[jacquesm]

Browsers can influence the power usage of a computer but just having the thing on all day is a significant base load that needs to be met. Compared to that the websites that you visit likely have relatively little effect as long as the pages don't contain javascript that keeps on running, video or animated content.

How much difference there is between browsers then becomes a function of the different base-loads for the various OS's + the base load of using the browser in the first place + the efficiency of the rendering engine when rendering the pages under test.

Finding the power usage of the browser alone is going to be very difficult since you can't really run a browser without an OS and the various bits and pieces of hardware in the laptop which are not necessarily direclty related to running the browser.