absolutely, dificulty depends on degree of accuracy you seek, ideally for watts you need the volts and the current to be measured, for most applications though given the volts are somewhat regulated the current is generally sufficient to give you an idea of load, of course you then need to know how much is going to the chrgine the battery and how much is running auxilaries, i basic way is to use a length of the feed cable to the altinator/generator as a shunt, then connect a light gauge wire at each end of the cable, a meters millivolts will be a ratio of the voltage drip along that piece of cable. you will need to do the math for your particular scenario.