Let's say that your PSU is exactly 70% efficient while pulling 139W from the wall outlet. If that were the case, then your computer was pulling 97.3W from the PSU (multiply 139 x .70). If it was 90% efficient while pulling 139W from the wall outlet, then that would mean that your computer pulled 125W from the PSU (multiply 139 x .90).
I feel that this pretty much sums it up. I see that it says "Standard Efficiency" on it. I don't know what they mean by that, but I would bet that it's not 90%! It might be 70 or 80%.
We also don't know what its +12V capacity is. Without knowing that, I don't know if this could be considered to be a Continuous or a Peak PSU. You see, I know that it shows two 16A +12V rails, but those are like trip points, which means that we can't just add the two rails together to find the +12V capacity. The only way to know the +12V capacity is by finding out from the OEM manufacturer of this PSU is, whoever it is (hopefully it's Delta!).
I can make some guesses though. If the 320W capacity is the PEAK capacity, then the +12V capacity is probably about 19 to 20A, which would be about 230-240W. If that were the case, then this would really be a modern-day equivalent to a 250W PSU since the biggest concern in a modern computer is the 12V power consumption.
If the 320W capacity is the continuous capacity, then the +12V capacity is probably closer to about 290W, or really, 24A which is 288W. Or, it might even be 25A, which is 300W. We will never know. If the 320W rating is a continuous rating though, then we could probably call this a 400W peak-rated PSU. However, I sincerely doubt that it has a 400W peak capacity. It's probably 320W, and a PSU's peak capacity can really only be pulled for a few seconds at most, unless it's a super high-end PSU.
I also find it to be suspicious or strange that they don't list the +3.3V and +5V capacities. heh