I swear by the watts per pound. I'm an advanced pilot and I personally find that I like a minimum of 100 watts per pound unless its a cub type. I generally look for at least 125 to 150 watts per pound. Keep in mind that most of the folks that comment about my planes say that they are well powered. I've flown other planes in the 75-80 watt per pound range and they do fly OK but lack the aerobatic proformance that I like.

I look at the empty weight of the plane to get an idea of the wattage of the system I need, then I look at the battery that I'll need to carry to provide the current draw and duration, and then I look at the motor/prop/gearbox combo to get the most out of the battery.

John