I2C power meter?

Hello,

I have a 18-22v 100w solar panel and I was wondering how could I use the arduino to log the instant power it supplies.
The idea is to be able to quantify how much power (in watts) am I getting with it and log that info somewhere (sd card or any other location via ethernet).

Since I'm amazed with how easy I2C works with the arduino, I was looking for an I2C power meter, but so far I could't find any.
Does anyone have any suggestion? Maybe other protocol like 1-wire?

Thanks in advance,
Joaoabs

Try:-
http://www.maxim-ic.com/datasheet/index.mvp/id/6454

www.ti.com/lit/ds/symlink/ina209.pdf

ics.nxp.com/literature/.../pdf/em773.smart.metering.training.pdf