All power supplies are running at peak efficiency at around 50% load. Also factor in if you want to potentially upgrade to a higher wattage Cpu/Gpu in the future.
so calculate your total system wattage. and double it to find the ideal PSU.
IE if you calculate 250 watts, find a 500ish watt PSU. If you plan on upgrading in the future maybe pick up a 600-650 watt PSU So that your current rig is still pretty efficient now and in the future.
Besides wattage, the 80+ bronze,silver, gold ratings just mean that they are even more efficient and will run cooler and probably last longer.
I would suggest 80+ Bronze as a minimum because it runs 85% efficient at 50% load and they usually don't have a huge price premium.
https://en.wikipedia.org/wiki/80_PlusThis post was edited by kclla on Oct 7 2017 11:41am