Calculating DC power usage

I am just wondering what voltage to use in this situation when calculating the power consumption of a circuit. (P=VI)

I have a 24V DC power supply with 2 x 3V DC motors in the circuit. I have transformed to voltage to suit the motors and have an Ammeter recording the current.

Would it be correct to use the 24V (supply) in the overall circuit power calculation or do you use 3V as that is what the motors are running at?

Thanks

• Depends if you want the motors power consumption or the entire circuit consumption. Also a 3V DC motor doesn't mean the motor is 3V all the time, it's just the maximum recommended operation voltage. If you want to vary the speed you vary the motor's voltage. May 31, 2015 at 2:19
• You can use the current from the 24V supply and the 24V. Your "transforming" of the voltage from 24->3 has unknown efficiency. May 31, 2015 at 2:21

If you wanting a figure for power you must measure current and voltage at the same place.

measureing the current at the 24V supply and multplying by 24V will give you the input power that the system needs.

measuring the current at the motors ans multiplying by the 3V the motor supply provides will give the power used by the motors only.