Well, power (measured in columbs) is still power. Here's the equation you are looking for capacitance * voltage= power Put your numbers in and whichever combination yields the larges power, is the one you should use. Here are your numbers-> 120uf * 300v = 36,000 or 1000uf *50v = 50,000 As you can see, the lower voltage setup will be better. Typically, a 50v 1000uf capacitor will be significantly smaller than a 300v 1000uf capacitor. Also keep in mind if you did go the higher voltage route, you are going to have a hard time safely making 300v. Much easier to stick with the lower voltage anyway. Remember anything over 30V can SHOCK you and capacitors arc and can KILL, so no touching.