How many panels can I put on 10x20 roof?

[crashintoty]

Roof is 10' 8" H x 21' 7" W (128" H x 259" W). The panels is question are 62" H x 42" W. I figured with just a bit of over hang I can fit 12 panels up there. Something tells me I'm being overly optimistic and it may be less due to mounting requirements (an inch or two gap between panels?), maybe more like 10. I'm looking to mount them myself and considering using Unistrut/superstruts (unless y'all have a better, yet still affordable alternative). Considering this roof size, panel size and the mounting plan, how many panels can I put up there best case scenario and the best way to do it?

[J.P.M.]

Check building code and fire regulations before you start. Chances are you won't be able to cover an entire area w/panels by statute, code or regulation, or anywhere near it.

For that matter, you may not want to, particularly if you ever need to get at one of the panels. Think long term, think maintenance and think accessibility.

[Ampster]

QUOTE=crashintoty;n412653]....... Considering this roof size, panel size and the mounting plan, how many panels can I put up there best case scenario and the best way to do it? [/QUOTE]
I think you need to understand your local building code before you get into that level of detail. There are three issues that come to mind. They are setback requirements, engineering of the rack system, and rapid shutdown requirements.
Check your local building department for any setback requirements. In several of the jurisdictions where I have systems there is at least a 3 foot setback at the peak of the roof. There may also be setbacks on the sides and bottom but that varies. Once you get those requirements you can easily layout landscape or portrait orientations to see which one gets you the optimum number of panels. Also a mix of those two could end up but it all depends on how close yuur usable dimensions are to multiples of 62.5 and 42.5 inches. The actual distance between panels may be greater or less depending on the fastening hardware. Speaking of hardware I have used a lot of unistrut for other projects but I have never used it for solar. I don't think you will find fasteners or readily available engineering for unistrut.

I have used UniRac and Iron Ridge on my self installed systems and they both offer engineering designs and diagrams that make getting permits easier. On my last DIY system is was more about finding a local supplier than one vs the other. Yes they are more expensive per foot that unistrut but the ease of assembly and getting a permit are the cost effective tradeoffs for me. If you are thinking of doing this without a permit you will get no further advice from anyone on this forum. You will also foreclose the opportunity of getting a Net Energy Metering agreement from your power company because they will need to see a final building permit when you apply,

BTW those dimensions sound like those are not the usual 60 or 72 cell panels. I used some 96 cell used Sunpower commercial panels on my patio cover PV system and they worked fine. I wired 4 of them with micro inverters and had to spend a little more on microinverters to accomodate the higher voltage of the 96 cell panels. The rest of them worked well on my string inverter.

As to rapid shutdown requirements for the protection of firemen you only have two cost effective solutions, microinverters and SolarEdge string inverters with oprtimzers. Since you have shadows you can solve RSD and shadow issues with either of the above.

[littleharbor]

3' setbacks at the ends and ridge turn it into a 7' x 14' roof.

[Ampster]

As your usable roof area gets smaller depending on your AHJ you may want to evaluate the cost of panels based on energy density. A recent number I have seen on some panels is 16 W per Sq. Ft..As a ballpark that means it could be as low as a 1500 Watt system unless you use more efficient panels. I forgot what kind of production PV Watts said you could get on the northern slope of your garage? It might be 1000 Watts after the AHJ haircut but not as much annual production as on the south facing roof.

[J.P.M.]

As a ballpark that means it could be as low as a 1500kW system unless you use more efficient panels. I forgot what ind of production PV Watts said you could get on the northern slope of your garage?

1500kW ?

[Ampster]

1500kW ?

Thought I had a dot there. Should be 1.5 kW or 1500 Watts. I will correct that post. That is just 6 of those 300Watt panels. Not much to offset his 20,000kWhr annual consumption.

[J.P.M.]

Thought I had a dot there. Should be 1.5 kW or 1500 Watts. I will correct that post. That is just 6 of those 300Watt panels. Not much to offset his 20,000kWhr annual consumption.

Last time I checked, 6 X 300 was 1,800.

Whatever.

Putting 1.5 STC kW on an unshaded 20 deg. tilt north facing roof in Seattle will generate ~ 1,200 kWh/yr. The shade will reduce that further for the OP.

If you're suggesting the OP could reduce that 20,000 kWh/yr. by 6% for a lot less toil and treasure through an energy audit, conservation measures and an energy use diet than installing 1.5 kW on a roof, I agree.

[Ampster]

Last time I checked, 6 X 300 was 1,800.

Whatever.

They are used panels so 1500 Watts was a conservative guess. In any event, at that latitude and with the expected cloud coverage plus the shading issue it could be as you say, "whatever".Precision is probably not possible in that environment or given the unknowns of the usable roof area.

Putting 1.5 STC kW on an unshaded 20 deg. tilt north facing roof in Seattle will generate ~ 1,200 kWh/yr. The shade will reduce that further for the OP.

If you're suggesting the OP could reduce that 20,000 kWh/yr. by 6% for a lot less toil and treasure through an energy audit, conservation measures and an energy use diet than installing 1.5 kW on a roof, I agree.

I don't think it is one or the other. He has already embarked on an energy audit and I don't have any idea how much he can reduce his consumption concurrent with adding generation. I don't know exactly what PV Watts estimated but you are correct, shading will reduce that. He has demonstrated a desire to try some solar to reduce his use of the grid. He has some challenges but there may be other south facing exposures on his property that could generate additional Watts. It may take a number of approaches to accomplish his goal. Whatever the result it will be informative to others.

[Mike90250]

Used panels, north roof, shade issues. I'd pass on it. WAY TOO MUCH bother to be usefull

[crashintoty]

Woooooow! That 3 foot set back requirement drops me down to 6 panels on the south slope. This make it necessary to put 6 on the north slope too. This is becoming less and less exciting LOL

[J.P.M.]

Woooooow! That 3 foot set back requirement drops me down to 6 panels on the south slope. This make it necessary to put 6 on the north slope too. This is becoming less and less exciting LOL

Putting stuff on a north facing roof in the northern hemisphere isn't all that exciting to begin with.

Get with your local AHJ for better guidance than non authoritative net sources. You may need to also contact the folks in your area responsible for fire protection. That 3 ft. setback for ridges and at least one side as well as other setbacks depending on roof layout is common in many areas but not all areas. It also may be subject to different interpretations for garages, outbuildings and such.