Selecting Modules for My Garage

SolarWorld offers U.S.-made modules. Its Hillsboro, Oregon, facility performs each step in the manufacturing process, from growing silicon crystals to module assembly.	The author with her PV-ready garage.

While the rooftop itself measures 25 by 18 feet, and setbacks are generally not required for detached garages, I would like an approximate 2.5-foot access path on both sides of the array (and about 6 inches of clearance at the top to the ridge and 12 inches at the bottom of the array to the eave). These criteria leave us with an available mounting area measuring 20 by 16.5 feet, providing an approximate 330-square-foot area. After estimating our annual electrical consumption, considering our Colorado solar resource, and figuring in typical system derating, I calculated that a 4.6 kW array would meet our electricity needs (see “PV Array Sizing” in this issue for details on sizing).

Using this number, I calculated the minimum module power density required:

4,600 W ÷ 330 ft.² = 13.9 W per ft.²

Of the more than 900 modules listed, 360 modules have a power density greater than or equal to 13.9 W per square foot.

My next criteria are that modules for this project have positive-only power tolerances and a minimum PTC-to-STC ratio of at least 0.90. That shrinks the possibilities to 147 different modules. If I opt for a minimum materials warranty of 10 years, this shaves off another 13 module options, leaving me 134 modules to consider.

I then looked at some of the many module options, like Canadian Solar’s NewEdge module line for rail-less mounting. These modules are backed by a noncancellable product warranty and the company has been manufacturing modules since 2001.

I wanted to see how they might fit on the roof. Two models matched up: the 245- and the 250-watt versions; both 64.5 by 38.7 inches. The mounting hardware requires 1/2-inch spacing between modules. I have room for 18 modules on the roof (three rows of six modules in a portrait orientation). Using the 250 W modules would give me a 4.5 kW array—very close to my 4.6 kW goal.

Another option is MPE MS08 Schüco modules, which have 245 to 260 W models—all of which measure 65 by 39.1 inches. I could also fit 18 of these modules on the roof. Using the 260 W models would yield a 4.7 kW array. However, a noncancellable warranty isn’t available for Schüco modules—a consideration. The same exercise could be completed from numerous manufacturers such as Jinko, Samsung, Trina, and Yingli.

A U.S.-made option would be to use 18 SolarWorld SW250 modules, which would yield 4.5 kW. While they are without a positive-only power tolerance (+/- 3%), SolarWorld’s U.S. manufacturing history dates back to 1977. They offer an all-black version (frames and back-sheet), which makes an attractive, uniform-looking array.

While I hope to utilize module-level monitoring via either microinverters or DC-to-DC power optimizers, I did check that array configurations of three strings of six modules would also work with several string inverters. If, for some reason, the module-level electronics don’t stand the test of time, I could easily run these three six-module strings to a conventional string inverter.

And finally, it is important to realize our module table only lists 200-watt and larger module options, so in reality there are many more modules I could consider in this exercise if I bypassed this limitation.

Several string inverters are suitable for our example 4.5 kW array. This Power-One string-sizing program shows that its 4.6 kW inverter can accommodate three strings of six 250 W Canadian Solar modules.