Will my roof handle solar panels?

fooliosfoolios Solar Expert Posts: 53 ✭✭
How do I figure out if my roof can handle adding solar panels to it?
I can not get underneath/inside the section of roof in question. Is there a way to check externally?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Will my roof handle solar panels?

    In my city, you need a structural engineer to sign off on panels and roof structural integrity.

    However, solar panels are relatively light (like adding another layer of asphalt shingles)--And most roofs probably do not need additional structure to support their weight.

    In your area, your roofs are probably rated for snow loads too (not my area).

    Are you planning on a large Grid Tied solar array (or large off grid system) that will require building permits--Or a few panels to supply some power for experimenting/a little emergency lighting/cell phone/laptop computer operations?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Will my roof handle solar panels?

    How old is the house? The period of construction will determine the load factors and standards that had to be met at time of construction... Ask your city building department...
     
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  • fooliosfoolios Solar Expert Posts: 53 ✭✭
    Re: Will my roof handle solar panels?

    Are you planning on a large Grid Tied solar array (or large off grid system) that will require building permits--Or a few panels to supply some power for experimenting/a little emergency lighting/cell phone/laptop computer operations?
    Prolly more the latter, just for some experimental power/emergency lighting/tv/internet connection/power some laptops/cell phones. Small stuff.
    Is there a certain size that you don't need permits for? I have 4 panels that I've been playing around with on the ground in the backyard.
    I have been wondering if I can just keep using them like that or if they had to be mounted. Someone at the permitting center is supposed to get back to me on how that might affect property taxes if they aren't on the roof. But that's if I go with 10 panels.
    Geez, everything costs money to own...
    I was just thinking of adding a few more panels to this lot and get them up on the roof.
    The roofer says it won't be a problem, that's the easiest part, he says. The roofer guessed that the electrical part would be the biggest hurdle.
    But so far the electrical inspector says there are no rules regarding low voltage. It's when I plan to convert it to 120v for household usage that I have to get an electrician involved.
    So now I'm wondering of all the possibilities this seems to present to keep this install cheap.
  • fooliosfoolios Solar Expert Posts: 53 ✭✭
    Re: Will my roof handle solar panels?
    westbranch wrote: »
    How old is the house? The period of construction will determine the load factors and standards that had to be met at time of construction... Ask your city building department...

    14 years old. I'll have to find out. Thanks for pointing that out.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Will my roof handle solar panels?

    14 year old home--You are probably golden. My house is ~75 years old--And the rafters are near the size of modern construction. It is holding the array just fine--And I re-roofed to remove several older layers of roofing (and weight).

    If you want a 1,000 WH per day (a "smallish" system that will usually give you enough lighting/cell charging/laptop/radio usage). A simple battery bank @ 12 volts with 2 days of no-sun:
    • 1,000 WH per day * 1/0.85 inverter eff * 1/12 volt system * 2 days no-sun * = 392 AH @ 12 volt bank (~4x 6 volt @ 200 AH golf cart batteries)
    And you can charge the battery bank at a rate of 5% to 13%+ rate of charge:
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge= 369 Watt array minimum
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge= 738 Watt array nominal
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge= 960 Watt array "cost effective" maximum
    For a experimental/weekend/seasonal system, a 5% rate of charge can work OK. If you want to run the system daily, then a 10-13% rate of charge is better.

    And then there is choosing the size of the solar array based on your needed loads... using PV Watts for Detroit with the panels tilted to ~42 volts from horizontal:




    Month
    Solar Radiation
    (kWh/m 2/day)


    1
    2.90


    2
    3.59


    3
    4.13


    4
    4.84


    5
    5.52


    6
    5.58


    7
    5.42


    8
    5.48


    9
    5.18


    10
    3.96


    11
    2.59


    12
    2.15


    Year
    4.28




    If you "toss" the bottom 3 months (assume backup generator for bad weather/low winter sun angles), then February would be the "break even month" at ~3.59 hours of "average" sun per day:
    • 1,000 WH per day * 1/0.52 end to end system efficiency * 1/3.59 hours of sun for Feb = 536 Watt array minimum
    So--That is where I would start. It is "smallish" system that will provide enough power for some lighting and electronics for ~9 months of the year.

    Use a nice little MorningStar 300 Watt TSW AC Inverter. Efficient, reliable, has "search mode" (power saving mode) and a remote on/off switch input (use a small 12 volt rated switch to turn on/off the inverter remotely).

    Add a larger MPPT (or possibly PWM) charge controller (because 12 volt system, the charging current can be around ~60 amps or so--depending on size of array you select) with ~600-750+ Watts of solar panel, you would have a pretty nice little system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fooliosfoolios Solar Expert Posts: 53 ✭✭
    Re: Will my roof handle solar panels?

    Thank you for all that added info.

    I was thinking 24v with the 10 panels strung in 5 parallel sets of two series 12v(100w)panels.
    This should then output ~40 amps at peak. I think that'll fall in line nicely with a 40 amp MPPT CC.
    I'm not sure how I would set up the 6V golf cart batteries for the 400AH though.
    Originally I was going to go with some 12v AGM batteries but after price shopping, I realize the only way I will approach this is with the
    6v lead acid batteries at Sams Club. I'll have to do a little more work with boxing/venting them but it'll be worth the savings.
    With the 12's I think I had an idea of how to hook them up in series.
    But with the 6v batteries, I'm not sure how I'd wire those up in series. I've read that there shouldn't be more than 3 or 4 batteries for an efficient battery bank, but I'm not finding any setups within that limit.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Will my roof handle solar panels?

    My personal recommendation is one series string of batteries--Using higher AH low voltage batteries (6 volt/4 volt/2 volt) in series.

    And ~2-3 parallel strings is still doable.

    If you go with 4 or more parallel strings--My personal opinion is that current sharing becomes more problematic, and maintenance goes up (more cells to check/SG water in, more connections to check, more voltage/current checks for open/shorted cells, etc.).

    Don't get me wrong--There are folks out there with larger parallel strings--But unless you enjoy the OCD of monitoring/servicing of your battery bank, it is not a lot of fun.

    And, for many folks, there is a limit of how large a battery they can move into their bank (unload from truck, move down stairs, etc.). The limit a high AH @ 2 volt cell.

    You can get large forklift batteries in 12 and 24 volt (and other) configurations--But moving a couple of tons of lead means you need a good size crane/forklift/concrete flow with no steps, etc.

    What is your question about wiring in series? How to make/buy the cables, fusing/breakers, proper layout of wiring (as described here by Smart Gauge website--Note that the "single batteries" in the drawing can easily be 2x 6 volt batteries in series for a "12 volt" battery string, etc.), or what?

    Regarding:
    I was thinking 24v with the 10 panels strung in 5 parallel sets of two series 12v(100w)panels.
    This should then output ~40 amps at peak. I think that'll fall in line nicely with a 40 amp MPPT CC.
    I'm not sure how I would set up the 6V golf cart batteries for the 400AH though.

    What size battery bank/system are you thinking of? Is this a 12 volt @ 400 AH or 24 volt @ 400 AH or what?

    And note that MPPT solar charge controllers are really like the DC equivalent of AC transformers. For example same 1,000 Watt solar array and MPPT charge controller into a 12 volt and 24 volt battery bank:
    • 10 panels * 100 Watts per panel * 0.77 panel+controller derating * 1/14.5 volts charging = ~53 amps typical max current into a 12 volt battery bank charging
    • 10 panels * 100 Watts per panel * 0.77 panel+controller derating * 1/29.0 volts charging = ~26.5 amps typical max current into a 24 volt battery bank charging
    So for a 12 volt bank, you would need ~60 amp MPPT charge controller.

    For a 24 volt bank, you could get away with a ~30 amp MPPT charge controller.

    This demonstrates one of the advantages of a higher voltage battery bank... Less charging current means smaller/fewer MPPT charge controllers, and smaller copper wires carrying 1/2 the current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Organic FarmerOrganic Farmer Solar Expert Posts: 128 ✭✭
    Re: Will my roof handle solar panels?

    My roof can not handle solar panels.

    I am not in a city, our town does not have building inspectors, as we refuse to pay taxes high enough to hire one.

    My roof's structural design exceeds our county snow-load requirement by a little over double.

    The problem here is that my roof slope is nearly flat at 1:12 [1 foot of drop / 12 foot of run]. We normally get between 1 and 2 foot of ice/snow pack on our roof, that slowly [like a glacier] slides down slope. It is a large roof [2400 sq ft] so this glacier-like body of ice/snow weighs a few tons. Our chimneys are up at the peak, so there is no weight [or shear-stress] on either of them. But anywhere else that you might put something on our roof, and it would be sheared off.

    Around here, this is not uncommon. I am not sure if you see this much where you are located.

    My solar panel array holds the panels at various angles:
    Summer - 21.7 degrees from horizontal
    Spring / Autumn - 46.7 degrees from horizontal
    Winter - 68.5 degrees from horizontal

    Notice that the winter angle is much closer to vertical than it is to horizontal. The sun's path is low in the sky, and by being so steep less snow collects on the panels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Will my roof handle solar panels?
    I am not in a city, our town does not have building inspectors, as we refuse to pay taxes high enough to hire one.

    My roof's structural design exceeds our county snow-load requirement by a little over double.

    The problem here is that my roof slope is nearly flat at 1:12 [1 foot of drop / 12 foot of run]. We normally get between 1 and 2 foot of ice/snow pack on our roof, that slowly [like a glacier] slides down slope. It is a large roof [2400 sq ft] so this glacier-like body of ice/snow weighs a few tons. Our chimneys are up at the peak, so there is no weight [or shear-stress] on either of them. But anywhere else that you might put something on our roof, and it would be sheared off.

    2+2=4

    Up here no one would use a roof with that low a slope for just the reasons you mention. Greater slope sheds the snow before the build-up gets too deep. There are commercial buildings with flat roofs, which develop leaks and need constant repair. It's better to prevent the problems before they begin.

    Your slope would barely shed water. We're it my roof the plans would already be underway for raising the roof.

    Which is not to say the roof is always the best place to put solar.

    And if I couldn't see the underside of the roof I wouldn't trust it for holding additional weight, especially on a low slope.
  • Organic FarmerOrganic Farmer Solar Expert Posts: 128 ✭✭
    Re: Will my roof handle solar panels?
    Up here no one would use a roof with that low a slope for just the reasons you mention. Greater slope sheds the snow before the build-up gets too deep. There are commercial buildings with flat roofs, which develop leaks and need constant repair. It's better to prevent the problems before they begin.

    Your slope would barely shed water. We're it my roof the plans would already be underway for raising the roof.

    Which is not to say the roof is always the best place to put solar.

    And if I couldn't see the underside of the roof I wouldn't trust it for holding additional weight, especially on a low slope.

    My house is a steel building, similar to an airplane hanger. A tank could be parked on the roof. We decided to go with a steel building as they are very low cost [per square-foot]. If we had large enough doors, an airplane could easily fit in our living room.
  • fooliosfoolios Solar Expert Posts: 53 ✭✭
    Re: Will my roof handle solar panels?
    BB. wrote: »
    What size battery bank/system are you thinking of? Is this a 12 volt @ 400 AH or 24 volt @ 400 AH or what?
    -Bill

    24V @400AH

    I am going to keep doing some research on the matter of the roof.

    Thanks for all the advice. Gonna keep prodding along with the learning until I get more comfortable with turning the idea into application.
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