Before obtaining an inverter, an analysis of power needs is required. Go around the house and write down all the appliances owned along with the wattage ratings listed on the appliance tags. Some of the more obvious appliances include refrigerators, microwaves and ovens. Also include lights, intermittent-use appliances like hair dryers and on-demand water heaters. Add up the wattage of each. This provides an idea of the maximum running wattage.
Calculate the average-per-day wattage use. The University of the Pacific states that the formula for average wattage is the quantity of a appliance times the running watts times the hours per day times the days per week divided by seven. This must be done for every type of appliance in the home. If you only use a hair dryer for 10 minutes a day, the hours per day usage will have to calculated. This would be 10 divided by 60 divided by 24, or .007 hours per day.
When an appliance first starts up, it draws a surge of power. this is especially true of appliances with motors, such as refrigerators, well pumps, or furnace blowers. This surge can be three to seven times more than the running wattage. For example, if a refrigerator has a running wattage of 800 watts, the surge wattage can be 2,400 watts.
To actually measure the surge wattage, DeAnza College cites noted energy author Ben Root when he states you need a meter with a memory capability, to record the momentary surge. If you are not comfortable measuring live electricity, a qualified electrician can do this for you. Size up an inverter to carry the momentary surge wattage. An electrical engineer can assist you with surge compensation analysis, since the equations are extremely complex.