# Cost benefit analysis of a PV array with racking to set optimal angle, vs a larger array

I'm thinking of installing some panels on the roof. It's unshaded but only pitched at 5 degrees on long run roofing iron.

Would I be better off racking at an angle to optimize for winter sun, or just lie them flat and add more panels to compensate for lack of racking?

I'm in Wellington, New Zealand.

According to the New Zealand Ministry of Business, Innovation, and Employment (pdf), the average home in New Zealand uses 7,046 kWh per year, and pays a rate of \$0.2879/kWh (NZD).

I used this as a starting point to set up a simulated solar array using PVWatts, a very useful tool developed by the U.S. National Renewable Energy Lab.

I set up a system for Wellington, New Zealand, with the following parameters. For your purposes you'd want to determine the correct values for your proposed system and utility, and adjust the highlighted parameters accordingly. I set the azimuth to 0 degrees, meaning the simulated system is facing due north, which I assume would be optimal for a solar array in the southern hemisphere.

With a bit of trial and error I determined that the optimal tilt for such a system is 32 degrees. The outputs for this system, and an identical system at 5 degree tilt, are:

``````32 degree tilt: 1,318 kWh     \$380
5 degree tilt: 1,211 kWh     \$348
-----         ----
107 kWh     \$ 32
``````

Thus the optimal tilt improves output by about 8%, equating to about \$0.32 NZD per watt.

If we ignore the time value of money and assume a 20 year lifespan, you'd have `\$0.32 x 20 = \$6.40` to spend on labor and materials (per watt) for setting up racking at 32 degrees, or additional panels. With quotes for panels and racking installation, you could quickly determine which approach made the most sense -- though I suspect the answer will be more panels.

• Roof is actually facing west if it can be called facing at all at this pitch. I guess there's the additional issue of dirt ( no snow ) not washing off the panels so easily if they're lying flat. I'll have a play with the calculator. thx. Mar 9, 2018 at 19:50
• @GrahamChiu the "system losses" variable includes panel soiling. You could increase this number a bit to get a better idea of what output would look like longer-term.
– LShaver
Mar 10, 2018 at 16:35
• More tilt means less dust build up (slightly)... Mar 11, 2018 at 22:11
• FWIW: over 30° they can basically be considered self-cleaning; below that, not so much. If your location is dusty, know that you may lose as much as 15%-20% in dirt-based occlusion between cleanings. Back when I was working for a renewable energy contractor, we'd advise the angle just based on that alone; the exception was that if the user was utility grid-tied and on time-of-use metering, sometimes the better \$\$ was to not angle, if their best sun angle aligned with their peak cost of power. If you are grid-tied, look at your power purchase agreement: if not time of use, angle them. Mar 12, 2018 at 22:57

FWIW: over 30° they can basically be considered self-cleaning; below that, not so much. If your location is dusty, know that you may lose as much as 15%-20% in dirt-based occlusion between cleanings.

Back when I was working for a renewable energy contractor, we'd advise the angle just based on that alone; the exception to that was if the user was utility grid-tied and on time-of-use metering, sometimes the better \$\$ was to not angle, if the timing of their best sun angle aligned with their peak cost of power.

If you are grid-tied, look at your power purchase agreement: if not time of use, angle them.

• You need a Another 10 degrees to be self snow shedding, and you need to have no lips or edges for sliding snow to catch on. Mar 27, 2018 at 2:54

I ran through a bunch of similar calculations here. Setting your panels at your latitude is a pretty good approximation. Here, we use more power in winter, so I would use latitude plus 20 to optimize for winter, although I take about a 15% hit for the year.

Racking is simple to do. If you use metal, make them up first, then get them powder coated for rust resistance. This includes having all holes predrilled.

If wood, then use treated wood, and figure out ways for water to dry quickly.

Given what a pain you have when you have to replace the roof I would put the panels on their own structure if room permits.

Here I can get used power poles 35 feet long for \$CDN 28 each. One pole provides 2 supports one 14 feet long with 4 feet in the ground, and one 21 feet with 5 feet in the ground. A 16 foot double joist is bolted on either side.

This structure is repeated every 8 feet.

16 foot 2x4s are set on edge as purlons on these frames with distance chosen to match the panel dimensions. All wiring is carried underneath.

Wiring is done in a modular manner with a connecting point under the shelter of the panels.

At refurbishment time (about every 10-20 years, you build a new frame at the end, move one set of panels to the end. refurbish that frame, move the next set of panels.