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Max Watts per panel for any date, given my location? (no obstructions, flat roof)

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  • pkellner
    replied
    This is all great information! Thanks everyone. I'm thinking, as usual, I've gotten in over my head and maybe just leave things well enough alone. Sounds like the cost of getting an engineer to sign off, designs, new racking, etc will probably overwhelm any gain I get. The roof is about 5 degrees sloped in the good direction (for drainage). that is, sloping down from south to north and more facing where the sun is.

    BTW, the reason I have two inverters is because the system started with the 5K inverter and 14 290W panels. (I bought the house with that). I personally prefer the micro-inverters cause it's simpler, but decided to stick with what I had and added 11 320W panels and a 3K inverter. I had thought that because I wasn't tilting the panels that I'd get no where near 320Watts peak out of any of the panels but am surprised that now (today), each of those panels is peaking at close to 300 watts (and we are only mid march).

    Leave a comment:


  • J.P.M.
    replied
    Originally posted by pkellner View Post
    Mostly related, if I want to tilt some of the panels west, I assume, because of stadium effect, I should just do the two on the East side of my roof? (the roof is pointed directly south now with a little bit of tilt. I'm attaching pictures of my existing solar mounting system. I know nothing about tilting panels. Does it require a new system, or can the existing one handle that?

    https://www.dropbox.com/s/r40ffxg0afhle34/P3.png?dl=0

    https://www.dropbox.com/s/ueunnw94q1ax45x/P2.jpg?dl=0
    IMO, the sooner you disabuse yourself of the idea of fooling around with stringing, the faster you'll be on your way to a better system. The inverters and what feeds them seem reasonably matched.
    However, and without knowing much about the design, I'm wondering why 2 inverters in the first place.

    From the photos, it looks to me like the roof has a slightly northern tilt.

    O racking: Tilting an array usually involves the use of racking similar to what you have now with one side (the north side) elevated. There are usually fewer anchoring points on a tilted array.

    The existing system you have might need entirely new racking but I'd wager it probably won't, except as a way to dazzle you out of making the change by quoting a ridiculously high price to avoid doing something the installer either doesn't want or doesn't know how to do, or it just ain't worth their time when they coiuld be doing more familiar work faster and more profitably.

    Whatever the required changes in the design, it'll need to be submitted to the county for review and approval of the structural calcs for both the racking and what that racking is attached to (the roof) that show sufficient strength in both the racking and the roof and the attachments to the roof to be able to withstand the greater wind loading due to the increased (or at least different) array wind profile.

    As it stands now, for several reasons already discussed, you'd have been better off tilting the array in the first place, but that train has left the station.

    Get with your installer for details, but given what they did to you, I wouldn't be surprised if they balk and try to B.S. their way out of doing a modification because they don't want to do a tilted array or have the necessary structural calcs done (which will most likely require a P.E. stamp).

    In the meantime, Google is your friend. Google "tilted solar racking" and follow the bouncing ball. Pretty straight forward stuff and done fairly often, particularly in more northerly latitudes.

    Leave a comment:


  • pkellner
    replied
    Mostly related, if I want to tilt some of the panels west, I assume, because of stadium effect, I should just do the two on the East side of my roof? (the roof is pointed directly south now with a little bit of tilt. I'm attaching pictures of my existing solar mounting system. I know nothing about tilting panels. Does it require a new system, or can the existing one handle that?

    https://www.dropbox.com/s/r40ffxg0afhle34/P3.png?dl=0

    https://www.dropbox.com/s/ueunnw94q1ax45x/P2.jpg?dl=0

    Leave a comment:


  • chrisski
    replied
    Originally posted by bob-n View Post
    chrisski, Will that E/W technique work if all of the panels are in the same string? My concern is that the panel getting very little sun will limit the output from the panel getting a lot of sun.
    It honestly is more work than its worth, and getting panels out of horizontal can increase output. I have panels on my RV roof that got 40% more output when I tilted them to 30 degrees versus being flat, so that may be an easier fix. What stopped me from tilting is sunset and sunrise winds at the places I go to can be in excess of 30 knots and that is like putting a sail up on my roof, and I don't want to find out the wind rating of my panels while extended.

    If you had two panels in Series facing East, Two panels in series facing west and these were in parallel to a charge controller, they would not limit eachother. Shortly after sunrise, the two panels facing the eastmay be maxing out with putting out 64 volts and 10 amps, where the panels in the West would be in the shape and putting out around 65 volts and 1 amp, and you'd add the production together to get 11 amps.

    With those 11 amps I mentioned, of you took six of the seven remaining panels and placed them south, running two in series with three sets in parallel, each set would make somewhere between the east and west panels, so I'll say five amps per set. That would get you 15 additional amps.

    So in the example I gave, there was an extra amp of production by orienting East / West versus placing 10 directly south, but what I'd like to do is come up with formulas for orientations of panels and make a spreadsheet that should show close to truth data.

    I'm getting closer to getting a property where I could do something like this, and I have a feeling having East and West Panels may help with my final design. I do like that based off the extra production I get from my portable panels based off the three times I move those when they are out.

    The other reason I'd like that is if I ever get solar on my current house, roof space limitations means I will need to place panels on an East West part of the roof. There's several houses that have a North Orientation, which to me seems like a terrible idea, but I'd like to be able to run a scenario like that.

    My big electric draw would be in the summer when the sun rises from the NE and sets in the NW, so I might be surprised by some of the results.

    Leave a comment:


  • pkellner
    replied
    Originally posted by J.P.M. View Post

    Like Bob-n's suggestion, I wouldn't don't change the stringing. You might lose a bit of performance due to clipping now, but my guess is it won't be enough to make up the $$/hassle spent on all the fooling around.

    But I would consider changing the panels' orientation. IMO, that horizontal orientation was a mistake.
    How long have you owned the PV system ?
    If not too long, you'll be finding out that horizontally oriented panels need much more frequent cleaning to keep efficiency and so output from slipping, particularly in the desert. Without cleaning, your clipping will be a lot less due to lost irradiance, but so will your production for the same reason.
    Anecdotally, there is an array with a mostly horizontal orientation on a mostly horizontal roof near my property at Indian Head Ranch. The panels are effectively shallow sand boxes that turn to the dust to mud with any dew that does form - or any of the ~ 5"/yr. or so of precip. that may fall on the panels and then cake up and stick in layers. An ongoing consideration for horizontal arrays, particularly id desert climates.

    I wouldn't mess with any configurations you have in terms of shifting panels around, but I would suggest you might consider a reorientation of the panels to a non horizontal tilt for one reason: Better annual performance as a result of better array orientation, and less loss of performance due to less panel fouling of the type horizontal panel orientation causes. If you're interested in some rationale for that opinion, read on.

    I often say welcome to the neighborhood to new posters. I can't say the same to you as we already are neighbors of a sort. My wife and I own some property about 3 miles north of Christmas Circle out Borrego Springs rd. near where the road takes the sharp right turn.

    Having done some research on the subject of PV production in Borrego Springs and elsewhere far that matter, prior to designing my magnum opus solar home in the desert which got through plan check and permitting but never got built - a long, boring story - the best array orientation for unshaded residential PV in/around Borrego Springs for max. annual array production is about 195 deg. az. and a 31 deg. tilt.
    The best orientation in Borrego Springs for max. annual PV T.O.U. bill offset at current SDG & E T.O.U. rates for the typical rate schedule for customers with PV arrays (Sch. DR-SES) is ~ 208 deg. az. and ~ 33 deg. tilt.
    That 195/31 orientation will improve your annual array output over the current horizontal orientation by ~ 12-14 %.
    Higher tilts to maybe 45 deg. while still at the 195 azimuth orientation will keep your panel temps down in the summer but will also lower summer output some from the 195/31 orientation by maybe by a % or 2.
    However, between the lower summer panel temps. and higher winter output, the 45 deg. tilt will provide, as well as the lower fouling, and also the lower clipping from the lower summer output, an annual output that may be close to or slightly more than the horizontal orientation you have now.

    Any tilt beyond maybe 10 deg. or so with a mostly southern azimuth will improve performance over the horizontal orientation by helping to minimize the propensity for the panels to turn into caked up sandboxes.

    3 kW inverter may clip a bit now - which BTW most likely won't harm it - when the ambient temps. climb on into the summer, but that'll only be for an hour or 2/day around solar noon and it'll be less of a loss than you think because the panels will be operating near their max. operating temp. so I don't think you'll have a big problem other than what Bob-n notes as some clipping - which, as described, probably won't be all that much.

    Ironically, mitigating that clipping some is the idea that, for several reasons, the horizontal array orientation will cause the array to operate at a higher temp. than a tilted array and so lower array output a bit. Specific to Borrego Springs or other hot desert climates, the additional heat won't help the longevity of the electronics, particularly, IMO only, SolarEdge stuff, but probably not in a fatal way, just more service requirements and lost production would be my guess.

    On you future plans to model array output, if you're doing it to learn about the solar energy and its applications, I'd say it's a great way to learn. But if you're doing it as a way to learn what's likely to happen with your array, your time will be, IMO only, much better spent reading and understanding all the background info contained in all the help/info screens that come with PVWatts. It's a powerful tool not only for preliminary design but also for learning. If/Once you get familiar with it, you'll see why screwing around with stringing/changing panels may not be the best way to go. If you get the bug and want more details on array performance I'd also respectfully suggest models such as "SAM" - an NREL modeling program which is best described as PVWatts on steroids.

    As for how SDG & E T.O.U. rates and array performance turn into annual billing offset with SDG & E, I've done a spreadsheet that takes PVWatts output, multiplies it by the SDG & E hourly rate for DR-SES rate schedule (the common T.O.U., rate for NEM customers) and spits out the value of a system's generation by hour for an 8,760 hour typical meteorological year (a "TMY"). As long as annual array production is less than annual use, that generation value mostly independent of annual use. The spreadsheet is quite large but the generation value sheet but even though it's about 9,000 rows, it's mostly is mostly a copy job of 2 ea. 168 hour weeks with a few tweaks for spring T.O.U rates. If you're familiar with spreadsheets, that part is a cakewalk. Understanding SDG & E's rate schedules is the tough - but necessary part.

    If I were you, I'd give PVWatts another try. It does take a bit of a learning curve, but once you read all the help/info screens a couple of times to learn what to input and how to do it, it can give a lot of information - info of the type that will help you make informed decisions and maybe see why moving panels around may not be the way to go.

    Good luck.
    Thanks JPM for all the helpful information. My wife and I have been here in Borrego since last March and bought a house up on Bending Elbow. When the plague is over, it would be fun to get together. We do ride our bikes "a lot" so you've probably seen us around, usually wearing the same outfit, but lately not since one of the locals stopped and yelled at us for something I don't want to discuss, but has my wife nervous.

    I think you have me completely upstaged as a solar fanatic (though I do look at my Solar output, and my rainforest SDGE usage almost daily. I've also been thinking about a small battery with all the power failures we've been getting, but concluded probably best to do something for just the refrigerator, as that's really all I care about.

    Anyhow, be great to connect if you are up for it. Send me your email by going to my blog at https://peterkellner.net contact page and we can continue from there.

    BTW, I do like the idea of at least re-orienting half of our 11 panels that are above our garage to face more West. I think that will totally solve my clipping problem, as well as is probably a good idea, especially at 60 cents a KWH after 4PM. I do appreciate the insight into the mud problem but not sure it's worth it as we have a stairway up to our roof and the panels are super easy to wash off.

    Thanks again

    Leave a comment:


  • J.P.M.
    replied
    Originally posted by pkellner View Post
    Thanks bob-n. That's really good input. I'm in Borrego Springs where in the summer, 110 is not unusual, and it's in the 70's now. Also, the part about the panels aging. I'll leave well enough alone and try not to loose sleep over it. Someday, when my day job isn't so important, I want to write a solar simulator that takes all kinds of inputs (including TOU rates) and does what-if analysis's like this by integrating over the averages. Probably, I'd be the only one who would care, but then... I'd have nothing better to do.
    Like Bob-n's suggestion, I wouldn't don't change the stringing. You might lose a bit of performance due to clipping now, but my guess is it won't be enough to make up the $$/hassle spent on all the fooling around.

    But I would consider changing the panels' orientation. IMO, that horizontal orientation was a mistake.
    How long have you owned the PV system ?
    If not too long, you'll be finding out that horizontally oriented panels need much more frequent cleaning to keep efficiency and so output from slipping, particularly in the desert. Without cleaning, your clipping will be a lot less due to lost irradiance, but so will your production for the same reason.
    Anecdotally, there is an array with a mostly horizontal orientation on a mostly horizontal roof near my property at Indian Head Ranch. The panels are effectively shallow sand boxes that turn to the dust to mud with any dew that does form - or any of the ~ 5"/yr. or so of precip. that may fall on the panels and then cake up and stick in layers. An ongoing consideration for horizontal arrays, particularly id desert climates.

    I wouldn't mess with any configurations you have in terms of shifting panels around, but I would suggest you might consider a reorientation of the panels to a non horizontal tilt for one reason: Better annual performance as a result of better array orientation, and less loss of performance due to less panel fouling of the type horizontal panel orientation causes. If you're interested in some rationale for that opinion, read on.

    I often say welcome to the neighborhood to new posters. I can't say the same to you as we already are neighbors of a sort. My wife and I own some property about 3 miles north of Christmas Circle out Borrego Springs rd. near where the road takes the sharp right turn.

    Having done some research on the subject of PV production in Borrego Springs and elsewhere far that matter, prior to designing my magnum opus solar home in the desert which got through plan check and permitting but never got built - a long, boring story - the best array orientation for unshaded residential PV in/around Borrego Springs for max. annual array production is about 195 deg. az. and a 31 deg. tilt.
    The best orientation in Borrego Springs for max. annual PV T.O.U. bill offset at current SDG & E T.O.U. rates for the typical rate schedule for customers with PV arrays (Sch. DR-SES) is ~ 208 deg. az. and ~ 33 deg. tilt.
    That 195/31 orientation will improve your annual array output over the current horizontal orientation by ~ 12-14 %.
    Higher tilts to maybe 45 deg. while still at the 195 azimuth orientation will keep your panel temps down in the summer but will also lower summer output some from the 195/31 orientation by maybe by a % or 2.
    However, between the lower summer panel temps. and higher winter output, the 45 deg. tilt will provide, as well as the lower fouling, and also the lower clipping from the lower summer output, an annual output that may be close to or slightly more than the horizontal orientation you have now.

    Any tilt beyond maybe 10 deg. or so with a mostly southern azimuth will improve performance over the horizontal orientation by helping to minimize the propensity for the panels to turn into caked up sandboxes.

    3 kW inverter may clip a bit now - which BTW most likely won't harm it - when the ambient temps. climb on into the summer, but that'll only be for an hour or 2/day around solar noon and it'll be less of a loss than you think because the panels will be operating near their max. operating temp. so I don't think you'll have a big problem other than what Bob-n notes as some clipping - which, as described, probably won't be all that much.

    Ironically, mitigating that clipping some is the idea that, for several reasons, the horizontal array orientation will cause the array to operate at a higher temp. than a tilted array and so lower array output a bit. Specific to Borrego Springs or other hot desert climates, the additional heat won't help the longevity of the electronics, particularly, IMO only, SolarEdge stuff, but probably not in a fatal way, just more service requirements and lost production would be my guess.

    On you future plans to model array output, if you're doing it to learn about the solar energy and its applications, I'd say it's a great way to learn. But if you're doing it as a way to learn what's likely to happen with your array, your time will be, IMO only, much better spent reading and understanding all the background info contained in all the help/info screens that come with PVWatts. It's a powerful tool not only for preliminary design but also for learning. If/Once you get familiar with it, you'll see why screwing around with stringing/changing panels may not be the best way to go. If you get the bug and want more details on array performance I'd also respectfully suggest models such as "SAM" - an NREL modeling program which is best described as PVWatts on steroids.

    As for how SDG & E T.O.U. rates and array performance turn into annual billing offset with SDG & E, I've done a spreadsheet that takes PVWatts output, multiplies it by the SDG & E hourly rate for DR-SES rate schedule (the common T.O.U., rate for NEM customers) and spits out the value of a system's generation by hour for an 8,760 hour typical meteorological year (a "TMY"). As long as annual array production is less than annual use, that generation value mostly independent of annual use. The spreadsheet is quite large but the generation value sheet but even though it's about 9,000 rows, it's mostly is mostly a copy job of 2 ea. 168 hour weeks with a few tweaks for spring T.O.U rates. If you're familiar with spreadsheets, that part is a cakewalk. Understanding SDG & E's rate schedules is the tough - but necessary part.

    If I were you, I'd give PVWatts another try. It does take a bit of a learning curve, but once you read all the help/info screens a couple of times to learn what to input and how to do it, it can give a lot of information - info of the type that will help you make informed decisions and maybe see why moving panels around may not be the way to go.

    Good luck.

    Leave a comment:


  • bob-n
    replied
    chrisski, Will that E/W technique work if all of the panels are in the same string? My concern is that the panel getting very little sun will limit the output from the panel getting a lot of sun.

    Leave a comment:


  • chrisski
    replied
    Something else you can do with the one that is clipping is to orient a couple to the East and a couple to the west. That way the east facing produce earlier in the day, die down as the sun moves, and then the south facing panels take over, and as the sun starts to set, production drops off in the south and picks up in the north.

    This will require some figuring and how many panels in series, and all strings would need to match. With 11 panels, this would mean taking one off, (2S5P) or buying at least one more (2S6+P).

    I’ve wondered about the simulator you mentioned, but am honestly lost at the math involved in that to include the different times of year.

    I’m only familiar with PVwatts and it is supposed to have weather built into the monthly averages.

    Leave a comment:


  • pkellner
    replied
    Thanks bob-n. That's really good input. I'm in Borrego Springs where in the summer, 110 is not unusual, and it's in the 70's now. Also, the part about the panels aging. I'll leave well enough alone and try not to loose sleep over it. Someday, when my day job isn't so important, I want to write a solar simulator that takes all kinds of inputs (including TOU rates) and does what-if analysis's like this by integrating over the averages. Probably, I'd be the only one who would care, but then... I'd have nothing better to do.

    Leave a comment:


  • bob-n
    replied
    Short answer: It's probably best to leave it as it is, but it would be an interesting experiment to try.

    Long answer: It is best to have identical panels in the same string. The weakest panel puts out the lowest current, and all of the other panels in that string are limited to that current. In your case, it may not hurt you much, because you would be adding strong panels (320W) to a weaker string (290W). It's not optimum, but it will only reduce the output of the one or two panels that you move.

    In addition to power output, you also need to know the voltage output for each panel and the maximum voltage rating of the inverter. You don't want the sum of the maximum open-circuit outputs from each panel in the string to exceed the absolute maximum rating of the inverter. Otherwise, you could destroy the inverter. Different panels have different output voltages and different inverters have different absolute maximum voltage ratings. Before any change, you need to research this.

    Another thing worth knowing is that solar panels produce more power on colder days. If the weather now is cool and you're getting lots of power, that may not be a reason for worrying about the summer. The sun will be higher, giving you more light, but the air will be warmer, reducing power output. It may come out a wash.

    There's yet another consideration. If your 11 panels are producing more power than your inverter can handle, it will limit. That's OK. It will only limit at the peak of the day (ie: 11:30AM to 12:30PM). Every other time, your inverter is taking all that the panels can give. So you won't be losing much. But if you move a panel to the other string, you will not be taking full advantage of the panel. You'll be derating it to the equivalent of a 290 watt panel all of the time, just to get a few extra watts at high noon. It may not buy you anything.

    Finally, panels don't produce the same power when they're a few years old as when new. They drop perhaps 0.5% output per year. After a few years, the 3kW inverter will not be your limiting factor. Especially if global warming continues.

    Leave a comment:


  • Max Watts per panel for any date, given my location? (no obstructions, flat roof)

    I currently have 2 Solar Edge Inverters, a 5K and a 3K. The 5K has 14 290W panels on it, and the 3K has 11 320W panels. I noticed today, that my 3K inverter was near max (3K) for a couple hours and it's only March. My question is, should I move 1 or 2 panels from my 11 320W panels on the 3K invert to the 5K inverter.

    Another way to ask is:

    My 11 320W solar panels are currently generating 2950 Watts and it's March 10th. I'm near San Diego, flat roof with no obstructions (panels mounted flat), how many watts might I expect to produce in May and June when the sun is the highest?

    I did look at hourly PV Data and the numbers are hard to make sense of as it seems it includes cloudy days.
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