You are correct. You are paying for watts. Look at the formula. Volts times amps = watts. In my example, 220 watts either case. 220 volts comes into play with heavy current loads or long distances. It is the amps that cause voltage drops. Voltage drop =amps times resistance of wire in ohms. Smaller wire has more resistance.
If you are running cable a long distance to a pond you either have to go to a higher voltage or larger cable. That is how electricity is delivered over long distances. It is stepped up to several thousand volts at lower current and the transformers in local neighborhoods step it back down to 220 volts. The power(kilowatts remains constant except for loses in transformers etc. I know this is a pond forum, but since I dont have much experience in ponds, if anyone has any questions about electrical devices I may be able to help.