I've been doing this for more years than I can remember and can tell you from professional and personal experience that buying a PSU that just gives you as much wattage as you need will come back to bite you in the arse later when you start having problems with your stability and performance. Most people do not buy and build a gaming rig for their minimum load. Also, 50-80% load is the sweet spot on a PSU when you are talking efficiency. At 100% load on the PSU, you will get less and also shorten the life of the PSU. This is why you get a PSU rated higher than what you need by 200 watts or so. Then when your system is at full load in gaming, you are only running the PSU at the 50-80% load and benefiting most from the efficiency of the PSU. At the same time you are not maxing the PSU at 100% which shortens the life of the PSU and also gives a much greater chance for stability problems. It is that simple. This is the advice I've been giving out for PSUs for years and regardless of the efficiency ratings and technical terms, that advice still holds true, even now.