Panel mounting - roof

Hi Everybody,

I'm about to take the plunge and start a self-installed grid-tie system. I've already got alot of info but I haven't found the answer to the following question. I'm in Los Angeles (around Lat 33)

The ridgeline of my garage points 35 deg E (adjusted for declination), so the 25 degree slope roof surfaces point to 125 and 305. The 305 side has less shading, though both sides are pretty good. Rather than mount a flat 4x4 or 4x5 array, wouldn't it be better to mount in 4 or 5 panel strips from eave to ridge using low profile legs tilted South-ish?

The 125 surface wouldn't need much tilt to add up to a generally south array, the 305 probably closer to 25 deg from the roof.

I haven't seen anyone that has tilted their arrays on an already sloped roof. Any special reasons that I'm unaware of?

TIA,
Eric

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Panel mounting - roof

    I have seen solar panels on sloped roofs that have been angle mounted.

    If you want to see how different mounting angles affect the solar energy captured in your area, see this link:

    http://rredc.nrel.gov/solar/pubs/redbook/


    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Panel mounting - roof

    there isn't anything wrong in angling them by the mounts, but if you angle too far you will find pvs shading pvs behind them. this may be more pronounced in the winter months when you would want and need more sun. i too have a main roof with this type of predicament. i do not use the main roof at this time as i found it easier to mount above my porch roof that doesn't follow the extreme east-west circumstances my main roof does. it is still off by 39 degrees though so angling is still a must for me. i compromised at 23 degrees west of south as it receives more overall sunshine from that sw direction due to a hill and the wooded areas more towards the se direction. my pvs are mounted off of the vertical slope of my 3 story brick home with aluminum angle mounts i devised myself that also have some play for adjustments for better seasonal use as opposed to just one permanent position.
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Panel mounting - roof

    A couple of other things to think about...

    1. Trees and other things (i.e., new construction) that happen over years... Try to place them where you can "control" the shading.

    2. I have Grid Tied with Time of Use metering... If you panels will output more power than you use during the TOU period (mine is Noon-6pm Mon-Fri), then you might be better favoring the more westerly exposure and generating more power during the peak power cost period (for me, is about $0.10 vs $0.30 per kWhr--or my panels generate ~3x the dollar amount of power during the "peak" charge period that I can use during my off-peak time--this is the "summer" rate, not near the difference in costs during "winter").

    3. Almost any shading causes dramatic loss of electrical generation (i.e., on my system with two arrays, 5% shading--one panel/of twenty--costs me 50% of my power). Try to avoid any shading you can during the 9am-3pm range.

    4. With Grid Tied power in California, you should be able to use a 1 year net metering plan (I have this near SF, CA). So, overall, unless you use a huge amount of power in the summer (like Airconditioning), I would suggest that you aim at maximum amount of kWhrs per year (instead of just Summer or Winter peak generation). Net Metering will allow you to spread the money (credit/debit) over a 1 year period.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RoderickRoderick Solar Expert Posts: 253 ✭✭
    Re: Panel mounting - roof

    A tip on TOU metering, check your utility's website and look at the exact plans for interconnect agreements immediately. That .10 vs. .30 cent deal was closed to new customers the the end of 6/2006 (I assume that being in the SF area, BB is referring to PG&E). I just missed it by a few days, and if I had been on the ball, and filed as soon as I got my panels, I could have got it. The new TOU is significantly worse, it seems, to the tune of several hundred dollars a year. The utility, at least mine, only needed the crudest of drawings with the application, so don't wait to make it pretty.

    Also, when filing for your California rebate, if you have not already done so, be sure and check your supplier's specs and math if they filled out the form for you. On mine, the supplier filled out the wrong efficiency figure for my inverter (check the CEC website for the authoritative value), losing me maybe $50 of rebate. Not a whole lot, but hey, could be a nice dinner for two, especially if you don't drink.
  • System2System2 Posts: 6,290 admin
    Re: Panel mounting - roof

    Thanks for the input.  The garage is at the NW corner of my property, my neighbor has already built as much as he can close to me, so there will be no additional shading in the forseeable future.  I had already realized the shading effect of the tilted strings and was planning on spacing them to avoid shading at Winter solstice.  The WNW roof has 2 power lines fairly close, 15 - 20 feet away(mine and my neighbor's) as the only shade that would hit the panels at all for more than the 6 summer months.  

    How much impact does a 1" shadow have on a 200w module's output? (my sailboat's single 46w panel does not like shade from even a single rope)

    I'm having a hard time calculating the actual slope and direction that results from tilting on a 90 degree axis from an already tilted surface, I'll probably just go up there with a board and measure it.  I could also mount the panels on the WNW roof of my house, same slope and direction, modules tilted, even less shading during prime time, though a longer run to get to the service panel on the far side of the garage.

  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Panel mounting - roof

    Yep, PG&E and E-7... Sorry to see it not available anymore. Although, I did think it was "too good to be true" and it did not make much sense for a company in business to sell power to me under this hybrid solar/TOU plan. Except, this year, because I don't have an electric vehicle (or AC) at this time and oversized my solar array, PG&E will get to keep my ~$200 1 year net metered credit...

    Roderick, what is the new TOU plan and does it affect E-1 residential with solar too? I did not quite understand how it works from the rate sheets. If it s Schedule "S", it sounds like a nightmare to calculate, measure, and understand how it interacts with solar (given that solar is not a consistent source of power from 10am through 8pm--as an example).

    I guess I now have to wait and see how long before they change me (and I lose the $277 E-7 meter they sold me).

    Regarding shading, I have a 3kW Grid Tied Array, however, it is on a second story and would not be fun to try and shadow my array.

    Ewdysa, if possible, I would see if the panel supplier (and/or installer, if there is one) and work with them to do some shadow tests with the panels/inverter/orientation and see how much it affects the output (use 1" blue painter's tape or duct tape across the arrays to test?). I would be concerned about a 1" shadow on my arrays during the summer and would try and test it. If you lose less than 10% power, then it probably isn't too bad.

    Is it possible that you can reroute the utility drops to better location (you would probably have to volunteer to pay for your neighbor's move too)... Sometimes solar electric is just not meant to be...

    Solar hot water (and/or hot air for space heating) may be a better (and more cost effective) application here.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Panel mounting - roof

    I too never see other people angle-mounting there panels on an already sloped roof, but I did it on my little 1KW array, I'm rotated about 20 degrees for a true S heading on my main roof's south-most facing side. There's still a giant pine tree I don't clear until about 9:45 in the morning but otherwise it's clear until near sunset or about an hour before when the power is really weak anyways. So I just have a little morning shade problem
  • mike95490mike95490 Solar Expert Posts: 9,571 ✭✭✭✭✭
    Re: Panel mounting - roof
    ewdysar wrote:

    How much impact does a 1" shadow have on a 200w module's output? (my sailboat's single 46w panel does not like shade from even a single rope)
    My site link on left panel. (LA also) I have a transmission line east of me, and I can graph the dropouts of the 3 wires shadeing my panels as the sun moves. It's not a huge amount of loss, but you should factor it in. Drop me a line if you want to discuss more. Plot from Oct 2, 2006 a nearly cloudless day.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Panel mounting - roof

    ewdysar,
    quoted from you:
    "I'm having a hard time calculating the actual slope and direction that results from tilting on a 90 degree axis from an already tilted surface, I'll probably just go up there with a board and measure it."

    i too have been trying to equate that as i can picture it and i'm having difficulty putting the math to it. for instance, my main roof doesn't face due south, but is off by 51 degrees east of south and it slopes at 45 degrees. a pv flat on the roof will face that 51 degrees with an elevation of 45 degrees. now lifting the pv to 90 degrees elevation like it has hinges (aimed at horizon) will swing the direction by 90 degrees so that it is now 90 degrees elevation and is aimed 39 degrees west of south. the inbetween i have troubles calculating as i suppose you are as i'd want to know for aiming at south what the elevation angle of the pv should be to achieve that. if you or anybody else comes up with something let me know.
    it is difficult to measure as the main reference is a true south direction at whatever elevation angle that is chosen based on your particular lattitude. this shows the importance of the math. there are also variable mounts as everybody knows and being mounted to a slope to begin with and not facing true south is quite a challenge in figuring the actual directions it would face and at what elevations for the usual 15 degree steps these mounts accomodate.
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Panel mounting - roof

    Might be easier to just build a model out of plywood and/or carboard and use a good protractor/level from the hardware store... I think my trig/vector calculus has gotten to rusty over the years to do-- :-P

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,571 ✭✭✭✭✭
    Re: Panel mounting - roof
    ewdysar wrote:

    I'm having a hard time calculating the actual slope and direction that results from tilting on a 90 degree axis from an already tilted surface, I'll probably just go up there with a board and measure it. I could also mount the panels on the WNW roof of my house, same slope and direction, modules tilted, even less shading during prime time, though a longer run to get to the service panel on the far side of the garage.

    PRIME TIME - I like it. Look at my graph, my prime time lasts about 3 hours (I just checked the spreadsheet) and that's the time I'd least like shadows on my panels. Go a gauge larger on the wires, get the best position you can.
    Mikes prime time:
    top 85% noon - 3 pm > 2500 W
    top 70% 11:30- 4:30 > 2000 W
    top 50% 10:30 - 5:20 > 1500 W
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • System2System2 Posts: 6,290 admin
    Re: Panel mounting - roof

    I don't think there's any shame in a protracter and a plumb line personally. I used it to double check my calculations and it turns out I was off by a bit (lumpy roof as it would turn out).
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