By David Spero
What makes a roof better or less suited for having solar panels mounted on it? How should you design a new retreat structure, and/or, if looking at buying an existing dwelling, how do you know if it is optimized for solar?
Answering these questions is reasonably straightforward. To start with, if you are looking at buying an existing retreat structure, it absolutely must have a southerly facing roof and an unobstructed view of the southern sky from directly south to about 75° either side of directly south. You don’t need a full 180° of clearance, but anything much less than 75° either side of south means you’ll start losing some morning or evening sun.
Ideally the roof should have a fairly steep pitch on it. The ideal angle for solar panels is to have them angled at the same number of degrees as the latitude the panels are at. That means, if you’re in a northern state, you probably want to have a 45° angle, or even possibly slightly more. If you think of a line between the two Portlands, the one in Oregon is at 45.5° N and the one in Maine is at 43.7° N, that gives you an intuitive feeling for your likely latitude, and remember that much of the Canadian border follows the 49th parallel, ie, 49° N. To be more exact, you can instantly see any latitude anywhere from Google Earth and other mapping programs.
It is acceptable to have a somewhat flatter pitch (or a steeper pitch, but that’s less likely!), but once your pitch starts to be more than perhaps 15° away from your latitude, you’re going to start to feel a loss in solar energy production. A 15° differential will cost you 3.5%, and the loss of power starts to quickly rise from that point forward.
If you are going to build your own retreat, choose a lot that will allow you to build with this southerly aspect, and design your roof for as close to your ideal pitch as is practical. One thing is likely – you’ll be getting a lot of attic space that way!
Indeed, if you don’t have height restrictions, rather than having a typical ridge line roof with two equal roof sides rising to meet in the middle, why not consider a single sloping roof, going all the way up to the top. This would give you a lot of extra space above your top level in your structure, and while this space would be facing to the north rather than south, it could surely be used for just about any normal purpose.
How Much Roof Area Do You Need?
Now, the next question becomes either ‘how much roof area do you need’ (if you’re designing a new dwelling) or ‘how much power can you get from the roof you have’ (if you’re buying an existing retreat structure.
The answer to both questions is very much ‘it depends’. But there are some simple rules of thumb you can use.
At present, it seems that a typical solar panel measures about 39.13″ x 65.04″, and typically generates about 250W according to its official specification sheet. Some panels will give you fewer watts for this panel size, and some higher priced ones will go up to 275W for the same size.
The panel is close to 18.35 sq ft in size. So, divide 250W by 18.35 sq ft, and here’s a rule of thumb : Ideally, with reasonably efficient solar cells in the panels, you can get about 13.5 watts of solar power per square foot of roof area. If you make adjustments to allow for not every square inch of roof space being usable, and leaving some maintenance walkway space and such like, we’d probably suggest that for quick guesstimate calculations, you figure on 11 watts per square foot of roof.
A 250W panel, which seems to be about the sweet spot for price vs performance, will cost about $250 (plus the associated costs for wiring, installation, control systems, and so on). This points to another rule of thumb – figure about $1 per watt of panel capacity, plus more to install, etc, the panels and power from them.
Remember that your total roof area will be greater than the footprint of your dwelling. The slope means it has more length on it, and there is probably some overhang that adds to the roof dimensions too. But remember also to deduct any parts of the roof that aren’t southerly facing – the ‘other side’ of a typical two-sided roof, the ‘hip’ sides of a hipped roof, and so on. Also, if there are corners and sides to your roof, possibly the sides might shade the main south-facing roof portion, potentially almost halving the power production on areas that would be shaded.
You probably have a target amount of power generating capacity you would like for your retreat (see our article on ‘How Much Solar Power Generating Capacity Do You Need‘ for more discussion on this). Multiply your roof square footage by 11, to see an approximate maximum generating capacity for your roof. Is that above, below, or close to your target capacity?
If your roof clearly has more than enough space for the generating capacity you need/want, then you can relax, and proceed with all the other things to consider when evaluating current retreats or planning your own custom retreat.
If your roof is marginally close to meeting your power requirements, maybe you should calculate things more carefully. In this case, we suggest the easiest thing to do is to get scissors and paper. Cut out a large shape that represents the portion of your roof that is southerly facing. Then cut out, to the same scale, the number of 250W panels you want to place on your roof (maybe, to make things quicker/easier, cut out larger shapes that represent strips of 2, 3, 4 panels).
Lay the panel shapes out on the roof shape. Leave some aisles for you to walk along (or up and down) so that you can access your roof for maintenance (hopefully seldom) and cleaning (depending on where you live, cleaning will be a reasonably regular activity). We suggest you allow about 2ft wide corridors, and you design things so you’re readily able to reach panels with a ‘window washing’ type extendable handled cleaning device (which indeed might be a window washer). Maybe you can plan to reach out 9′ or so from where you are standing. So that would allow for aisles every 18′ if you access the panels from both sides, and perhaps you’d want the first aisle 9′ from the edge of the roof.
We don’t know why, but we see very few roof installations that leave aisles to make it easy to access the panels, but we feel this to be essential. It doesn’t take much dust or dirt or leaves or branches or bird poo or whatever to massively reduce a panel’s power output, so we believe regular panel cleaning is essential. Perhaps the designs with no walk-ways assume that you’ll do the cleaning from a ladder or from the other side of the roof, and those are both possible options. But if you’re like us, the easier something is to do, the more likely you are to do it, and so we’re keen to make this as easy as possible for us.
So, lay out the panels as best you can and see how many will fit. The good news is the panels can be laid in either direction – long side horizontal (ie ‘landscape’) or long side vertical (ie ‘portrait’). While it mightn’t look so nice aesthetically, you can even have a mix of different orientations, any way that will allow for best space utilization.
Measuring Roof Slope and True Roof Surface Area
If you can conveniently climb onto your roof and safely walk around on it, then the easiest thing to do is measure it directly.
But if this is not so practical, you’ll need to measure what you can on the ground and then adjust based on the roof slope for the actual roof surface area.
There are two typical ways of measuring roof slope. One – less common in the US – is to talk about the angle of the roof slope. The other is to talk about the slope in terms of units of vertical rise per so many units of horizontal run.
You probably know – or can easily measure – the horizontal length of the building footprint, and you also can probably measure the vertical rise. It is also possible to measure the degrees of inclination with only some relatively simple tools, but for most of us it will be easier to measure the horizontal length and rise.
Let’s look at a worked example. Say you have a roof that has a 30 foot ‘footprint’ – ie, it covers 30 ft of horizontal level floor. It has a single ridge in the middle, and the rise from either end to the middle is 6 feet.
If you remember way back to your trigonometry days, you might remember Pythagoras’ Theorem for finding the length of the third side of a triangle. The sum of the squares of the other two sides equals the square of the hypotenuse, right? And in the case of your roof, you now know the two sides around the right angle (ie 6 feet for the rise and 15 ft for the horizontal length). So
62 + 152 = 36 + 225 = 261, and √261 = 16.2
The roof length is 16.2 ft – not much more than the length on the ground in the case of what would be a fairly moderate slope on the roof.
Oh, and for the sake of completeness, if you do know the angle of the roof and the horizontal length to the ridge point, then you can calculate the roof length by the formula
Roof length = Horizontal length divided by the cosine of the angle.
For example, a roof with a 30 degree pitch and a 15 foot horizontal length to its ridge would have a length of
15/cos(30°) = 15/0.866 = 17.4 ft.
A Sample Calculation
Say you have a 1250 sq ft building footprint (perhaps 25′ x 50′). Say you extend your roof one foot over this footprint for eaves/overhang (generally it is common to have greater overhang). And you give the roof a 45° degree pitch.
Of course, you want the long side of the house to be facing south.
If you have a standard single ridged roof, with no hips, and if the roof is in equal halves about the central ridge, then the actual dimensions for each half will be 52′ long (the 50′ width plus an extra foot at each end) and the width will be 37.4′ (the 25 ft flat length becomes a 35.4 ft length on a 45° angle, plus an extra foot of overhang at each end). But remember that only half of this is facing the sun, so in total, you have 972 sq ft of roof area facing the sun.
Now let’s allow for some maintenance aisles. Should these lanes run along the roof, or up and down it? We’re not sure which is better, you can decide. But let’s simply, for now, set aside 20% of the gross area to leave you room for these aisles. So your 972 sq ft of panel area has a net usable area of 778 sq ft.
We’d round that down a bit further and call it 750 sq ft. Or, alternatively, because you are using real dimensions rather than trying to give a generic example, now is a good time to start mixing and matching the actual dimensions of panels to the space on your roof.
For this exercise, we randomly chose a fairly standard size panel, measuring 39.13″ x 65.04″, which we’ll call 40″ x 66″ for our calculation. These panels are rated at about 250 watts, which means that each ten square inches of panel is giving you almost 1 watt, or, if you prefer, each square foot is giving you about 13.5 watts.
Now let’s first do a ‘perfect world’ calculation. Our roof has 52′ x 18.7′ dimensions, or 624″ x 224″, which is 139,776 square inches. Our panels are 2640 sq inches each, so in theory, we can somehow fit up to 52.9 panels on the roof. If we do the quick rule of thumb and reduce that by 20% (for aisle-ways), that points to 42.4 panels, which we’ll round down to 42.
That suggests our roof can provide a maximum of 250W x 42 panels, = 10.5 kW of power. That’s actually a reasonably good number for most retreats and most purposes. These panels would cost about $9,500, plus extra for mounting accessories, mounting, wiring, and so on.
If you were keen to maximize the power from your roof, you could get slightly more efficient panels that generate 275 watts from the same surface area. But these more efficient panels are also very much more expensive – your cost for 42 panels is likely to increase from about $9,500 up to about $14,600, while your power output will go from 10.5kW up to 11.55 kW. You’re paying an extra $5,100 for 1 kW of extra generating capacity – that’s a lot of extra money, and maybe it is better to think about spending the money to adapt your roof so it can accept four more of the standard panels (which would add the same additional capacity), or perhaps, use the money to build a shed and put the panels on top of that. You need an extra 75 sq ft for the four extra panels.
Another approach is to have more of your roof pitching up in the southerly direction, and less or none in the northerly direction. This will raise the maximum height of the structure, but if that’s not a problem, then go for it. You’re sure to find a use for the extra internal space you are creating, too.
Personally we generally prefer to have more low efficiency panels rather than fewer high-efficiency panels. Not only is it cheaper, but the loss of a single panel is not so serious, and our sense is that lower efficiency panels might be more reliable and ‘less stressed’ than higher efficiency panels. But we have nothing to back up that perception.
If your target power generating capacity is around 10 kW, then you don’t need to do anything more at this stage. You know that for 10 kW, you’ll need 40 panels, and you know that your roof has enough space for up to 52 panels, depending on layout and service lanes, so clearly that is going to work.
But if you are keen to get every possible watt you can, and you’re thinking of paying a great deal more for higher efficiency panels, now is the time to do an actual layout diagram for how your roof could be set out, using the cut out shapes.
We provided a couple of rules of thumb in this article. There’s one more rule of thumb, or perhaps assumption, that seems fair. It is probable that you’ll want to cover your entire roof with solar panels; especially if you have a multi-level retreat (ie more total floor area, and more living space, but with a smaller footprint and roof area).
The information in this article helps you understand how to calculate how many solar panels you can get on your roof.
This article first appeared at Code Green Prep: Designing Your Roof for Best Solar Panel Use