Stepper Motor Power Calculations

Hi, I'm new to this board so I've been watching for a couple of days, and it
looks like the discussion gets pretty technical, so I'll run this one up the
flagpole:

I recently built a stepper motor control box for PC parallel-port stepper
control using some motor controllers that incorporate PWM chopper-type
current regulators for the motor phases. The regulators operate at a range
of voltages, so I selected a power transformer within the voltage range of
the regulator with a current rating equal to the sum of the current
requirements of the motor coils. But I think I may have overkilled it using
this approach.

Now that I think of it maybe I should have calculated the power consumption
(watts) of the motors at maximum current, and used that to determine the
current rating of the transformer, since the motors run at a lower voltage
than the transformer supplies.

For example, if the motor coils are rated at 3 amps max, and the resistance
is 2 ohms, the coil will drop 6 volts when running at max current, which
means they are dissipating 18 watts. If I have four of them that's less than
80 watts total worst case. A transformer with a 20-volt secondary would
therefore need only supply 4 amps, not 12 as in the simple-minded approach.
Oh-well, live and learn.

I guess I was thinking the chopper acts like a linear voltage regulator, but
on reflection I think it acts more like a transformer, and so the power
calculations would apply. Has anyone seen a difinitive answer to this
question? Thanks,

--Phil M.