Here is a fictitious system in idle/load conditions demonstrating my point that cards can draw more than what the kill-a-watt difference shows:
Idle: 130W at the wall, power supply outputs 95W (73% efficient) = 15W to the video card + 80W to the rest of the system
Load: 200W at the wall, power supply outputs 176W (88% efficient) = 96W to the video card + 80W to the rest of the system
yochdog would conclude from the kill-a-watt readings that the card under load is drawing at most an extra 70W (200-130), but in fact, it is drawing an extra 81W (96-15). This is why subtracting the idle from the load wattage at the wall is not as accurate as you all seem to think.
At the very least, if a kill-a-watt is all you have, I suggest you:
- publish your power supply specs to look up the energy efficiency curve at different loads
- measure "idle" condition with the card physically removed from the system
While I get your desire to have 100% accurate info, I don't fully agree that it is more useful.
1- virtually everyone is going to be running mining rigs on computer power supplies, which are never 100% efficient
2- knowing the
actual real world power usage from the wall is very useful for calculating cost of mining, while knowing the
exact power usage from the power supply is an interesting mental exercise it doesn't actually benefit anyone who is merely trying to do a cost/benefits analysis.
3- most miners use 80+ certified power supplies, if not 80+ silver or greater spec. The difference between actual power usage and exact power usage is going to be less than you indicate. For example, 80+ silver gets between 85% and 88% efficiency guaranteed, a 3% difference even on a 1000W scale is only 30W and not really a huge inaccuracy.
Dropping 40-50C will certainly save a good chunk of power. 
