because the voltage rating just says how much the cap can handle, higher voltage rating the longer it lasts.
Not true!
Electrolytic Caps are rated for a Working voltage. You should run them at 20% under the rated voltage - when running new designs. IF you replacing caps in an existing design.. be at LEAST as good as the original part.
Running a 500V cap at 50V is a waste of cash and does
NOT prolong it's life:
"Among these factors, temperature (ambient temperature and internal heating due to ripple current) is the most critical to the life of aluminum electrolytic capacitors.
Voltage within the allowed operating range has little effect on the actual life expectancy of a capacitor. However in certain applications or misapplications, the applied voltage can be detrimental to the life of an aluminum electrolytic capacitor.
When in service at voltages equal to or below the rated value, the life of electrolytic capacitors is affected less by applied voltage than by operating temperature. [...] The curves show that the life of the capacitor has not been significantly increased by a reduction in voltage. "
http://www.chemi-con.com/u7002/life_ms.php
The best thing to do is spend the cash on higher temp rated caps... IE longer times at 120C instead of the cheaper low life at 80C.
For example; If I was designing a new Powersupply with a 5VDC input... I'd run the next higher voltage grade. usually 6.3V or worst case 10/16V. IF I know the caps going to be in a hot monitor or some other non-airflow enviorment... I'd get 120/150C rated caps with as long hours as I could.