It's a little different than a cell phone charger taking 110V AC input and 5V DC out...
For the conventional motor controller, you can make the assumption that Iin = Iout. Here's Why:
If you look at conventional motor controller designs, the current that drives the motor flows from the input of the motor controller, through a large output device (Power MOSFET or BJT), through the load (motor, or bank of resitors), and then back into the motor controller, through another large output device, and finally, back out the negative battery input on the motor controller.
You can think of the output devices in this case like a switch. When the output devices are "on" (transistors are in the saturation region), they have a small resistance (this is what causes the voltage drop between the input and output of the motor controller). This resistance here is in series with the load. Kirchhoff tells us that current through all components in the loop is the same. Operating on the assumption that the SD540 is in fact built like most conventional motor controllers, the input current will be the same as the output current (assume extra current consumed for control circuits etc in the motor controller is negligible). You know the resistance of the resistor bank, and you know the total power output, so you can easily calculate current through the motor controller. Now, knowing Iin and the delta V across the motor controller, you can calculate the amount of power consumed in the motor controller. This power is dissipated as heat.
As you can see, from the 50A load test on the MindSensors site, this motor controller does, in fact, get HOT (125C after 5 minutes at 50A), and still climbing... Seems to me like a lot of energy lost to heat in the SD540, not to mention a potential safety hazard...
http://www.mindsensors.com/content/7...haracteristics