contractor wants the roof slope changed

Hi all,


I have a shop with a 2.5 /12 pitch roof (about 25 degrees), a "sales engineer" says the roof is too flat and wants 4 /12 or even 6/12 if possible.
If necessary I can reengineer the shop but I'd rather not if I can get by with out.

I'm in Seattle at 47 deg N, if it matters much. and plan to do most of the installation using 240 watt panels and micro inverters in a grid tied system.
I thinking between 5 and 10 kw.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 28,155 admin
    Re: contractor wants the roof slope changed

    What will your net metering plan be like... Summer afternoons, I get $0.30 per kWH. Winter, I get "paid" about $0.10 per kWH--So, having an array with better summer numbers is actually a much better deal for me.

    It also depends on what your generation profile will look like... If you use lots of summer afternoon power (store, shop, A/C, etc.), then you want to make sure that your Time of Use billing is not killed by high summer loads. Lots of winter production will not make up for excessive summer billing/usage.

    Using PV Watts for Seattle WA, fixed array at 25 degrees and 47 degrees, and 10kw array. 47.5 degree first:
    "Station Identification"
    "City:","Seattle"
    "State:","Washington"
    "Lat (deg N):", 47.45
    "Long (deg W):", 122.30
    "Elev (m): ", 122
    "PV System Specifications"
    "DC Rating:"," 10.0 kW"
    "DC to AC Derate Factor:"," 0.770"
    "AC Rating:"," 7.7 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 47.5"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 6.4 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 1.54, 333, 21.31
    2, 2.50, 503, 32.19
    3, 3.71, 837, 53.57
    4, 4.37, 957, 61.25
    5, 5.31, 1177, 75.33
    6, 5.52, 1166, 74.62
    7, 5.88, 1271, 81.34
    8, 5.17, 1120, 71.68
    9, 4.98, 1048, 67.07
    10, 3.00, 657, 42.05
    11, 1.76, 370, 23.68
    12, 1.26, 256, 16.38
    "Year", 3.76, 9697, 620.61

    And 25 degrees:
    "Station Identification"
    "City:","Seattle"
    "State:","Washington"
    "Lat (deg N):", 47.45
    "Long (deg W):", 122.30
    "Elev (m): ", 122
    "PV System Specifications"
    "DC Rating:"," 10.0 kW"
    "DC to AC Derate Factor:"," 0.770"
    "AC Rating:"," 7.7 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 25.0"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 6.4 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 1.33, 278, 17.79
    2, 2.24, 449, 28.74
    3, 3.55, 803, 51.39
    4, 4.50, 993, 63.55
    5, 5.74, 1287, 82.37
    6, 6.14, 1312, 83.97
    7, 6.43, 1400, 89.60
    8, 5.41, 1179, 75.46
    9, 4.77, 1007, 64.45
    10, 2.73, 597, 38.21
    11, 1.56, 321, 20.54
    12, 1.08, 211, 13.50
    "Year", 3.80, 9838, 629.63

    Make this a pure dollars and cents calculation...

    Note that on a year over year basis, the two system performance results are about the same. PV Watts is acutual ~20 year measurements--So marine layer and weather definitely are taken into account.

    And remember that winter does not supply much power anyway... 20% increase (of a small amount) in winter is not a big deal.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: contractor wants the roof slope changed

    Too flat for what? Shedding water or panel angle?
    Year-round angle for panels there would be roughly 39 degrees.
    If the panels need more tilt you adjust the mounts; you don't re-engineer the whole roof. A competent installer would know this.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: contractor wants the roof slope changed

    i have to say that i've gotten confused a few times by this in the past, but you don't have about 25 degrees there. remember if you go out 12in and up 12in that it's not vertical, but it is 45 degrees. see this for an idea,
    http://www.pole-barn.info/roof-pitch.html

    now with that out of the way i have to ask why you have to change your roof pitch? there are mounts that can accommodate flatter roofs such as these for example,
    http://www.solar-electric.com/romo.html
    no doubt he is looking to just run all rails the same height near the roof surface for ease on mounting on his part. i do agree that the angle of the pvs should be a bit higher than your roof angle.
  • BB.BB. Super Moderators, Administrators Posts: 28,155 admin
    Re: contractor wants the roof slope changed

    Inv Tan (2.5/12) ~ 11.8 degrees

    Will have to come back with new pv Watts calculations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 28,155 admin
    Re: contractor wants the roof slope changed

    Same as above, but 11.8 degrees:
    "Station Identification"
    "City:","Seattle"
    "State:","Washington"
    "Lat (deg N):", 47.45
    "Long (deg W):", 122.30
    "Elev (m): ", 122
    "PV System Specifications"
    "DC Rating:"," 10.0 kW"
    "DC to AC Derate Factor:"," 0.770"
    "AC Rating:"," 7.7 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 11.8"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 6.4 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 1.13, 219, 14.02
    2, 1.97, 385, 24.64
    3, 3.25, 736, 47.10
    4, 4.34, 960, 61.44
    5, 5.70, 1284, 82.18
    6, 6.20, 1330, 85.12
    7, 6.42, 1401, 89.66
    8, 5.27, 1152, 73.73
    9, 4.36, 920, 58.88
    10, 2.42, 520, 33.28
    11, 1.36, 268, 17.15
    12, 0.92, 164, 10.50
    "Year", 3.62, 9339, 597.70

    Does not affect summer peak much, but you do lose a good amount in winter...

    But a ~100 kWH per month is about $10-$20 per month (December)... But overall, you are only losing ~500 kWH per year over the over the other settings. Somewhere around $50 to $100 per year at current power prices. Is the "extra work" worth it to you financially?

    Again, your billing options may make a difference in your interpretations too (in California, we are forced to use Time of Use billing. This means that "small systems" may actually increase costs as the summer usage can be billed at 3x winter rates--and if there is not enough panel to reduce summer usage--the high TOU rates for summer can increase billing costs in those months.

    In California, our complex/Time of Use/Tiered Rate plans make understanding solar PV power production very difficult to accurately model the interaction.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarMusherSolarMusher Solar Expert Posts: 172 ✭✭✭
    Re: contractor wants the roof slope changed
    Inv Tan (2.5/12) ~ 11.8 degrees
    11.8° is not a problem for a tilted array with rails and adjustable or fixed rear legs.
    It would be very easy to install, easier than a 10/12 or 12/12 slope...
    Erik
  • BB.BB. Super Moderators, Administrators Posts: 28,155 admin
    Re: contractor wants the roof slope changed

    Keeping the panels at ~25 degrees will optimize your summer output (and overall output a bit too). And keep the panels a bit low and possibly less visual impact (and less chance of catching storm winds--if a problem in your area).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,108 ✭✭✭✭
    Re: contractor wants the roof slope changed

    In thPac. NW the loss in winter may not matter too much, since you don't hve. Lot of productions form Nov-March

    Tony
  • toddtodd Registered Users Posts: 22
    Re: contractor wants the roof slope changed

    looks like I'll need to modify the roof slope, it's current slope is 2.5/12 and according to an online calculator this equals aprox 11.75 degrees, a 4/12 comes in 18.4 and 6/12 26.5.
    Granted, there are mounting racks that allow for change in angle, I suspect the additional cost and shading might be an issue.
    I think the contractor wanted a steeper slope to get more mounting space. The shop is currently it's 22x22 with 10x20 usable mounting space,
    Which I think it will be tight just fitting 4kw of panels.

    I'm planning on enlarging the shop anyway to 30 x 40 which will provide a mounting space of 28 x 20 at 0 degree , when elevated to 4/12 or 6/12 it's more like 28 x 26.

    btw: power costs are now .10 per kw.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: contractor wants the roof slope changed

    that's a different story if you are redoing it anyway.
  • toddtodd Registered Users Posts: 22
    Re: contractor wants the roof slope changed

    THANKS ALL FOR THE INFO.

    Yup it's not so much that the shop's roof need to be resized, it's more what slope to build it to. I'd be tempted to leave is as is but with a 16kw utility average I suspect a 4- 5 kw install won't make the huge difference I'm looking for. Summer time A/C make a fairly big hit to the summer bill. Winter time use is tied to a hot tub heater.

    I'm guessing a 4/12 roof 26 degree, will be a fair compromize. I don't expect winter production to make any front page news, it's pretty dark, cloudy or rainy during 5-7 months of the year.
    6/12 might be slightly better but the steeper the slope the harder the install gets. Is I were having someone else do the mechanical then it's a moot point, but I have personal issues falling or sliding off a roof.
    But I expect to do the install and leaving the final inpection to the professionals.
  • icarusicarus Solar Expert Posts: 5,108 ✭✭✭✭
    Re: contractor wants the roof slope changed

    Even a 4/12 steel roof can be very slippery! A drop of dew or some moisture on your boots, and it is ski time! Be careful.

    Tony
  • toddtodd Registered Users Posts: 22
    Re: contractor wants the roof slope changed

    I plan to use asphault rolls over 30 pound roofing felt and 1/2 in plywood, cheap, effective and doesn't require special tools to install...
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