Just some quick basic electronics for you...
[u]Ohm's Law[/u] determines the relationship between voltage, current, and resistance. Resistance is the resistance to current flow... More resistance means less current.
In most situations the voltage is approximately constant. For example, there is always 5V on the USB port and 120 or 240V at your wall outlet but no current flows until you plug something in and turn it on. A 1000W hair drier "draws" more current than a 100W light bulb and if you plug-in (and turn-on) two hair driers you'll draw too much current, the circuit breaker will blow, and the voltage will drop to zero.
You won't find a resistance spec for a servo motor, but you should have a current spec (Amps or milliamps).
Power (Wattage, related to heat) is calculated as Voltage X Current.