[jreuter]

Richard:

With regard to climb rate modeling, there is a simple approach that skips the conversion to energy.

For your example of 22.5 W net output power (two motors @ 15W x 75% efficiency:

Converting units gives 22.5W * 0.7375 = 16.6 ft-lbf/s

Divide by the weight (in pounds) that you want to lift, and you get the theoretical top speed
16.6 /5 lb = 3.3 ft/sec,
or 2.8 seconds to climb 110 inches.

The trick, as you point out, is to get the right wheel radius and gear ratio to get to the peak power point for your weight. The estimate of efficiency is key, since it needs to include all sources of friction/torque. As I learned (again) this year, mis-estimating this can put you near stall, and smoke a motor.