battery stats in watts?

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http://www.linuxquestions.org – I'd like an app, whether graphical or command line, that will tell me my laptop battery's charge or discharge rate in watts. The goal is to calculate my needs for a solar panel or other off-grid energy source. It doesn't have to run continuously, typing something on the command line and getting watts usage would be acceptable. I do want it to display watts as that makes my mental gymnastics about power requirements a lot easier. I could probably fudge all the numbers on paper myself but I am lazy and see the purpose of an "app" as to do this sort of work for me. (HowTos)