Out of curiosity, what are you running (CPU, video card, etc) and what figures did the manufacturer list for power consumption of your system? It's usually possible to estimate the peak/average power consumption of a system based on the components.
In reality, most manufacturers tend to overstate numbers by listing the max power consumption numbers vs. averages. For example, while my CPU alone is rated at 85W, I've found that on average my entire system actually only consumes about 65W.
The larger the max power draw, the more overstated these numbers tend to be. For example, when I tried swapping out my 85W CPU for a 35W CPU, the average power consumption of the system only dropped by about 10W - much less than the 50W difference you might expect to see looking at the manufacturers TDP ratings. My takeaway from the whole experience is that unless you are dealing with some specialized application - i.e. an always-on file server - it's not really worth sacrificing CPU power for power efficiency.