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  • How many

    Newbie here. Looking at panels for our place. Have 3 estimates so far, ranging from 2 to 2.55 per watt. My question is, ev estimate is for a different number of panels, 28,31 and 40. Of course, they won't send an engineer out to go up on the roof until I'm closer to committing. But what's with the variance in number of panels?

  • #2
    An array with 10 @ 300W (larger) panels could perform the same as
    an array with 12 @ 250W (smaller) panels. They are used interchangeably
    here. Bruce Roe


    • #3
      Yeah, guess I wasn't really very specific. Thanks Bruce. Surprisingly, I get the math. In my case, though, the panels are all about the same strength, 3-400 watts per. I told the companies I'm fine with overproducing a little. So how is it they're saying 28 panels in one case and 40 in another?


      • #4
        Sounds like you would benefit from buying Solar Power your Home for Dummies (or find an older PDF on the web). It will answer many of your questions.

        Unless you gave the designer accurate long term load data, the two firms may be working with different sizing assumptions. They also need information on your roof and surrounding vegetation to do an accurate shading analysis. If thy do not have that info from a site visit, they may be using different assumption in a modeling program. Its worth searching for PVWatts and playing around with panel angle to see how much the numbers can vary.


        • #5
          It isn't the number of panels, it's the nominal (STC) wattage of each panel multiplied by the number of panels.

          Before you do anything else, download a free PDF of "Solar Power Your Home for Dummies" and read it. The way it is now, your solar ignorance will result with you being in a bad place, and poorer for having overspent on a crappy, unfit for purpose system.