Reduced Power Output

[ATSman]

I purchased a 600W grid tie inverter about 2 wks ago and connected
(2) 240W solar panels to it. After doing some testing & measuring the output of the panels separately I measure 32VDC open circuit and around 8 amps short circuit current(256W.) With both panels connected in parallel at inverter input I am only getting around 270W max AC output mid day when sun is at optimum angle. With inverter effeciency stated as 85% I calculate I should be getting around 408W max output. If I connect only 1 panel to inverter the max output is around 170W. Multiply Times 2 panels should be 340W. I measure the current from 1 panel with both in parallel to inverter and read 4A DC. It seems the output current drops to half of panel rating when connected in parallel with inverter. Can you explain why and what I can do to get more power out? Is this because of the impedance power matching between the panel output and the inverter input?
Would connecting each panel to a separate 300W inverter be a better way for me to get the most out of the panels?
I can supply the panel and inverter specs if required.

 

[Smart $]

Several factors go into this...

First off, your measurement is open circuit voltage and short circuit current. These values drop to low 80s and mid 90s percent respectively when in circuit (Vmpp and Impp in test conditions). In actual use, it will depend on inverter input impedance.

Then you have actual sun intensity. Directly overhead on a clear day is about 1400W/m².... and that only occurs between the Tropics. When not overhead, a fair approximation of intensity at midday is to multiply that value by the cosine of the difference in lattitude degrees between your location and where sun is perpendicular to earth. At this time of year, the latter is approaching the Tropic of Capricorn. The difference between Bay area and Tropic is about 60?.

1400W/m²*cos 60?=700W/m²

Check your IV curves for the panels.

 

[ATSman]

Ok, with all that said you didn't answer my question. Forget all that about the suns
rays, I gave you the OC & SC volts and amps of the panels and inverter power rating.
We are talking about panel vs inverter design, before and after connecting the two.
Is it the impedance matching error between panel and inverter that is causing the
less output that I am expecting?

 

[Smart $]

Ok, with all that said you didn't answer my question. Forget all that about the suns
rays, I gave you the OC & SC volts and amps of the panels and inverter power rating.
We are talking about panel vs inverter design, before and after connecting the two.
Is it the impedance matching error between panel and inverter that is causing the
less output that I am expecting?

Actually I did answer your question. Just not in a way you want to hear it. To be blunt about it, you seem to be expecting way more output than you could possibly get. Let's examine (per panel):

32V*81% * 8A*94% * 85%eff = 166W

The above is obviously a more conservative estimate than below.

700W/m² ? 1000W/m² * 240W * 85%eff = 142W

 

[jaggedben]

To be blunt...
I suspect your inverter is junk.

85% efficiency for a grid-tie inverter is horrid by today's standards and no reputable brand produces such a product. Is it UL listed? Even if it says yes I'd worry that was fraudulent. Is it CEC listed? That is easy to check. If it's not, then the advertised efficiency may not be for real.

So, with that out of the way, and recognizing that it may not be reasonable to expect the product to perform as advertised...

The only other thing I can think to try is to hook up the panels in series instead of parallel. Maybe the specs don't recommend that, but if you fry the inverter you likely haven't lost anything of much value.

Regardless, I don't recommend experimenting near flammable materials.

 

[ATSman]

Impedance Matching

Impedance Matching

Thanks for both of your responses and Jagged for the good inverter list. I guess maybe I didn't want to admit
I bought a piece of junk I paid $150 on ebay and its made in China. Although it is over 2 yrs in operating and has not quit yet. I have installed an external cooling fan to help out the little internal fan and am operating it on the low end of its power range.
I have attached an email from a fellow engineer that may explain the physics behind what is happening.
Pic shows power out close to what Smart$ calculated.
Thanks again!

 

[jaggedben]

A good inverter varies its input impedance to find the voltage for the best power output. It's called Maximum Power Point Tracking (MPPT), and the top companies are quite secretive about exactly how their algorithms work to find the MPP.

Your cheap inverter may either not do MPPT at all, or not have a real MPPT algorithm, or have an absolutely terrible one.

 

[GoldDigger]

And if your inverter, like most small crap import GTIs, was wired for plug and pray connection to your mains, it is not UL listed and totally illegal.
PS: I have seen YouTube reports of people with two 100W panels and a 300W POS who plan to get two POSs instead to improve efficiency from the 60% they were getting.
Good money down the rabbit hole.
No current legal GTIs can be installed in the US without hard wire, inspection, and POCO interconnect agreement.
PPS: there is one manufacturer that claims a legal plug in GTI, but if you read the fine print the "plug-in" is to a unique dedicated receptacle installed on a new branch circuit by a licensed electrician.
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