Power Inverter Sizing

Make a list of all the appliances you intend to run from the power inverter, and write down the wattage of each appliance.  Highlight all the appliances that you may need to have running at the same time, and add up their wattages to determine the maximum combined wattage that your inverter will need to support. Add 20% to get the continuous rating required. 
Continuous rating = maximum combined wattage + 20%.

For large appliances with motors, you will need to calculate a surge rating or a startup load. This allows for surges that occur during appliance start ups. Usually motor starts are 2x the wattage of the appliance and 3x if it is a large induction motor.
Start up load = (wattage of largest appliance x at least 2) and (wattage of largest induction motor x 3).