"A 500W PSU won't necessarily draw more than a 250W PSU"
Mostly true, but not exactly. Most computer PSUs are more efficient when operating around 50% of their rated load. So if a computer consumes 125W internally, a 250W PSU would translate to lower power consumption measured at the wall than a 500W PSU, typically by about 2-5%.
For example see the chart https://www.sunpower-uk.com/files/2014/07/What-is-Effciency.... (115 VAC input) : 88% efficiency at 25% load, vs 90.5% efficiency at 50% load. In practice if the consumption is 125W at the PSU's DC output, this translates respectively to 142W vs 138W measured at the wall.
This 2-5% difference may not seem much, but it's similar to upgrading 1 or 2 levels in the 80 PLUS ratings (Bronze, to Silver, to Gold, to Platinum, to Titanium).
Mostly true, but not exactly. Most computer PSUs are more efficient when operating around 50% of their rated load. So if a computer consumes 125W internally, a 250W PSU would translate to lower power consumption measured at the wall than a 500W PSU, typically by about 2-5%.
For example see the chart https://www.sunpower-uk.com/files/2014/07/What-is-Effciency.... (115 VAC input) : 88% efficiency at 25% load, vs 90.5% efficiency at 50% load. In practice if the consumption is 125W at the PSU's DC output, this translates respectively to 142W vs 138W measured at the wall.
This 2-5% difference may not seem much, but it's similar to upgrading 1 or 2 levels in the 80 PLUS ratings (Bronze, to Silver, to Gold, to Platinum, to Titanium).