When it works the power inverter will consume a part of the input power. The output power will be lower than the input power. That efficiency of the power inverter is the ratio of output power and input power. If the input power is 100 watts (from DC power), and the output power is 80 watts (for AC power). Then the efficiency of power inverter is 80%. When we using a power inverter, it is better not powered the device for more than 80% of the rated power, and not to be continuously used for a long time.