The easiest way to explain the mechanics of a capacitor is to compare it to a battery; where energy is stored to either be released suddenly or over a period of time. The energy or capacitance of an electric capacitor is measured in the form of micro-farads. A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. It is used to maintain a charge. In AC units, there are dual run capacitors. One capacitor provides power to the fan motor. The other sends power to the compressor. Run capacitors measure in at approximately 7-9 micro-Farads. A 440 volt capacitor can be used in place of a 370 volt, as it is built better, but the 370 can’t be used in place of a 440 volt. It will work for a while and will fail prematurely, as exceeding the capacitor’s rated voltage will cause the dielectric to break down and the capacitor to short out. Same as an electric wire- 600V rated wire is better than 300V one .The capacitance(µF or MFD) must be the same or stay within 10%+ or 5%- of its original value . Example: 50 µF cap can be substituted by 47.5 to 55 µF with the same or better voltage ratings capacitor . Round oval-shaped run capacitors are commonly used for air conditioning, to help in the starting of the compressor and the condenser fan motor.