If you were running a 1000W PSU at full capacity, meaning your components are pulling 1000W from the PSU, your Kill-a-watt would register somewhere close to 1175W (assuming your efficiency dropped to around 85% at full power). That's a lot of wasted energy due to heat.

So even if your kill-a-watt is sayin 950W, your PSU is probably only delivering 810ish watts.

The 80% rule is a very broad term that came from server PUDs and also applies to household fuses which will flip due to excess heat generated by running continously at over 80% of the circuits max load (usaully 15a or 20a for households).