backup power and gas-generated systems is to calculate your home's emergency electrical needs. Start by making a list of every appliance, fixture and device that you want to keep going in the event of an outage. On each item, look on the manufacturer's nameplate or stamp to find the wattage (watts) rating. If only the amperage (amps) is given, multiply the amp rating by 120 to find the wattage.

Note: For any appliances that use a motor (including refrigerators, furnaces and heat pumps, air conditioners, fans, etc.), you'll have to find the startup wattage and use it instead of the rated wattage on the nameplate (a.k.a. running wattage). Startup wattage is the amount of power it takes to get a resting motor up and running and can be several times higher than the running wattage.

Add up the total wattage of all the items, then add in 10 to 20 percent as a safety margin: That's how much power your generator must provide to meet your needs.

Based on the factors mentioned above, do some calculations and figure whether you're better off with a battery backup power or gas-powered backup.

Source: http://www.networx.com/article/battery-backup-power-vs-gas-generators