That concept is correct. However, if (example) a 500W PSU can actually output 500W or if it can only draw 500W and deliver a 500W*efficiency % inside the system, depends on the manufacturer of the PSU. Most generic PSUs state their maximum power DRAW, not output. So a generic 500W PSU with 75% typical efficiency can output only 375W max. Tricky, isn't it? The Power Trio 550W PSU is superior by every aspect, I see no reason why not to get that one.