Taking a Bosch 35 motor as an example; the motor's maximum power output is 35W and you want to run it at 12V. The current (I) is power divided by voltage;

So 35/12 = 2.91

This means your motor will be drawing 2.91A when running at 12V.
Ummm ...NO

Nominal voltage 12 V
Nominal power 35 W
Nominal current 9 A
Maximum current 24 A


The 35 watts is nominal ( the power it can produce all day long without over heating ) and the efficiency at 35 watts OUTPUT is probably going to be around 30% ..meaning that your 2.91 amps is 30% of the actual amps consumed to give 35 watts OUTPUT ..you actually INPUT is 9 amps....with 6 amps being wasted as heat.

PEAK Watts is around 45 @ 12v.

http://www.boschmotorsandcontrols.co.uk ... /index.htm

http://www.boschmotorsandcontrols.co.uk ... nlinie.htm