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Well your results are a little different than what I see for systems 4-6 years old. Most are performing below what they should be and they were sized with the EPBB tool. For the last 14 years that calculator has been the most accurate overall in the predicted output here. I am sure that you can tweak PV Watts to prove any theory and I have already seen that done. Or you have some calculator that is more accurate than what the State of Ca used for federal rebate calculations. You say it is 10% difference by your calcs right off of the bat, awesome, where is your proof of this claim? And I do not see that in the real world. When I size a system my quote is usually smaller than what the EPBB calculator says the customer needs. It is closer to what PV Watts says. But in the end the customers are usually more educated than you are portraying and want a larger system, that is close to the 10% difference you are referring to. I am not adding another 10% on top either. It seems like you are putting a lot of effort into this to prove what? Everyone who uses this calculator is a scumbag who rips the poor uneducated customer off? Hilarious. Tell me, are you active in the PV industry? Or just spouting calcs from your desk from a few systems around you?
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I'm in SCE area and most of my friends' system are in the same area. SCE only allows 100% coverage when you design you system. Yes, you can expand up to 10% and less than 1kW without re-apply net metering. oversizing/undersizing what what I have observed is pretty much: cash vs lease(monthly payment). Among of my 8 friends who bought their system size "properly" (good size) other 7 friends who is on lease payment undersizing their solar. I know it is not a good statistic with very small samples, but I'll stop right here.
POCOs like TOU simplly they can make more profit off 95%+ of their clients without solar. I like TOU plan, because I can use more kWh without paying more $$$. but I will not build a solar base on TOU plan.
My 3 yrs average is about 9,700kWh prior than solar installed. CSI EPBB calculator estimates 9973kWh and SAM estimates 10,599kWh on a Bosch 255W X 26 with SMA 6000TL-US-12, SW245 deg with 23 deg tilt. I have add 2 more panels within 6 months and gave me 11,500kWh real output in 2015. Am I oversizing? Not really, I still withdraw 3852kWh from SCE in 2015. Yes, I did endup with negative credit and pay almost nothing, but if I'm not in TOU plan I probably had to pay few hundred dollars. I have managed to charges the EVs in super off peak hours. If I cannot switch the loads to super off peak, how much more $$$ do I end up with?
Do your installer know all these infos? If you pay a fee and someone might come up with a more precise calculation base on your hourly usage, but people like to get info for free. If you like to save money, then learn what you don't know. You pay higher price on bottle of water than gasoline, have you complain about the B.S.?Comment
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A good source for real world data in our area is Team San Diego, on PVOutput.org. It is very clear when looking at actual system data that the older version of PVWatts, on which the EPBB tool is based, was underestimating performance. NREL has said that much when they reduced the default loss percent in the latest version, and even 14% is too much loss for most systems going up today.
Oversizing a system, whether intentionally or not, costs your customers money. You can choose to perpetuate the ignorance and myths that are embedded in the industry, or you can choose to objectively look at the data and try to help people understand that they don't need to offset 100% of their usage to come out ahead, especially (in many cases) with TOU plans.
PVWatts can not be tweaked to "prove" any theory. It is a model that builds specific relationships between inputs and outputs, using equations and principles established through science. If a user intentionally (or through ignorance) runs the model in ways that don't reflect reality, that doesn't prove anything. The output of PVWatts includes irradiance and environmental data that can be used to assess how accurately the inputs into the model reflect reality.CS6P-260P/SE3000 - http://tiny.cc/ed5ozxComment
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1.) As I and others have noted, NREL revised PVWatts in response to a lot of comment that it was too conservative with respect to measured and observed reality. Apparently they (NREL) agreed with those comments. You, and/or your data, seem to not agree. So be it. This is a forum where information can be exchanged. Some of that information is opinion.
2.) It seems that we agree that I, you, or anyone can tweak PVWatts, or any model for that matter, to bolster an opinion. Thanks for the insight on a blinding flash of the obvious. FWIW, I'd not call an algorithm that estimates system output a calculator as much as a model. By design, (good) models can be tweaked - for good or for not so good purposes. If for good purposes - that's one way they're improved.
3.) I have a lot of tools. Some are models. As such, those models can be adjusted when warranted by observation and data to better reflect reality. That's what I'd call a good tweak, or at least a tweak with a well intended purpose. A bad tweak might be thought of as one that intentionally decreases how well a model's output compares to that of similar systems in similar environments, often capitalizing on customer ignorance for the purpose of financial gain.
4.) To clear up something: I don't believe I wrote, or meant to imply that the 10% # is set in stone, and I never said or implied "right off the bat". That 10% underestimate is indeed an estimate. FWIW, you seem to agree with that estimate in your statement: " But you are right about the difference at 10 or more %." (your post of 05/09/2016, 08:39 P.M. (sic on time stamp)). Also,a # like that 10% seems a rough consensus among those knowledgeable in such matters and possessing broad powers of observation. Is it 8% ? 12% ? More ? Less ? Maybe. Maybe not. It's a gut, a feel, a dart throw. Whatever it may be numerically, the majority (including NREL, or they probably wouldn't have acknowledged the reasons for the PVWatts revisions as they did) seem to think that PVWatts, and thus the EPBB tool are probably not the best in terms of estimate vs. observation. BTW, while on that part of your above post, I don't think the state of CA uses much of anything to calculate federal energy tax credits as you seem to think. For starters, I believe that is the responsibility of the taxpayer. Also, CA has nothing to do with the federal tax credit.
5.) As for any "proof " : Without seeming to duck out under the cloak of brevity, this is not the place or venue for long streams of data. However, as the guy in my HOA who reviews and recommends action on solar additions to the HOA Arch. Rev. Comm.,I've got about 95 or so files of neighbors in my HOA. Each one contains most all there is to know about their PV systems. Among other data/information, all contain vendor quotes, many contain several quotes from competing vendors. All have copies of the contracts, either purchase, lease or PPA. All have copies of docs required for county/SDG & E approval. All have system performance estimates from both the vendors, and by me - for my purposes only. My estimates include a PVWatts run (by me, or the bidder(s) or both), a SAM run (by me), and often, other estimates based on stuff I've written. All that's where some of my opinion about the ~ 10% number for PVWatts came from.
Some of the rest of my opinion came from comparing output from cooperative neighbors' systems with both PVWatts estimates and SAM estimates. Given the same inputs to the greatest extent possible, SAM estimates of long term system output have been about 8-10% higher than the PVWatts output using TMY3 Miramar weather, the 14 % default system loss parameter, all the while trying to be as true to actual conditions as the models' inputs will allow. As for neighbors' actual output vs. the models' estimates, I've used clear day estimates from the models for actual clear day output on a same date basis and compared that to actual output for that clear day. Sam is usually within a few %. I then "tweak" the PVWatts system loss parameter to match both actual output and SAM' s estimate. The PVWatts system loss parameter usually come in at about 8 - 10% for a reasonable match, vs the 14 % default.
I'm not out to prove a position. I'm looking for a way to accurately estimate current and future system performance. My experience is that the EPBB calculator underestimates likely system performance by something of the order of 10 %, and that a PVWatts system loss parameter of something like 8-10% has shown to be better # to estimate long term average performance than the 14 % #. Opinions vary, but until I find otherwise, that's my direction. I think I'm in good company with that opinion.
6.) Where's the "proof" of your claim: " For the last 14 years that calculator has been the most accurate overall in the predicted output here." ? That door swings both ways.
7.) We also seem to disagree on customer sophistication and knowledgeability with respect to solar energy, its applications, potential and limitations. I'd sum my opinion up by writing if more people had any clue, and knew some of what I think I know, there would probably be fewer PV systems on residential property, and those systems that did exist would be smaller, better designed and more fit for purpose. Or, to use your phraseology, maybe more systems would kick booty. I'll refer you to a pretty healthy portion of the postings to this forum for further reasons for my somewhat skeptical opinions with respect to the general public's knowledge of things solar. It's a target rich environment for shysters.
8.) Please do not continue to try to twist what I write into an inference that I think everyone is dishonest with such statements as: "Everyone who uses this calculator is a scumbag who rips the poor uneducated customer off". Not everyone rips customers off. But to think all vendors are scrupulous is naïve. My apologies to all the scrupulous vendors. Don't get me started on how many of the 30 or so vendors I've dealt with might know something of what they're doing. The brainless ones who hit/run force the good ones to drag the anchor of poor reputation and drag the industry down. At least we may agree on that point.
9.) I am not active in any industry. I have never worked in, what you call, the "PV industry". Since you seem curious, I'm retired with more time than money and more money than brains. FYI, I spent the first 10 years of my adult working life as a commissioned peddler and made a fair living. When my curiosity and interest in solar energy overtook me, I took a pay cut, returned to school, got a BSME, and was trained, worked, licensed and titled as mechanical engineer, designing industrial heat transfer equipment and pressure vessel and piping systems for power plants, refineries and other energy intensive industries. Solar and alternate energy was the driving reason I became an engineer, but no one was building solar equipment in my area when I started the engineering odyssey. Solar has always been more than a hobby and more like a second job. Being retired, and not having any skin in the game, I now get to bloviate all over the planet via the net, pretty much with impunity. Interpret my situation as you please.
Take what you want of the above. Scrap the rest.Last edited by J.P.M.; 05-10-2016, 05:11 PM.Comment
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I posted a new thread about panel clipping with data. Since it relates to this inverter and panel I'll do a repost here for general interest, hope it doesn't violate some rule or another
Had some perfect days here and noticed that at peak my system flatlined. Nearly straight lines don't often happen in nature so I suspected clipping, let's see ...
You can see my system below- Enphase S280 inverters
- LG 315W panels
Now to the data- Under STC (Standard Test Conditions) the panel voltage/current gives 315.4W, conditions are 1000W/m2 irradiance and 25C.
- My Davis science grade weather station measured 998 W/m2 at peak, convenient!
- Microinverter temperature at that time is 111F/44C, inverter temp isn’t panel temp but assume it’s close
- Temp derating then gives the panel output at about 90%, so now it’s a 283W panel
By the way Enphase has a great paper on sizing inverters for panels
Their conclusion - bigger panels perform better year round. And in fact, in this case I did a rough integration and figured I lost maybe 500w-1kW or say 25 cents worth of energy at peak, but at non peak am generating more power than I would with smaller panels. Which corroborates their study.
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I point out that it is biased because many people bring up that paper as proof that larger modules on small inverter is proof that clipping gets more production which is 100% not true.
Larger modules produce more DC, and LARGER inverters produce more AC from the larger DC.
yes a 350W module paired with a 250w inverter will produce more than a 300w module with the same 250W inverter. True. But ALSO F'n true that the same 350W module paired with an inverter large enough to have no clipping will produce even MORE power. -
Fair enough. Let's take it in pieces though ...
| proof that larger modules on small inverter is proof that clipping gets more production which is 100% not true
Not sure what that means. The paper demonstrates that money is better spent on larger panels than larger inverters. Given that micros tend on the small side this goes against Enphase if anything, I think.
| Larger modules produce more DC, and LARGER inverters produce more AC from the larger DC.
Yes ... but that's a bit of a tautology and sidesteps the point of the paper, which is a larger inverter relative to panel will just you marginally better actual output, from their real-world data. Basically they just clip just under ideal conditions which rarely occur.
| But ALSO F'n true that the same 350W module paired with an inverter large enough to have no clipping will produce even MORE power.
Oh seriously you're still not getting it, despite the bluster. Go read the paper and report back, better yet study it and ponder the different between an integral and a derivative. -
no you are not getting it. I never claimed that a larger inverter on the under sized module would produce more. I claimed that a larger inverter on a larger module would produce more than that same module on an inverter that clips.
lets use an example. say with a S280 vs SolarEdge and 350W pv module and 300W pv module:
the enphase report is claiming (correctly) that
an S280 will produce more power with a 350W module vs the same S280 with a 300W module
the S280 with 350 would produce more than S280 with 300W correct
however solaredge with 350W module would produce even more than the S280 with the 350W module
As you state the paper demonstrates that funds are better spent on a larger module than a larger inverter, However, they assume that the inverter is the same cost. Solaredge and other larger micros are cheaper than enphase, so you would have more funds to put towards a larger module....Last edited by ButchDeal; 05-12-2016, 02:00 PM.
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More later, but the inverter temp. has little if any relation to the panel temperatures. FWIW, this time of year, and from a lot of measurements I've done w/ an IR thermometer from under the array, the ave. array temp. runs about 25 - 30 C. above roof ambient under clear skies at min. incidence angle time, a bit less as f(P.O.A.) off min. incid. angle.
The roof ambient runs about 1- 2 deg. C or so above ground level ambient for early/late day or cloudy skies and ~ 3 - 6 C. above ground level ambient location for clear skies, depending on wind/sun. The panel temps. are primarily F(P.O.A. irradiance, wind vector, ambient air temp. at the array) and a few other minor effects.
What's the orientation of your Davis solar sensor ? If horizontal, that 998 W/m^2 needs to be adjusted to P.O.A. irradiance for what you're measuring. For a bit more precision, the solar sensor reading needs to be corrected for temp. effects (~ -1/1500 per deg. F. above/below 77 F.).
I've got a Davis Pro II +, located about 4 ft. north off the N-S axis of my array. Been measuring/recording this type of stuff for a bit over 2 years or so.Comment
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Yes agree but as sensij said it's probably not a horrible rough idea
Good to knowFWIW, this time of year, and from a lot of measurements I've done w/ an IR thermometer from under the array, the ave. array temp. runs about 25 - 30 C. above roof ambient under clear skies at min. incidence angle time, a bit less as f(P.O.A.) off min. incid. angle.
HorizontalWhat's the orientation of your Davis solar sensor ? If horizontal, that 998 W/m^2 needs to be adjusted to P.O.A. irradiance for what you're measuring. For a bit more precision, the solar sensor reading needs to be corrected for temp. effects (~ -1/1500 per deg. F. above/below 77 F.).
Awesome! I've got a Vantage Pro2 with the trimmings. I'm also soon mounting mine above the array, let's compare notes.I've got a Davis Pro II +, located about 4 ft. north off the N-S axis of my array. Been measuring/recording this type of stuff for a bit over 2 years or so.Comment
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Better than using the inverter temp.
Short ans: Without knowing the apparatus and method of attachment of the temperature sensing method(s) used, I can't offer an intelligent response to your question in the form of an opinion.
See: "Photovoltaic Array Performance Model", King, et al., Sandia, 2004, and other sources for some discussion/information on array or cell temp. measurement. I've found the Sandia method workable for my situation, and it seems to give reproducible results. In general, my opinion is that stand off (detached) methods of temp. sensing - IR thermometers - have advantages over "attached" methods (thermocouples, etc) that may be more numerous that the disadvantages of the attached methods. Lots to consider.
I'm closing in on a short description of my results of a comparison of estimating array average instantaneous cell temps. using array voltages vs. actual measured panel temp. measurements (I know, it's been gestating for a long time now - give it a pit drip induction as my bride would say). Film at 11 or when bulletins break.Comment
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See my last post. While Sensij's comment describes a method that is for sure better than using the inverter heat sink temp. (as I assume what you were referring to), I'd not put a lot of faith in reported microinverter temps. without getting up on the roof and getting some data for comparison and/or calibration, or understanding more about how Enphase, or any monitoring system measures or estimates cell temps.
So far there are 3 posters I know of with Davis Pro II + instruments - you, me and Sensij. I'm working on pyranometer changeout and calibration check. I'm SWAGing mine's drifted ~ 1.8 - 2.3% or so over the last 8-12 months and is underreporting GHI by about that much. Can't/won't speak for Sensij, but it looks like he may have high readings due to albedo effects. This is not an exact science by a long shot.Comment
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Interesting statement (I don't sell solar, btw). For someone considering solar in San Diego (or California in general), and say they were reasonably able to project their usage, what defines "oversize"? Or, maybe better asked, what is the "right size". Is it 60%, 70%, 99%? For me, the lowest rate on my bill is ~18c kwh, I'd want to cover that and up because the ROI is good. Certainly current TOU periods are not likely to shift, right, so that shouldn't affect calculations much? What specifically is the ignorance and what are the myths? Would there be any scenario for a larger system today that might make sense later?
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I want my system oversized for my current usage because if I had lower-cost power, I would not suffer with minimal air conditioning going forward. Also I have a Tesla Model-3 on order. 10,000 miles on the car will be about 3.5 MW a year.
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Also SREC2 in MA are worth about $250 per MW. Electricity costs about $230 per MW, so the SRECs are worth more than the electricity. So even if your system is larger than you need, and even if you run a negative balance on your electric bill, you still won't lose due to the SREC value.
And besides, if I had a negative balance on my electric bill, I would just turn on resistance heaters in the winter and use up the power, saving on heating oil.Comment
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My thoughts exactly. I wanted to use about 500kw extra worth of AC per year. I bought a Volt before solar, so I knew my EV usage. Sadly, I'd like a Tesla 3, but with 500k now on order I think that's a 5+ year wait and the tax credits will be gone. Too late to the party for me. I am considering a base Model S, but I'll likely talk myself out of it.Comment
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I was 16th in line on the first day and got my order in around 10:07am EST. Given the 50 or so dealers on that time zone, I should be in the first 800.
They accidentally charged my card twice, so I have two orders. I was going to cancel one, but then didn't.
When preparing my load center for solar, I moved some circuits to a sub-panel and freed up enough space for 200 amps of car charging.
If I have an electric credit, I can always use electric space headers.
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