You’ll see lots of figures and factors in a solar proposal. But there are five really worth focusing on when comparing offers.
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Price per watt or kilowatt. That’s the upfront cost divided by the size of the system. The lower the cost per watt, the better the system’s value. You can use each company’s estimated cost per watt to compare proposals. “It’s like unit pricing at your supermarket.” explains Vikram Aggarwal, EnergySage CEO.
Warranties. They may differ for the workmanship, the solar panels, and the inverters—that is, the mechanisms that take the direct current (DC) that the solar panels create and convert it to alternating current (AC) electricity that our homes use. A standard solar panel warranty is 25 years, Aggarwal says. Inverter warranties range from 10 to 25 years.
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Rated power. This is a measure of the system’s efficiency—that is, how much electricity it puts out under ideal conditions. Rated power of at least 400W is preferable; Aggarwal recommends 420W to 440W, because he says it’s the most efficient. The solar company should also give you a projection of how much the power production will degrade by the time the warranty expires. Solar panels’ productivity degrades at a median, 0.5 percent a year, according to the Department of Energy’s National Renewable Energy Laboratory. At the end of a typical, 25-year warranty, that translates to productivity of 87.5 percent.
Annual production. You may notice that the proposals you get from different companies show dramatically different estimates of the amount of power they can provide—from, say, 100 percent of your current needs to 125 percent or more. Why bother with all that excess power? If, say, you add an electric vehicle to your power consumption, planning for more power may be worthwhile. “If you think you’re going to buy an electric car in next four to five years, you may want to oversize your system now,” Aggarwal says. “Most installers won’t be willing to add new panels and inverters in the future.” Depending on the arrangement with your utility, you may be able to sell back the excess power, reducing your electric bill further.
Quality of the solar equipment. You can look on EnergySage and SolarReviews for comparisons and judgments of solar panels and inverters; names like Canadian Solar, LG, and QCel show up highly rated there. For inverters, experts we talked to preferred microinverters, individual units attached to each panel, versus string inverters, which are connected to both the panels and each other like Christmas lights. As with Christmas lights, string inverters are annoyingly interdependent. “If one panel goes out, Aggarwal says, “the whole system goes out.”
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