I've had SolarGuard monitoring running on my PV system since November. You can see my system's performance at this new url: http://solarguard.solarcity.com/kiosk/SolarGuard.aspx?ID=800001F1-1172101504&AutoDemo=1&Timeout=90&ChartDate=3_1_2008&RangeType=Week (There is a 12-day gap in the record due to a firmware problem that was fixed on Jan. 23.)
SolarGuard sends my inverter's output statistics to SolarCity's computers. There is a transmitter attached to the inside of the inverter that sends the data wirelessly to a receiver in my home office. The receiver is plugged into my router and sends data via our cable modem.
SolarCity can use the data to monitor the performance of my system, and soon will be able to compare my output to that of other systems they have installed in Mountain View. If there is an unexpected decline in production that isn't mirrored by other nearby systems, they will alert me to the problem.