It depends on various on what device you are trying to run and how much energy it uses. At full capacity 200w inverter will use 25% more amps than 150w inverter. You can use this formula to calculate the amps or current drawn by any inverter of any capacity.  Current (amp) = Watts/Volts For example, if your inverter is 200 watts and has a voltage of 10v then the current drawn will be = 200w / 10v = 20amp