I think it used to be that PSU efficiency went way down as the load went down, so if you were using a 1000W PSU, but drawing only 150W of power, its efficiency would be crap -- you'd be, say, drawing 300W from the wall outlet to make 150W of power for the computer. So, the PSU would be delivering 150W of power to the computer (and generating 150W of heat through useful work) plus an additional 150W of power into just waste heat.
80 plus PSU's, though, are at least 80% efficient down to 20% load, meaning if you are drawing 200W from a 1000W PSU, it is drawing at most 200/.8 = 250W from the wall, so not bad.
However, 80 plus generally isn't rated at 10% load. So, if you have a 1000W PSU, but draw less than 200W normally, it's not clear what the efficiency is.