One Step Closer to Solar Electric Panels

Solar Pathfinder Reading of Our RoofThe good folks at SIREN gave a stirring presentation last month about how now is the time to switch to solar electricity.  Panel prices have fallen significantly due to the recession, electricity prices look poised to raise (at least here in Indiana where Duke Energy is spending several billion dollars to construct a coal gasification plant in Edwardsport and pass costs on to customers), and there’s a brisk market for renewable energy credits.  We had already caught the fever a bit and the talk just convinced us it was time to get an actual site assessment and cost estimate.

We’re working with Alex Jarvis of Solar Systems of Indiana, a quirky guy who knows a heck of a lot about solar.  He came out to discuss some different options and to do a site reading of a few potential spots using his handy dandy Solar Pathfinder.  It’s a very simple little device that maps shade to determine if a particular spot has a good solar window.  The image above is from the middle of our roof, which has a very good solar window of about 85% between the key sunshine hours of 9:00AM and 3:00PM – solar time.  (See rant on Daylight Savings Time below.)

pole-mounted-solarHe also measured a few other spots in our yard so we could think about doing a pole-mounted solar panel.  You could argue that the pole-mounted systems are a little on the ugly side but they are very practical in terms of maintenance – no climbing on the roof, no trying to patch the roof under the solar panels – and they are great for houses that have a shaded roof but sunny yard.  We are leaning in that direction simply because our roof is 20 years old and will need to be replaced well before the solar panels.  Alex actually has his panel (shown at right) configured so he can move it around during the day to maximize its exposure to direct sunlight and therefore maximize electrical generation.  He is the first to admit that is way too hardcore for most people but he is a tinkerer and enjoys fiddling.  Most people just retilt their panels twice a year at each solstice.  At the spring solstice, the panel is tilted closer to horizontal since the sun is higher in the sky during the summer.  At the fall solstice, the panel is tilted more vertical since the sun is low in the sky during the winter.

After that is the decision of what kind of solar panels and inverters to get, which I must confess is mostly Greek to me but Will is drooling a bit at the thought of enphase microinverters that broadcast all kinds of exciting data for him to analyze with a fine-toothed comb.  We’re looking at getting six modules that are each rated somewhere around 240 watts, giving us a system that is around 1.44 kilowatts.  Here in southern Indiana, we average about 4.7 hours of direct sunlight per day once you factor in cloudy days and the fact that our days are significantly longer in the summer but shorter in the winter.  In a perfect world, our solar panels would produce 6.8 kw-hr per day (1.44 kw x 4.7 hours) or 2,470 kw-hr per year.  However, there are some losses that we have to take into account with our lovely fudge factor friend, the derate factor.   The derate factor has several components:

  • Shade on our system.  Our solar window is about 85% open, 15% shade.
  • Losses from the inverter, connections, and wiring
  • Losses from dirty solar panels (we’re going to assume we can keep ours clean – with a hose if needed)
  • Losses from improper angling (again, we’re in good shape with a pole-mounted system that can be tilted at least twice a year)

In the end, we figure our derate factor is about 0.8 (meaning we lose about 20% of the ideal production level), which reduces our expected output to about 5.4 kw-hr per day or 1,970 kw-hr/year.  That should still just about cover our needs and in our area it’s not worth overproducing since the electric company will just keep rolling over our credits until we move.  If you’d like to try this game at home, check out the PV Watts calculator developed by the National Renewable Energy Lab.  Soon we should have some prices to go along with our power estimates but for now we’re excited about the possibilities and also trying to brainstorm how we might landscape our yard to make the solar panels blend in a bit.  Any and all suggestions are welcome!

Daylight Savings Time Rant:  Indiana only recently adopted Daylight Savings Time while staying in the Eastern Time Zone, and I am not a fan.  Today the sunrise was at 6:30AM and sunset at 9:15PM, which means our solar noon is really about 1:50PM and our key sunshine hours are 10:50 to 4:50.  It also means that the fireflies don’t come out until 10:00 and it’s awfully hard to schedule fireworks, bonfires, or drive-in movies that children (or I) can stay awake for.  I liked it better when we were in straight-up Eastern Standard Time and never had to worry about changing our clocks. this!

5 Responses so far »

  1. 1

    Eric said,

    July 18, 2011 @ 11:39 am

    One thing to know about the Enphase inverters (which I have, and which I think are great) is that the M190’s top out at 199W.

    There are newer M215’s that top out a bit higher.

    It’s not as bad as it sounds, because you do get other efficiencies from the microinverters, and your 240W panels will rarely produce 240W anyway; that takes perfect skies, perfect angles, perfect temps, etc.

    If you do a pole-mount, you could also do an automated tracker system. It’d be more expensive but you’d get more yield. Maybe ask about the feasibility of retrofitting a tracker later, to keep expenses down now?

    I have a 2.5kW rooftop system with 11 235W panels and Enphase M190s.

    I’ve blogged a bit about its performance here:

    and you can see it realtime here:

    If you have any specific questions, please ask, I love talking about it 😉

  2. 2

    Maggie said,

    July 27, 2011 @ 10:03 pm

    We talked to Alex about the Enphase inverters and decided that since the panels will rarely produce 215W (after efficiency losses and conversion to AC) we will be okay using them. I think the automated tracker systems are very cool but not quite worth it for us right now. We will look into the retrofit option; good idea. Our biggest challenge is just pulling the trigger – solar still has a pretty long payback and it’s hard to decide if we’re ready to take the plunge. I enjoy hearing your experiences – keep sharing!

  3. 3

    Eric said,

    July 28, 2011 @ 6:38 pm

    Yep, 215W panels would only max out those inverters on extremely good solar days. 🙂 There’s nothing really inherently wrong with upsizing those panels; mine are 230W and while they do sometimes clip at 199W, which is a bummer, they are more efficient for the other 90% of the time too, when they are NOT clipping, and they will degrade a little over time. Enphase has a whitepaper on sizing the panels, and they recommend something in the 230W range, if I recall. So it’s not a huge issue, just a bit of a surprise if you didn’t know about it.

    I know what you mean about pulling the trigger… that might -possibly- be one other advantage of microinverters – starting small and adding on later might be a little more possible.

  4. 4

    Eric said,

    July 29, 2011 @ 12:53 pm

    The module sizing whitepaper is here, FWIW:

    but I suppose you’re not quite that deep into things yet 🙂

  5. 5

    Ken Clifton said,

    November 13, 2011 @ 4:31 pm

    Great blog and articles. I can second everything Eric says about the Enphase microinverters. I have 24 of the Enphase M190s running that have been up about a year. I just love the performance. In fact, I could never bring myself to do solar until the microinverters came out — just too many issues otherwise.

    I am about to add another 1.4 kW of capacity to my system. My wife and I are currently charging an all-electric Nissan Leaf using solar, as well as running the house. By the way, the charging takes much less time than we expected. I have graphs on my blog at

    Best Regards,
    Ken Clifton

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