How to Measure a Micro PV Grid Tie System Level's efficiency

[qis_mo]

I started this educational journey with one slightly damage 250 Watt PV panel that still delivers it spec'd 8.0 Isc amps and 27~32 operational (across inverter) volts. For test purposes I chose a cheap 500 Watt Chinese GTI inverter that claims a peak performance factor of 95% or so. I am trying to determine the best way to measure this micro pv system's overall efficiency.

The last time I checked, I measured the Isc current across the GTI DC terminals at 7.08 amps. The volts across the GTI DC terminals was 28.5. Can I assume that the pv power available for use (28.5 v x 7.08 a) is 201.8 watts? (1)

Next I measured the DC current drawn thru the GTI inverter was 4.28 a, or approximately (28.5 v x 4.28 a) 121.98 watts. At this moment my Kill-A-Watt meter was measuring 108.4 AC Watts going into the GFI protected grid. Now if I can measure the GFI's efficiency by calculating AC watts-out divided by the DC watts-in (108.4 w/121.98 w) I get 88.9%? (2)

But is it reasonable to estimate this micro pv system's overall efficiency at GTI's AC watts-out divided by the DC watts-in available at the pv panel (108.4 w / 201.8 w) or 53.7 %? (3)

I have yet to read of an efficiency this low before adding in all the system's environmental derating factors.

I would like to learn where I error, and also how should I calculate this overall efficiency? (4)

Thanks for any comments of suggestions.

[Sunking]

Efficiency is very easy math. To find the percentage is:

[Output/Input] x 100 = X%

So if your inverter is outputting 100 watts, and input is 200 watts from the panel, then [100 / 200] x 100 = 50%

If you ever get 100% or more, you messed up because that is impossible.

[Mike90250]

While measuring Isc is good to verify the panels functionality, you will need to measure differently to get accurate results.
You will need to measure the dc voltage at the input of the GTI and use a clamp-on meter to measure the DC amps AT THE SAME TIME you measure the voltage. If you don't measure both these at the same time, you will not get the most accurate results.

Then use a Kill-a-watt or some AC watt meter, to measure the output of the GTI.

[qis_mo]

Thanks for your quick response

I believe that the Kill-A-Watt meter is an acceptable measure of a micro pv system's GTI output.

But how do you correctly measure the this micro pv systems input?


Thanks again

Moderator Mike Says:

..
You will need to measure the dc voltage at the input of the GTI and use a clamp-on meter to measure the DC amps AT THE SAME TIME you measure the voltage. ..

Don't make me repeat myself

[Sunking]

I believe that the Kill-A-Watt meter is an acceptable measure of a micro pv system's GTI output.

But how do you correctly measure the this micro pv systems input?


Thanks again

Mike just told you. You have to measure the DC current and voltage at the same time to the input of the inverter. You will need two meters. A volt meter and a current meter. Isc is not going to tell you anything because at Isc power = 0 watts.

[qis_mo]

Thanks Mike90250 for your response

I just caught your response. Thanks for your help.

Doesn't the clamp-on ammeter just measure the current going through the GTI inverter?
Isn't this used to measure the efficiency of the inverter only..and not the overall pv system?

In my original question ..."current drawn thru the GTI inverter was 4.28 a, or .... (28.5 v x 4.28 a) 121.98 watts. At this moment my Kill-A-Watt meter was measuring 108.4 AC Watts ....measure the GFI's efficiency by calculating ...(108.4 w/121.98 w) I get 88.9%"

Is there a way to measure the systems overall efficiency...
so can we match the panels to the inverter to maximize the systems output?

PS I am frustrated by the fact that I have a 250 Watt rated panel and a 500 Watt rated GTI producing 108.4 watts.
If the inverter is running at 88.9% efficient, where is the pv panels power being lost?

[Sunking]

Doesn't the clamp-on ammeter just measure the current going through the GTI inverter?

The DC clamp-on ammeter measures the DC current being supplied by the solar panels to the GTI. You take that current and multiply it by the DC voltage at the input terminals of the GTI to get your input power measurement.

Isn't this used to measure the efficiency of the inverter only..and not the overall pv system?

Yes it is part of the equation. You have to know both input power and output power at the same moment in time. You would need to be reading the DC input side with a clamp-on and DC volt meter. Then use the kill-A watt meter to measure the AC output power.

In my original question ..."current drawn thru the GTI inverter was 4.28 a, or .... (28.5 v x 4.28 a) 121.98 watts. At this moment my Kill-A-Watt meter was measuring 108.4 AC Watts ....measure the GFI's efficiency by calculating ...(108.4 w/121.98 w) I get 88.9%"

Is there a way to measure the systems overall efficiency...
so can we match the panels to the inverter to maximize the systems output?

For the last time that is what you are doing measuring the input and output power at the same time and run the numbers [output/input] x 100 = Efficiency Percentage.

PS I am frustrated by the fact that I have a 250 Watt rated panel and a 500 Watt rated GTI producing 108.4 watts. If the inverter is running at 88.9% efficient, where is the pv panels power being lost?

It is not being lost, it is just not there to be had. You have made a common mistake of assuming a 250 watt panel generates 250 watts. It does not. A panel power output is directly proportional to the amount of sunlight. It is winter, the sun sets low on the horizon going through a lot of atmosphere attenuating the sun pays and your panel is not facing directly into the sun at solar noon.

[qis_mo]

Thanks Sunking for the assistance

I am still learning on the steep part of the curve...
I have discovered that I really dont yet understand how the MPPT function of the inverter works.
I wiki article I read provides a theoretical I-V curve starting a Ics on the Y axis and Vco on the X.
This article states that the MPPT (maximum power point) is somewhere on that curve. I need to learn more.
I will spend some time collecting more data (Like Ics = 7.08 on a sunny winterday about 12:45pm @ 28.5 v.)
I will plot the MPPT function and how it changes at different sun angles.
Maybe I'll discover why some youtubers can get 80-90% of the panel ratings and I can't.

Thank you to everyone for your kind assistance
Back to school for me..

[russ]

[QUOTE=qis_mo;38642Maybe I'll discover why some youtubers can get 80-90% of the panel ratings and I can't.[/QUOTE]

Quite possibly it is because about 99% of the stuff on you tube is total BS - the people generally have no more idea than you but they like to chatter.

The wikis are semi useful - at the bottom of the article there are real references normally. The wiki does not constitute research but rather research 'lite'.

I very rarely even bother to go to you tube - a total waste of time trying to figure out whether the information is accurate or not.

[Sunking]

I have discovered that I really dont yet understand how the MPPT function of the inverter works. I wiki article I read provides a theoretical I-V curve starting a Ics on the Y axis and Vco on the X. This article states that the MPPT (maximum power point) is somewhere on that curve.

Here is your problem. The test equipment to make those I-V curves cost more than a luxury home. The MPPT is real easy to spot on a I-V curve, it is the knee portion of the curve. The reason you are having such a hard time understanding it is because a solar panel is a current source, not a voltage source like a battery. The current a panel produces is directly proportional to the amount of sunlight striking its surface.

Lastly some facts about the I-V curve:

• Voc or Open circuit voltage the power equals 0 watts because V * 0 = 0 where V = voltage, and 0 represents amps.
• Isc or current short circuit power equals 0 watts because A * 0 = 0 where A = Amps, and 0 represents voltage.
• Power = Watts = Voltage * Amps = Amps * Amps * Resistance