The easiest way to calculate the amount of power you need is to use an online calculator that asks you questions about the number and types of appliances you have, details about your heating and air conditioning system, and information about the square footage of your home. While this is normally enough to be in the right ballpark of how much power you need, the most accurate way to calculate power needs is to add up the wattages of all the electricity-drawing devices and appliances in your home.