Measure available Power

Hi all
I am doing a project of switching in between power sources using a micro controller, but now I first need to determine available power from Grid tie,Generator and also PV Solar ,I believe the method would be more the same for all of the them, whats the easiest way to measure available power from these sources?

If you anyone knows how to use the small reference cell method for measuring from PV Solar please enlighten me

Have you looked at the emonTx and emonPi?

The emonTx can measure the real power from your 3 sources - either operating singularly or simultaneously, and the emonPi can save and display the data as a web page. The emonPi could handle two sources, but not three, which is why you need the emonTx. Alternatively, if you can or wish to use, you could use the emonTx with an ESP8266 for WiFi, or an Arduino with the emonTx Shield and an Ethernet shield for wired, to send the data via your router - see the concurrent threads on this forum for more about the ESP8266.