Home Construction Performance Live Data  Page Maintenance Economics


Photovoltaics Xantrex Inverters SMA Inverters After 7 Weeks After 18 Months



After making the sizeable investment in my PV generating plant, I wanted to assure maximum performance. So I collected the necessary equipment and began taking measurements. Here's what is needed to assess performance with reasonable accuracy:


First, some basic instruments, such as

Analog AC and DC voltmeters (oldies but goodies)
Digital AC and DC voltmeters
AC and DC clamp ammeters
Non-contact infrared thermometer
Calibrated silicon PV reference cell.


Some information is gathered from five kilowatt-hour meters. One, of course, is the utility company meter, and the PV system is equipped with a private meter. A third meter is dedicated to our electric domestic water heater, another for the heat pump, and finally one for our EV charger.



Data for evaluating the Xantrex inverters is obtained from their displays. The original model ST2500's displayed instantaneous power output and total energy delivered to the moment. The newer STXR2500's displayed only the total energy delivered.


Early in the project the Xantrex inverters were replaced with SMA Sunny Boy inverters. The Sunny Boys are equipped with RS-485 datacom ports that convey a wealth of information. SMA provides a free software product called Sunny Data. It is Windows-based, easy to use, highly configurable, and quite powerful.

For those viewers who like to add numbers to check totals, be aware that the individual inverter values (shown in red) will be totaled into the system wide fields (shown in green) at the next sample interval. Green is always one sample behind red.


The RS-485 interface is accomplished with a small conversion adaptor that connects to a COM port. I first used Sunny Data under Windows 98 with an RTS-enabled RS-485 converter. It worked well, but failed miserably when I upgraded to Windows XP. The scope traces of the RS-232 signals show why.

This image shows the end of the outbound query packet issued by Sunny Data. The traces were triggered by the de-assertion edge of RTS (lower trace) which disables the RS-485 line drivers. The final 26 milliseconds (including stop bit) of the query packet was not delivered so of course no inverters replied.

I replaced the RTS-enabled converter with a self-enabled model and the problem was solved.