You could do a test of the drag by measuring the spindown of the wheel suspended off the ground. First, measure the moment of inertia of the wheel with a torsional pendulum. Then impart spin to the wheel and measure the record the angular velocity of the wheel over time to calculate get the instantaneous deceleration at various angular velocities. Add the lights and do the same. The differences in the decelerations will give you the torque required to drive the lights at those angular velocities.

Torque = moment of inertia x angular acceleration. Etc...