Portable generators are sold according to how much power output they can generate, which is measured in watts. If you want to be able to power a range of essential domestic appliances in case of a blackout emergency, then you will require a minimum of 5000 watts.
The best way to figure out how much power you need before you go ahead and purchase a generator is to physically write down a list of all the essential appliances you would want to continue using. Add up that total wattage and go for a generator that meets those needs. As a handy reference, below are some of the most common domestic applianced and how much power they utilize:
Refrigerator - 600 watts
Portable heater - 1500 watts
Lights - 60 to 600 watts
Air conditioning unit - 1000 watts
Computer - 60 to 300 watts