Selling home, solar inspection issue with VOC measurement.

[jschner]

Sold my home with a 5.72K grid-tie system and the buyer had an electrician come out to inspect the solar. They did a VOC measurement on the strings and had some concerns.

The system is 22 260W Suniva panels about 5 years old. There are 11 panels on each string feeding into a ABB/Aurora PVI-5000 inverter, dual MPPT. The spec VOC is 38.3V per panel. With 11 panels in series, that would be 421.3V. The electrician measured the same 378V on each string. The panels are pointed due south at an angle of 28deg at 40 latitude and had already been producing for a couple hours that morning when the electrician made his measurement at 8AM. Ambient temperature was around 75 to 80 degrees, high for the day was going to be 107. I also noticed light powerline shadows on the panels in the electricians photos. My production for the last 5 years has been inline with the GoSolar projections as the system produces about 35kW to 38kW per day in the summer and usually exceeds the yearly projections.

The electrician wants $2500 to pull apart the whole system and verify each panel. It has also freaked out the buyer who also now wants verification. It seems to me the conditions were not ideal for this measurement and my question is, is this an accurate way to measure performance or is there possibly something wrong with this system?

[max2k]

Sold my home with a 5.72K grid-tie system and the buyer had an electrician come out to inspect the solar. They did a VOC measurement on the strings and had some concerns.

The system is 22 260W Suniva panels about 5 years old. There are 11 panels on each string feeding into a ABB/Aurora PVI-5000 inverter, dual MPPT. The spec VOC is 38.3V per panel. With 11 panels in series, that would be 421.3V. The electrician measured the same 378V on each string. The panels are pointed due south at an angle of 28deg at 40 latitude and had already been producing for a couple hours that morning when the electrician made his measurement at 8AM. Ambient temperature was around 75 to 80 degrees, high for the day was going to be 107. I also noticed light powerline shadows on the panels in the electricians photos. My production for the last 5 years has been inline with the GoSolar projections as the system produces about 35kW to 38kW per day in the summer and usually exceeds the yearly projections.

The electrician wants $2500 to pull apart the whole system and verify each panel. It has also freaked out the buyer who also now wants verification. It seems to me the conditions were not ideal for this measurement and my question is, is this an accurate way to measure performance or is there possibly something wrong with this system?

IMO the only performance the buyer should be concerned with is how many kWh (energy, not volts amps or even kW) the system produces. Can you show them recent history of that? This is the only value they should be concerned with as it is the unit they will be using/selling back to POCO/getting energy credits for.

[Sunking]

It is called leverage. The buyer has leverage and using it to drive down the asking price.

[sensij]

The buyer's electrician is fundamentally misrepresenting the electrical health of the system. The Voc of 38.3 V per panel is at STC... 25 deg C. The datasheet also provides a temperature coefficient for Voc, as well as the NOCT (normal/nominal operating cell temp). If you use the NOCT of 46 deg and apply the -0.335% / deg C coefficient, you'd get about 7% drop in Voc, reducing 38.3 to 35.6 Voc per panel, reducing the expected string Voc to 391.6 V. If the powerline shade is causing any segment of the panel to be bypassed, or if panel temp is hotter than NOCT (which is based on 20 deg C / 68 deg F ambient temp) suggests, dropping another 15 V of string Voc would be expected for a healthy system.

If their concerns are in good faith, hopefully a more technical explanation will help. If they are just using every available tool to negotiate, good luck.

coefficient.JPG

[SunEagle]

I agree the electrician is not providing useful information to the potential home owner concerning the production output of the system.

Where is the house located?

[max2k]

The buyer's electrician is fundamentally misrepresenting the electrical health of the system. The Voc of 38.3 V per panel is at STC... 25 deg C. The datasheet also provides a temperature coefficient for Voc, as well as the NOCT (normal/nominal operating cell temp). If you use the NOCT of 46 deg and apply the -0.335% / deg C coefficient, you'd get about 7% drop in Voc, reducing 38.3 to 35.6 Voc per panel, reducing the expected string Voc to 391.6 V. If the powerline shade is causing any segment of the panel to be bypassed, or if panel temp is hotter than NOCT (which is based on 20 deg C / 68 deg F ambient temp) suggests, dropping another 15 V of string Voc would be expected for a healthy system.

If their concerns are in good faith, hopefully a more technical explanation will help. If they are just using every available tool to negotiate, good luck.

coefficient.JPG

I still think the recent monthly kWh numbers would help more- not so many ppl can conduct that technical discussion in a reasonable way when money is involved. kWh are hard to argue with because this is the value system is providing. I could care less if it has Voc of 0 and still managed to rake up 1500 kWh month after month without maintenance costs .

[sensij]

I still think the recent monthly kWh numbers would help more- not so many ppl can conduct that technical discussion in a reasonable way when money is involved. kWh are hard to argue with because this is the value system is providing. I could care less if it has Voc of 0 and still managed to rake up 1500 kWh month after month without maintenance costs .

Yeah, I meant my post to supplement the point you made about kWh, not replace it.

[peakbagger]

I agree the electrician is clueless on PV and is just making work (or leverage for the buyer).

Unfortunately the electrician has "poisoned" the deal, in many states you need to disclose it to any potential buyers as a potential defect unless the buyer is willing to buy as is. The only way out is hire a professional that has a clue render an opinion that the system is operating properly. If you wanted to be litigious you probably could go after the original electrician for making a statement not supported by facts but expect the only winner in that one is the lawyers on either side.

[jschner]

The system is located in Shasta Lake CA. It was connected to PVoutput until I had the internet turned off in June. https://pvoutput.org/list.jsp?userid=15767 Power is still on and it produced 943kW the last billing cycle ending June 25th.

I provided a three year history of the production meter readings from the city and the PVWatts and GoSolar projections and I'm hoping that works. I finally got a hold of the actual inspector and asked him what his methods were and he did not use an ambient light meter or log the temperature of the panels. I let him know the VOC can change based upon the amount of light and temperature of the cells. Hence the reason for the solar panel chart with different lines at different temps and light. He seemed like a reasonable guy and I'm hoping he looks the specs over and clarifies his wording a little.

I also called around and got a lot of different opinions. One said you should measure VOC at 10am. Not too hot and a little more sun. One said as long as the sun is up, you can measure VOC and should be within 0.5V of the spec all day long. One said my measurement seemed about right. The last one will be my backup inspector if I need one.

[J.P.M.]

Sold my home with a 5.72K grid-tie system and the buyer had an electrician come out to inspect the solar. They did a VOC measurement on the strings and had some concerns.

The system is 22 260W Suniva panels about 5 years old. There are 11 panels on each string feeding into a ABB/Aurora PVI-5000 inverter, dual MPPT. The spec VOC is 38.3V per panel. With 11 panels in series, that would be 421.3V. The electrician measured the same 378V on each string. The panels are pointed due south at an angle of 28deg at 40 latitude and had already been producing for a couple hours that morning when the electrician made his measurement at 8AM. Ambient temperature was around 75 to 80 degrees, high for the day was going to be 107. I also noticed light powerline shadows on the panels in the electricians photos. My production for the last 5 years has been inline with the GoSolar projections as the system produces about 35kW to 38kW per day in the summer and usually exceeds the yearly projections.

The electrician wants $2500 to pull apart the whole system and verify each panel. It has also freaked out the buyer who also now wants verification. It seems to me the conditions were not ideal for this measurement and my question is, is this an accurate way to measure performance or is there possibly something wrong with this system?

Agree with others here that the electrician is clueless about PV with respect to voltage as f(temp.).

Unless the electrician has verifiable, and complete, and accurate panel temp. data the measurements are meaningless as a measure of system operation. See Sensij's posts for explanation on Voltage and max2k on realities. My system runs fine and I've measured voltages between 410.5 V and 359.3 V while nearly coincident average measured average panel temps. were 33.6 C. and 67.1 C. respectively. That series of 60 measurements, both winter and summer, has served as my basis for estimating average array temp. using voltage, and it seems to produce very consistent results after several hundred measurements. That electrician needs to either get it right or get lost. Unfortunately, I'd agree that unless the buyer can be enlightened (and not likely if $$ are involved), what that electrician did may have killed the deal.

[NorthRick]

Maybe the lesson here is to quiz or somehow confirm that an electrician selected by a potential buyer fully understands PV before agreeing to allowing them to inspect the system.

[max2k]

Agree with others here that the electrician is clueless about PV with respect to voltage as f(temp.).

Unless the electrician has verifiable, and complete, and accurate panel temp. data the measurements are meaningless as a measure of system operation. See Sensij's posts for explanation on Voltage and max2k on realities. My system runs fine and I've measured voltages between 410.5 V and 359.3 V while nearly coincident average measured average panel temps. were 33.6 C. and 67.1 C. respectively. That series of 60 measurements, both winter and summer, has served as my basis for estimating average array temp. using voltage, and it seems to produce very consistent results after several hundred measurements. That electrician needs to either get it right or get lost. Unfortunately, I'd agree that unless the buyer can be enlightened (and not likely if $$ are involved), what that electrician did may have killed the deal.

The seller can prob hire his own electrician and demonstrate the opposite- it should work especially backed by recent production figures. It's just negotiation, that's all.

[J.P.M.]

The seller can prob hire his own electrician and demonstrate the opposite- it should work especially backed by recent production figures. It's just negotiation, that's all.

Along with some backup documentation/literature to support the voltage behavior as f(temp.).

[Sunking]

Makes no difference if the EC knows what he is doing or not. All that matters is the buyer willing to accept a solar system and pay up for it. The Solar System limits potential customers. The house is only worth, what the buyer is willing to pay. He could care less about the seller. In many many states, solar has negative net equity.

[adoublee]

Makes no difference if the EC knows what he is doing or not. All that matters is the buyer willing to accept a solar system and pay up for it. The Solar System limits potential customers. The house is only worth, what the buyer is willing to pay. He could care less about the seller. In many many states, solar has negative net equity.

Not if the home was already sold. OP says the house was sold. The question is if the buyer who already agreed to buy is justified in using this as a reason to exit the deal.