With solar, it's super easy to measure impact:) 
We generate kWh's of clean electricity which we sell directly to our commercial customers, generating plenty of documentation on exact generation, transmission, etc.
We're happy to share generation reports, site photos, bank records etc

We're also currently working with Marc & Alan from Filecoin Green to implement their verification certificates for both Impact and Security.
 
We create tons of extra impact by investing in emerging markets, paying local developers to install projects, funding solar on schools/hospitals etc, but energy generation is by far the easiest to track, the rest is icing on the cake!