I've got 2 separate PV arrays on my roof (installed in different years), 2 different makes of inverter and I was looking for a way to easily monitor the performance of both systems, as well as how much electricity I was actually using at any given time.
I was going to go for the emonPI system, but it seemed overkill for what I wanted, and all the different components you need quickly add up in cost:
then I found the EmonWRT system, which uses some of the same components, but in a very neat pre-built unit that combines the emonTx and a WiFi connection:
for a total cost of £88.73 I got the EmonWRT unit, 3 mains sensors (one for the grid, one for each of the solar PV outputs), and an AC power adaptor (this is needed so the unit can tell if the grid cable is importing or exporting electricity). Obviously a bit cheaper if you only need to monitor a single PV array.
It is very fiddly to set up, and the instructions are really aimed at hardware geeks who put them together from kits. Hopefully they'll improve the instructions so the hardware & software is much easier. Once it is all set up though, it's amazing!
I can now see all manner of live numbers & summary graphs/reports from my smartphone