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A solar detective story: Explaining how power output varies hour by hour

The views expressed are those of the author and are not necessarily those of Scientific American.

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Graph of array power output with timeEditor’s Note: Scientific American’s George Musser will be chronicling his experiences installing solar panels in Solar at Home (formerly 60-Second Solar). Read his introduction here and see all posts here.

Solar homeowners’ favorite topic of conversation is the performance of their arrays. As part of the sales pitch, the installer estimates how much power you’ll generate, and most systems come with a meter (separate from the utility meter) to monitor the power output continuously. But how can you tell whether your array is really living up to expectations? That simple question set me off onto a mathematical hunt that other solar homeowners might enjoy — and which would make a good term-paper project for a high-school science class.

There are lots of tools out there to estimate the performance of a solar array. NOAA provides an applet and Excel spreadsheet to determine the sun angle at any time of year based on the astronomical geometry, with corrections for atmospheric refraction and the offset between clock and solar time. The PV Watts website combines solar geometry with historical weather data to calculate the expected performance for a panel at a given location with a given orientation. PV Performance also factors in the properties of the solar cells and the inverter that converts their DC output to household AC power. For site-specific issues such as shading, you need to make more detailed measurements. The New Jersey state inspector who signed off on my system used a nifty 360-degrees light meter called the Solar Pathfinder.

So much for the predictive tools. To measure the actual output minute-by-minute, I have two independent energy monitors that are installed in my electrical service panel: the TED5000 and the Locus. They display the total array electrical output through a web interface.

Useful though they are, these devices don’t separate out the myriad factors affecting power output: solar geometry, weather, diffuse vs. direct illumination, shading, reflection off the glass surface of the panels, temperature dependence of solar cell output, solar cell and inverter efficiency, and so forth. These effects work in concert. For instance, if the temperature gets so hot that it cuts the output of the solar cells, the total array output can drop low enough that the inverter chokes.

To weigh some of these effects quantitatively, I put together my own spreadsheet to calculate the expected power output for June 30th — a nice, mild cloud-free day when the air temperature at our site stayed fairly constant, reducing the confounding effects of weather. Those who want to choose a similarly opportune day can look up their local weather records at Weather Underground.

Over the course of a day, the intensity of sunlight varies sinusoidally with time. The amplitude and phase of this sinusoid changes with the seasonal cycle, but in retrospect I didn’t really need to worry about that. I could just have drawn a cosine curve and stretched and shifted it to match the midday peak power. Between 9 a.m. and 3 p.m., the array output followed a sinusoid almost perfectly (see the blue curve in the above plot).

In the early morning and mid- to late-afternoon, however, the output fell off more rapidly than the illumination conditions alone would explain. So I went down the list of other factors.

First, I took a bit more care with the geometry of my roof. Usually, panels in the Northern Hemisphere are assumed to point in the general direction of south, tilted upward. My roof, however, also slopes toward the west, so my panels are tilted along both their long and short axes. It took some linear algebra to account for this orientation. The effect was to give the sinusoid a slight skew.

Second, sunlight passes through a greater mass of air in the early morning and late afternoon, attenuating it. Bradley Hibberd, director of engineering at Borrego Solar Systems, pointed me toward a model adopted by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). (For those who care about the technical details, the model uses the Beer–Lambert law, whereby the solar intensity is reduced by the exponential of the secant of the angle between the sun and the zenith.) This helped to explain why the power fell off so dramatically outside the peak hours.

Third, I considered losses due to sunlight glinting off the panel glass. A paper by Sandia Labs has a graph showing how reflection becomes a serious issue when the incidence angle of the sun’s rays is greater than 55 degrees, which in my case meant before 9 a.m. and after 4 p.m. The drop-off goes roughly as the cube of the angle. Incorporating this into my spreadsheet further reduced the off-peak power.

Finally, I took into account the efficiency of my inverter. Below a certain power level, the inverter isn’t able to convert the DC to AC very effectively, giving you the double-whammy of less sunlight and less power per unit sunlight. I approximated this fall-off using as a power law.

Between inverter, reflectance, and atmospheric attenuation, I was able to account for the diminished array output during the morning hours (see red curve). That left the abrupt decrease in the mid-afternoon to explain. I suspect it might be shading. A tree and chimney are located just west of the array, and although they’re not very tall, even partial shading would cause a sharp loss of power. The panels are wired in electrical series, so that whatever affects one of them affects all of them. The Solar Pathfinder measurements showed somewhat more shading in the west than in the east, which should cut into the power output beginning about 3 p.m. One of these days when I find myself at home in the mid-afternnoon, I’ll go up to the roof to see what’s going on.

My analysis was really just the first step and I’m not sure that I’ve really isolated the important effects among all the possible ones. I’d love to hear how other people have sought to explain the performance of their systems!

Graph courtesy of George Musser

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Comments 8 Comments

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  1. 1. jnrowell 1:39 am 07/31/2010

    Or set up a web cam to watch the shadow creeping up on your solar array. It would be more fun and geeky :D

    Link to this
  2. 2. MrSustainable 10:10 am 07/31/2010

    I have a similar fascination. My 3.6kW microinverter array gives me detailed stats per panel and I have been tracking output on a graph that compares real vs PV Watts estimated power. The variance in interesting here in Maine, weather is rather unpredictable. See the details on my web page:

    Link to this
  3. 3. gmusser 5:53 pm 07/31/2010

    @ MrSustainable: That is one of the best descriptions I’ve ever seen of how to do a home solar installation. Extremely detailed and informative. I’ll give it a shout-out in a future post.

    Link to this
  4. 4. 8:56 am 08/4/2010

    A low-tech engineering solution could be found where the entire array is motored East facing to West over 10 hours, then reset overnight. If it was cheap it would work, the major problem nowadays is the extreme cost of a simple solution.

    Link to this
  5. 5. colinkeenan 5:12 pm 08/15/2010

    I don’t think you can rule out heat even on a day where the temperature stayed the same from 6am to 6pm. Your house would’ve cooled overnight and gradually heated during the day. So, I think increased heat should be part of the model for the afternoon.

    Link to this
  6. 6. diysolarpv 3:24 pm 08/26/2010

    I suspect you’ve answered your question, but the limitations of publishing in Sci Am limit speculation. How come though you don’t describe your data if only in one sentence? What’s the accuracy of your power sensors? How many days did this cover, one? Which day was it? Because that looks like the plot of afternoon cloudiness during summer–not a chimney’s shadow. Local shadows don’t come all at once (with intermittent energy spikes) but cloud shadows do. But then you didn’t ask for comments on your data, did you?

    Link to this
  7. 7. Jonathan Cole 1:25 pm 01/20/2011

    While marginally interesting to compare predicted vs. actual output on a clear summer day, it may be a bit misleading in terms of its relevance. Why do I say that? Well if you look at the charts in this article,, you can readily see that solar output on days with mixed clouds go way above and way below the predicted curve. That’s why its not too wise to get too hung up on short term analysis in systems which are constantly variable. Its the seasonal and annual conversion that is ultimately important in a grid tied system. Unless you can optimize every factor at the outset, then the way to increase the solar energy that you gather is with more capacity. I have been living on solar, off the grid, with all modern appliances and amenities for more than two decades. It is definitely affordable and systems can be built that are durable, maintainable and user-friendly. Please visit my web site at and my web log at for in depth factual information that the homeowner can rely upon.

    Link to this
  8. 8. Dr. Hait 11:55 am 01/30/2011

    I would like to help the President in his stated goal of electrofying America with alternative energy. I hope to go on the lecture circuit teaching how to produce electricity at 70% efficiency (proven) instead of their less than 20% today.

    I’ve written an ebook, "Saving the Planet, while making a profit." (available at But I haven’t found out how much local utilities are paying for alternative energy. Please send me a list, so I can publish it.
    No one will invest unless they have a good idea of how much money they can make.
    Thank you,
    Dr. Hait

    Link to this

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