steve,
Good question.
The Power company installed 2 meters at my home, on turn-up day, Feb. 9, 2010. I installed Sun Power panels and there free monitoring system. I divided Sun Powers one year kWh total into meter monitoring the solar output. I got the number 99.7, After that I started to believe Sun Power's numbers, have a display on my home office desk. I'm also running TED, which monitors daily electric cost.
I've ask for the new Sun Power 2.5k (2.45k) system to be added to the existing free Sun Power monitoring system. There's advantages for having individual inverters for each panel, but also with 50 inverters, there's more things that can go wrong.