I have always been trying to calculate how much energy is consumed by each of the electrical appliances I have at my home. I had a first hand experience of Wattmeter, a device that calculates the wattage of a device at Electrical Systems Lab. during my second year of undergraduate studies. But I never dared to buy it, as an estimate cost of it was around USD 100. So, I would normally estimate it comparing it to ratings of a standard device.
But thanks to a lovely research initiative at Microsoft, a windows application called Joulemeter is a reality. Joulemeter can quickly estimate the power consumption of your computer based on the screen’s brightness, the microprocessor, etc. And now I can measure the wattage of my computer to be 74W. I still need to verify this, but as the application is vendored at Microsoft, I can have some faith on the data.
Note: You Need Microsoft VS DotNet Framework 3.5 for running the application