San Marcos San Diego new System Quick

[thejq]

If it's ok, I'd like to ask a question or two more.

I had another company come out and during the discussion, I got confused as he started talking about the output AC numbers vs my power needs. So, basically he used the system power in DC to say that my system would then produce so much power AC and use that to calculate it to my 9900 kWh/year.

From what I gather in PV Watts just using my SE roofing, a 5.75 kW DC system would give me 9700 kWh/yr. He quoted a 6.46kw system and I asked him why it was so big, and he went back to the AC numbers.

Thoughts?

Your consumption is in AC Wh (because AC is what comes out of your outlets). But solar panel produces DC current, hence all the rated power is in DC W. The inverter converts the DC current to AC current. All modern inverter has really good efficiency, eg SolarEdge is >98%. So 1 DC W can be converted to 0.98 AC W. So for all practical purposes, they are pretty much the same.

[katass1031]

So the quote was for a

6.46 kW DC
5.8?? kw AC

using an SMA6000 inverter. What am I missing? Seems like a huge loss here. Or maybe he is figuring system loss over some amount of years.

[thejq]

So the quote was for a

6.46 kW DC
5.8?? kw AC

using an SMA6000 inverter. What am I missing? Seems like a huge loss here. Or maybe he is figuring system loss over some amount of years.

I think you should ask him (or her) to explain how those two numbers are related. According to SMA, the 6000-TL has a efficiency of 97.6%. But based on your quote, it's more like 90%. Maybe there's a valid reason or assumption, or maybe he just want to sell you more stuff.

[sensij]

If it's ok, I'd like to ask a question or two more.

I had another company come out and during the discussion, I got confused as he started talking about the output AC numbers vs my power needs. So, basically he used the system power in DC to say that my system would then produce so much power AC and use that to calculate it to my 9900 kWh/year.

From what I gather in PV Watts just using my SE roofing, a 5.75 kW DC system would give me 9700 kWh/yr. He quoted a 6.46kw system and I asked him why it was so big, and he went back to the AC numbers.

Thoughts?

It sounds like you are saying that when you look at PVWatts, you come up with a 5.75 kW system to produce 9700 kWh a year, but when you ask you installer how big the system needs to be to generate that much energy, they are saying 6.46 kWh.

You installer should be able to do a better job of explaining how they've generated their estimate than just some hand waving around DC vs AC efficiency, especially once you've indicated your awareness of PVWatts. There might be good reasons for the discrepancy, but it might also be a sign your installer is trying to oversell you.

Without knowing more about your array than "San Marcos" and "SE" (is that true SE at 135 deg azimuth?), I'd say that a system sized to generate 9700 kWh in a typical year would be something between the size you've suggested and what your installer recommended. If you share more details about the specific azimuth, tilt, and shading, we may even be able to find a close match in PVOutput.org data so you can see what a real system has produced over the past year.

[katass1031]

This was his response.

Hello, The DC rating is set at 6.480 watts. The AC wattage is set at 5.650. The AC wattage is derived by taking the PTC rating of the panel which is 242.7 watts, multiplied by the number of panels, multiplied by the effeciency of the inverter. This is how AC wattage is calculated. I was able to avoid the shade while keeping the same number panels, just need to adjust the array accordingly.

[katass1031]

It sounds like you are saying that when you look at PVWatts, you come up with a 5.75 kW system to produce 9700 kWh a year, but when you ask you installer how big the system needs to be to generate that much energy, they are saying 6.46 kWh.

You installer should be able to do a better job of explaining how they've generated their estimate than just some hand waving around DC vs AC efficiency, especially once you've indicated your awareness of PVWatts. There might be good reasons for the discrepancy, but it might also be a sign your installer is trying to oversell you.

Without knowing more about your array than "San Marcos" and "SE" (is that true SE at 135 deg azimuth?), I'd say that a system sized to generate 9700 kWh in a typical year would be something between the size you've suggested and what your installer recommended. If you share more details about the specific azimuth, tilt, and shading, we may even be able to find a close match in PVOutput.org data so you can see what a real system has produced over the past year.

I have not figured out the tilt, but the azimuth is 145 on the SE and 215 on the SW. I have been using only the 145 as worst case scenario right now.

How do I figure out shading? According to the vendor today (he took all the reading and went on the roof), he was able to avoid the shade. So maybe it is not applicable here.

[J.P.M.]

This was his response.

Hello, The DC rating is set at 6.480 watts. The AC wattage is set at 5.650. The AC wattage is derived by taking the PTC rating of the panel which is 242.7 watts, multiplied by the number of panels, multiplied by the effeciency of the inverter. This is how AC wattage is calculated. I was able to avoid the shade while keeping the same number panels, just need to adjust the array accordingly.

All that smoke/mirrors may or may not be true depending on a lot of other things, but it's mostly not relevant. If you have no shading, run PVWatts for each orientation for 1 kW systems and use a 10% system loss. Then scale up each size as the amount of roof area allows, maxing out the more productive orientation first. AC output from PTC testing is not something to be concerned with for preliminary sizing. It mostly muddies the H2O. As Sensij notes, a system somewhere between the two will probably produce about 9,700 kWh/yr. I'd get the correct az. and tilt and rerun. PVWatts seems to work well for most, including your application.

[katass1031]

Another 2 options came in today. One that was way too much money, but the other one is

6.3 kW
20 LG315NC1-G4
SE6000
$24255 ($3.85/W)

I think business is really good right now and they have no reason to have low pricing because they can always get another job site.

This vendor presented me how they calculate the system power. It was a website/program that he input all the variables just like PVWatts and they have all the panels in there as well. I told him about PVWatts and he said they find it to be 10% off from actual real world. He said this other program was within 2%. Of course this is what he is trying to sell me.

[J.P.M.]

Another 2 options came in today. One that was way too much money, but the other one is

6.3 kW
20 LG315NC1-G4
SE6000
$24255 ($3.85/W)

I think business is really good right now and they have no reason to have low pricing because they can always get another job site.

This vendor presented me how they calculate the system power. It was a website/program that he input all the variables just like PVWatts and they have all the panels in there as well. I told him about PVWatts and he said they find it to be 10% off from actual real world. He said this other program was within 2%. Of course this is what he is trying to sell me.

That vendor has either found out on their own or been lurking on this forum as some do. That 10% can often be dealt with by changing the 14% system loss parameter to something less - maybe 9 - 11% or so.

That size will probably get you a bit past 10,000 kWh/yr. for the orientations you have. For several reasons and unless shading is more of a factor, I'd think about favoring the SW orientations a bit more if you have a choice.

If this is a quality vendor that you believe capable of giving you the quality and timetable you want, take a deep breath and offer $3.50/Watt on a firm, ready for sign off finish date with the vendor picking up your electric bill for the time between the missed date and actual startup. That's a possible and fair price for that equipment as long as the vendor is established and reputable. Use the date and bill reimbursement as a negotiating give away point before you bend on price. You might not get $3.50/Watt, but there is often/usually about 10% or so wiggle room in a lot of pricing around here. You've got nothing to lose in the attempt, and you won't get if you don't ask..

Winter is usually slow season, but circumstances this year will probably change that and impact pricing. How much ?? You don't get what you deserve. You get what you negotiate.

[katass1031]

Basically, times and business are good right now and they don't need to offer discounts to people to get the work because they can get more work somewhere else. I tried a little to see where he stood on a better deal and he wasn't really moving at all. I told him that there are a few of us in my office that all want to do solar and if there would be a discount. He came back to me with their referral program which is very good. The thing about group discounts he says is that usually not everyone comes through.

[J.P.M.]

I don't think you'll find any posts in which I've stated that a particular array size is dogmatically correct for everyone considering solar. What you've posted here is similar to how many people eventually size their array, and I fully support the freedom for anyone to make their system as big or as small as they'd like. However, there is a common belief that a "correctly" or "properly" sized array achieves 100% offset. My posts on this subject are intended to challenge that belief, by showing that from a financial point of view, it is difficult to come up with a reasonable set of assumptions about the future that makes it true.

In this thread in particular, there was some discussion of the minimum bill and how to consider it in array sizing, for which I could demonstrate how stark the payback gets for increases in array size that offset energy paid for by the minimum bill. There are ways in which the example I presented is simplistic, and I really encourage anyone who wants to understand the financial consequences of their decision to go solar to take a much closer look at expected generation or consumption on at least a monthly basis, but perhaps even hourly or sub-hourly for consideration of TOU rates (as they are designed now and also how they are projected to change).

Going solar doesn't have to be a financial only decision. However, most who are seriously considering solar have an expectation of some kind of financial return, and are starting from what is frequently a misinformed idea of "financially optimal" based on what they've been told by a salesperson or read in a poorly thought out forum post . Want 100% offset for non-financial reasons? Go for it... just know that it is not likely to be a sizing strategy that minimizes the cost of energy over a 10 or 15 year timespan.

There are many products in the world sold as cheap insurance, and in some cases, buying that insurance can be a great decision. Collectively, in most cases, the party benefiting from that insurance the most is the one selling it, not the one buying it. Like any insurance, the cost of buying it should be weighed against that risks of not having it and the peace of mind it provides. To me, paying for baseline tier electricity is not a catastrophic outcome worth insuring against, but like most extended warranties, I would not object to those who consciously choose to buy it for peace of mind despite the poor outlook for actual financial benefit.

I would also challenge the suggestion that *now* is the best time that will ever exist to install solar, and therefore it is better to go big now than to wait until some point in the future when the range of cost effectiveness assumptions have narrowed. For sure, there are incentives that may not be available soon that could make *now* better than 3 years from now. However, if you look at projections on installation costs over time used by those in the industry (Attachment A), you'll see expected drops in price of over 40% over the next 10 years, which could potentially compensate for the loss of the ITC, and perhaps even structural changes to NEM. 10 years from now, my guess is that we'll be having much more serious conversations about the cost effectiveness of local energy storage for peak demand shaving, and very little of the equipment being sold today supports it. The idea of "future-proofing" using today's equipment seems like a non-sequitor to me. I'd rather spend as little is as possible today in a way that maximizes my chances of getting a positive return on what is invested, and keep my powder dry for modifications to the system in the future that benefit from the inevitable changes in technology over time.

Given the gap between the perceived want/need to acquire residential solar and lack of knowledge about availability and use of tools helpful in the economic portion of the decision making process, perhaps this post or something similar could be the start of a sticky thread dealing with concepts of cost effectiveness considerations and their place relative to other considerations in the solar go/no go decision making process. Knowledge is power.

[katass1031]

Update...last vendor quotation

6.3 kW
20 LG315NC1-G4
SE6000
$21652 ($3.44/W)

This is better than the latest quote with same equipment with great reviews. He also gave me another option

6.21kW
19 SunPower X20-327
SE6000 with P400 optimizers
$23109 ($3.72/W)

He told me that these X20-327s are the same panels as the X21-345 that do not meet the criteria for the 345s and are calssified as a step below because they do not produce enough watts. Basicall the 345s need to create greater than 346W, if not, then they are classified as the 327s.

Is it worth the $1500 premium for the SunPowers?

[solar pete]

Howdy katass1031, re the SP panel premium, I dont think its worth it but there are quite few that do, they do have a good warranty. To me it's more about paying the extra for US made equipment if you are really keen on supporting local manufacturing. Opinions really do vary on this one. Thanks for the update and good luck with it.

[J.P.M.]

Howdy katass1031, re the SP panel premium, I dont think its worth it but there are quite few that do, they do have a good warranty. To me it's more about paying the extra for US made equipment if you are really keen on supporting local manufacturing. Opinions really do vary on this one. Thanks for the update and good luck with it.

To be clear, S.P. panels are mostly made in the Philippines, Malaysia and S. Africa. They have a small mfg. facility in Milpitas, CA. They are majority owned my the French energy (oil) co. Total.

[J.P.M.]

Update...last vendor quotation

6.3 kW
20 LG315NC1-G4
SE6000
$21652 ($3.44/W)

This is better than the latest quote with same equipment with great reviews. He also gave me another option

6.21kW
19 SunPower X20-327
SE6000 with P400 optimizers
$23109 ($3.72/W)

He told me that these X20-327s are the same panels as the X21-345 that do not meet the criteria for the 345s and are calssified as a step below because they do not produce enough watts. Basicall the 345s need to create greater than 346W, if not, then they are classified as the 327s.

Is it worth the $1500 premium for the SunPowers?

You'll get about the same annual output from 1kW of LG 315's or SP 327's. I did a cost/benefit analysis for S.P. vs. a more standard panel like an LG 300. Short answer: A Sunpower paneled system is worth roughly about a 5% or so premium over 12 years with what I believe are reasonable but still variable assumptions, starting with neither system will have a failure. 8% is obviously outside that premium margin, but other non financial factors may come into play to tip the balance.

If I went further w/ the S.P. quote, I'd make sure of the details on that S.P. price and make sure they're not used, sloppy seconds, or counterfeit. $3.72 is lower than usual for S.P. by a lot. I'd love to see it, but I'd sure trust but verify. If a deal seems too good to be true, it probably is.