With regards to the Inverter Comparison article in HP #91 one thing that wasn't entirely clear is whether the significant performance issues with the ST and STXR Xantrex series is solely related to the MPPT features of the ST line or whether these performance issues exist across the entire Xantrex line, PS, SW, DR, UX, etc. An important distinction, since a possible takeaway of the article is that perhaps the entire crop of Xantrex inverters don't measure up to their advertised specs. It certainly left me asking myself that question.

all the best,

My testing was on the Gridtie ST/XR and it shares no hardware with other Xantrex Products.

The SW/DR were designed before the ST/XR and neither product has any type of mppt software.

I'll Let Xantrex response to the overall question of spec’s and there accuracy ....

Hellow Solor-guppy,

I just read your inverter comparison in Home Power. Well, actually I just scanned it, jumping to the end. I have complaints about the ST2500 too.

My system includes a One-Meter from Brand Electronics. I can sample the ST every second and watch a five minute chart recorder type display on my PC. So when the system shuts down, when the sky is clear, I can see it in real time. I have captured screens of this problem and emailed them to Xantrex but they just do not respond.

My career was spent programming computer controlled test equipment for testing integrated circuits. I?m an electronic engineer by education. With that background by sense is that the MPPT software is not what it should be.

For example: With one second samples you can see that every time the ST drops power the recovery slope is constant. So, what the software is doing is a linear search for the correct loading. I suggested they try a binary search in my last email, but they don?t respond.

The fan control, I have a fixed speed fan, is also dumb. I proposed to Xantrex that they sample the power output every second. The sample result, greater than 100W ?, could be stored in an array. The array could be parsed, say also once a second, and if the power was ever over 100W, for 120 samples, then the fan should be on. This function is similar to the ?watch dog? circuit in almost all electronic equipment.

Another thing I wrote them about was their mechanical drawings. My ST was backordered and the wiring/conduit was done waiting for the inverter to arrive. Well, the conduit punch outs were not where the drawing said they would be. I actually got an email response on this complaint. My email was forwarded to someone within the organization for action. This is, to me, a clear example of no active quality control group. Documentation errors are the easiest problems to fix. Why not thank the customer for his time and trouble and fix the problem?

I am really impressed that you actually had a house call from Xantrex. Since they don?t reply to my email I thought they were just too unprofessional to care.

Thank you for your time and trouble in putting this article together.

Please find below a copy of my letter that I sent to my salesman on 9-11-02. He forwarded the letter to Xantrex the next day. I have not received any reply as of this date. Has anyone been compensated for an inverter that does not meet specifications? If this was a car it would be covered under lemon laws.


Hi Joel, have you read the inverter comparison article in the latest Home Power Magazine, issue # 91? It compared the Xantrex STX and the STXR with a Sunny Boy. The author (solar guppy) concluded that the Xantrex inverters are 14% to 28% short of kilowatts depending on a sunny day or a cloudy day. It appears that the Maximum Power Point Tracking (MPPT) program is not working properly.

As you know I have a STXR-2500 watt with a remote meter. Without the remote meter I would not have suspected that anything was wrong but I have observed the STXR crash many times on a cloudy day. I live in the high desert of California and work in the Los Angeles area. In the high desert, the sun is always more intense without the smog and haze that exists in Los Angeles. I do not know what my insolation is, but on a sunny day I have recorded a daily 11.5 to 12.5 kw-hrs which agrees with the Xantrex white papers that predict an approximate efficiency loss of 25% when taking into account the photovoltaic tolerances, panel termperature, dirt in the atmosphere, dirty panels, voltage losses, photovoltaic mismatch tolerances and inverter losses.

There is a reason that the desert is so dry, and that is due to the mountains that prevent many of the clouds from entering my area compared to the Los Angeles area. Many times I have driven home thru the rain or clouds in Los Angeles and when I travel from Los Angeles to the high desert there are no clouds and definitely no rain in the high desert. So, when I say that there are clouds it is not thick or heavy clouds that would block all of the sun. When this partly cloudy condition exists, I have seen the watts on my remote meter at 0 watts. This crash condition is a short duration but then the Xantrex program will take many minutes to recover and bring the watts back up to its maximum watts again. Sometimes by the time it has recovered it will then crash again and start the process all over. My daily ouput on a partly cloudy day has varied from 3.5 kw-hrs to 11 kw-hrs. I do not know how much loss is attributed to my Xantrex system but the Home Power article made a lot of sense.

DO YOU KNOW OF ANY PLANS BY XANTREX TO CORRECT THEIR (MPPT) PROGRAM AND SEND A NEW CHIP? I HOPE IT IS THAT SIMPLE. DISCONNECTING MY STXR FROM THE WALL WOULD BE EXTREMELY TIME CONSUMING, NOT TO MENTION THE LOSS IN ENERGY WHILE IT IS AT THE XANTREX FACTORY.

Also, I believe that they have a fan program problem. The author (solar guppy)mentioned the variable speed fan on the STXR. I do not believe that it is a variable speed fan but a 2-speed fan. It may seem variable due to the constant changing of speeds. It appears to have a poor control loop or PID (Portional, Integral and Derivative) loop. The fan will run on high for 5 seconds, low speed for 5 seconds and then off for a few seconds. Then it runs on high again. The fan is all over the place in regards to its speed. I suspect that I will be changing this fan more than once during the lifetime of the inverter. Constantly changing a fan from off to low to high speed will surely shorten the life of the fan motor.

DO YOU KNOW OF ANY PLANS BY XANTREX TO CORRECT THEIR FAN PROGRAM AND SEND A NEW CHIP? I HOPE IT IS THAT SIMPLE. DISCONNECTING MY STXR FROM THE WALL WOULD BE EXTREMELY TIME CONSUMING, NOT TO MENTION THE LOSS IN ENERGY WHILE IT IS AT XANTREX.

Do not get me wrong. I am very happy with my system and received an Edison invoice last month of $21 versus $70 last year. No maintenace is required except possibly cleaning the dirt from the inverter once a year due to the dust that enters the unit thru the fan and hopefully I will have many years of trouble free operation.

I only hope Xantrex steps up to the plate and fixes these problems with minimal inconvenience to the customer. The ST series has been out for more than 3 years now. That is a lot of time to resolve any programming issues. I am sure many of you in the industry are watching the progress.

Gary Gneiting

[ September 26, 2002: Message edited by: Ggneiting ]

In response to Lopez,

The Sun Tie is an entirely different machine than the other models Xantrex sells. The ST is a maximum peak power tracking machine. It changes the array DC voltage to extract maximum power from a PV array over the course of the day. All other Xantrex grid-selling inverters available at this time sell from a battery bank which is charged and discharged at a specific BATTERY voltage. Henry's article purely points out a challenge in the MPPT software, which only affects the ST.

To the long letter posted above, please see the xantrex website for information on the upgrade program at www.xantrex.com.

Best, Tobin Booth, Xantrex Technology