Wattage pick for longevity

I'm about to buy a new gaming system and would like to get a solid PSU with it. I recently figured out that most of the ATX PSUs are most efficient while load is around 50%. Assuming *cough* the PSU will last the longest if it ever only gets to output around 50% of it's rated capacity, I could calculate the total power draw of the system during gaming and then double this amount to get a wattage number for a PSU that would basically always run at 50%, give or take a few %. Is this a good approach to picking a PSU? Does this make sence or is this wrong? Or badly explained?