Because current is inversly proportional to power.It's basic Ohm's law.
Let me see if this will work.
P

I * E
P,being power is over E(voltage) and I(current).In this formula,the current and voltage are proportional,but both are inversly proportional to power.
You can use this formula to find Power by multiplying I * E.You can also use it to find the current(I) if you know the Power and (E)Voltage by dividing the E(Voltage) into the Power(watts).
Or,if you know the power and the current(I),you can find the voltage by dividing the current(I) into the Power.Hopefully the formula above helps.
