Thanks @sihui 
The Sonoff Pow module is quite new and there was no firmware available yet. However, I contacted arendt, the developer of the Sonoff alternative firmware and he was amazingly quick and professional in adding support for the Sonoff Pow:
@hipohop If you are interested in the Pow and are not afraid to do what was described in the article linked by sihui, it’s a pretty good choice to get your consumption measurement started
Update: I just remembered, that this could be of interest to @TommySharp. Check here for more details.