Site surveys are the first step towards determining how much solar can fit on a roof. But how do you wrap your head around it when there are obstructions on a roof everywhere?
Here are a few simple pointers. The first?
Think Zones.
Here is an original drawing of what my original site survey appeared like. With solar panels, they are usually installed in blocks. By thinking in terms of these blocks, you can map out how much space is viable for solar on a complicated commercial roof. Yes, zones.
- Get the entire perimeter. Subtract 2' from the edges.
- Measure out the "zones," which are areas of space that are square which do not have any obstructions.
- Verify with distances to the air vents.
I then label the zones by the roof surface. In the illustration to your right, you see that this is "Roof B" and thus there is an area marked "B-1, B-2," and so on. Solar panels, on flat roofs, are often counter-ballasted and require space between rows. Typical amount of space is 2' and the panels are mounted with a 10° to 15° angle
 |
| Air Handler |
Then I take these drawings home and digitize them on a simple graphics program, such as "Flash" or "Photoshop." One thing that helps? The original drawings are to scale, where each square represents 50 square inches (with solar, measure in inches, not feet).
For a smaller roof, the scale may be 10" per square. That would also be fine. In this instance, though, I'm using 50" per square.
So here we go.
 |
| Satellite Photo |
This is an actual look at the roof. As you can see, there are many vents and other complicated parts about it. By examining a roof with an aerial satellite photo, you can't really determine the accuracy of space available, with the same level that you can if you're actually standing on the roof.
 |
| The actual roof |
This drawing, to the left, was made using the data that I created from actually measuring the roof with a 25' measuring tape.
 |
| What is the angle of the roof? |
This is now with the areas marked out which are suitable for solar. Imagine walking around the roof after the panels are installed. What if you need to get to one of the air handlers? That's the reason why it's important to be conservative about the amount of space that you give for your array in a situation like this, with a large complicated commercial roof.
The zones are also listed by their area. Note that they're all rectangular (no rhombus! no parallelograms!).
So here's your final design. This is how many panels will reasonably fit on this roof surface. Note the distance between the rows, for the inter-row shading. That's important.
The next question though is, how much power will that make them? What about the rest of the math?
How many panels is that?
We're about to get into some of that math really soon in just a matter of moments.
How many panels is that?
We're about to get into some of that math really soon in just a matter of moments.
The Math Behind The Scenes
Ok. So you know how many panels can fit.
I devised the above layout for Sunpower 310w panels, which are 41.5 by 61.5 inches in dimension. I don't know how expensive they are at the moment, but this is just a hypothetical site survey. Here is the electrical data:
What can you gather from that information? Well, here is some other information to consider. You have an open circuit voltage (60.3v) limits the number of modules you can put in series. If you match that with the temperature derating factor in the NEC, you must over-rate your voltage by a factor of 120%. Therefore, 8 in series is 482v (x1.2) = 578.88 which is as close to the maximum 600v allowed by law!
In other words, you can group the array. 5 in series is too low, because the rated voltage is 50.1 and the inverter kicks in at 250v (see inverter operating voltage). In this case, I'd aim for 7 in series for the most optimal operating voltage for the inverters.
A - 1 : 500 x 80 (10)
A - 2 : 200 x 200 (6)
A - 3 : 200 x 200 (6)
A - 4 : 500 x 100 (20)
Areas B: 108
B - 1 : 200 x 225 (4)
B - 2 : 100 x 300 (5)
B - 3 : 200 x 200 (4)
B - 4 : 300 x 400 (16)
B - 5 : 300 x 550 (25)
B - 6 : 150 x 350 (9)
B - 7 : 200 x 350 (15)
B - 8 : 200 x 250 (12)
B - 9 : 200 x 250 (18)
Areas C, D, E: 46
C : 550 x 250 (20)
D : 750 x 250 (20)
E : 100 x 300 (6)
That's 196 panels. Divisible by 7? Yes. 28 strings of 7, which will fit if each of the 4 strings go onto an inverter. That's 7 inverters. Here are some more awesome questions you can ask.
How Much Power is This System?
It's technically a 60kw. But that's if the panels were in a laboratory at exactly 1kw/m². In reality, it's not going to reach that number ever. What it's more likely to perform at is based on the angle, orientation, and shading, along with wire transmission loss and inverter loss. A more accurate number?
60kw x .97 (angle/orientation) x .8 (shading) x .75 (other losses) = 34kw. Yes, life isn't a laboratory, and the .8 rating for shading is because of all of the tall air handlers which will block the sunlight from panels at certain parts of the day. It might be smart to eliminate some of those panels, in order to streamline the system and increase its functionality. A rule? Count 3x the height of the obstruction if the panels are directly north of the object, and 2x the height to the west or east of the shading object.
In the Northeast United States, you get 4.4 hours of sun on average per day, according to NREL. That means this system will average about 150 kWh per day. That's 54,750 kWh per year, or 54 mWh. At current rates, this system is worth $12,000 in savings per year. In 10 years, that's $120,000.
Do you think this system will cost more than $120,000?
