my power measure is power1 = consumption, power2 = grid .
so I want to measure power1 - power2 for measure PV generation power.
any advice would be highly appreciated.
The power1pluspower2 addition is done in the “emon” front end of your emonPi, and the 3 values are passed separately into emonHub.
In practical terms, all you can do is reverse the c.t. for Power 2, which will make Power 2 negative, then put a minus sign ( × -1) into the process list on the input page to make it positive again.
If you can edit the sketch inside the “emon” front end of your emonPi and then upload it, it will be overwritten the next time you update emonCMS (unless you remember to update as “emonBase” and not “emonPi”).