Thursday, May 15, 2014

Solar Panels: One Year Later

We get questions about our solar array fairly often.  So, I hope this blog post answers some of them.  Our 3.36 kW array has been converting sunlight to electricity for just over a year.  Puget Sound Energy (PSE) hooked it into the grid in late April 2013.

Our Barn/Garage with 14 south-facing solar panels.   April 2013  

These panels do not replace our need to purchase electricity from PSE; they only supplement our power needs.

At its most productive, during the summer's long hours of sunshine, the array earns up to $22 each day. However, it's much more informative to look at a year's worth of data to get a realistic view of the payoff.

One year of power generation, by month (May 2013 - April 2014)

In the past 12 months our array generated over 3650 kWh (per PSE).  It has earned about $2100 (5% from a reduction on our monthly bill, and 95% from Washington State's Renewable Energy Cost Recovery Program at $.54/kWh).    The chart below provides a monthly comparison between our electric bill as though we didn't have solar, and the amount we earned each month from the array.  (Note: Although the Renewable Energy Payment is received in a yearly check, I broke it down monthly for this chart.)

Month Bill if No Solar Solar $ Earned
May 13 $52 $224 
June 2013 $56 $271
July 2013 $71 $307
August 2013 $61 $224
September 2013 $63 $182
October 2013 $57 $121
November 2013 $68 $89
December 2013 $118 $55
January 2014 $73 $68
February 2014 $80 $106
March 2014 $47 $165
April 2014 $47 $223

As you can see, our earnings exceeded the cost of our usage for all but 2 months.

In addition, we received almost $5800 as a tax credit on our federal return.  And, we saved about $1650 in Washington State's sales tax waiver.

At this rate, we should break even after 7 years of power production.  We will earn the equivalent to a savings interest rate of 2.6% at 10 years; or 3.2% at 15 years; or 4.4% at 20 years. That's a decent long-term financial investment, but certainly not the best.  For me, however, our investment in sustainable clean power trumps the mediocre financial returns.

Here are some details about our 3.36 kW Photovoltaic Grid Interface System:
  • Spec'ed out and installed by Western Solar, 2 year installation warranty
  • 14 mono-crystalline solar modules from Itek Energy, 240W each, 10/25 year workmanship/power warranties
  • 7 Blue Frog Solar micro-inverters, YC-500 (1 micro-inverter per two panels), 25 year warranty
  • APS Energy Communication Unit (ECU), 3 year warranty
  • We do not have a bank of batteries, the power is fed directly into the grid
Other components include the junction box, combiner panel, breakers, meters, mounting hardware, etc.

The ECU is networked so that we can monitor the health and production of our array via the Internet.  The bar graph above is from our ECU.

I've made the information on our ECU public.  If you'd like to see both real-time and historic production data from our solar array, click on this link: Carlson Solar Energy Monitoring and Analysis or on the photo of our barn/garage in the right-hand column.


  1. The contribution to a cleaner world is priceless. More of us should be doing this.

  2. You provided a massive amount of information here. This is absolutely wonderful. Thank you so much for sharing this. It answered so many questions I have had. I did have one question. I noticed that at peak power, your 14 panels produced no more than 2785 watts. This averages out to be roughly 199 watts per panel. However, your panels are rated at 240 watts. So, I had to wonder, what happened to the other 41 watts per panel that it supposedly should produce. I had to wonder if the difference had to do with the direction of light falling on some panels more than others as the sun shifts across the sky, but that seemed unlikely. Or, maybe a leaf is on one of the panels, or your Blue Frog inverters really only allow up to 400 watts of combined energy between 2 panels. So, I looked at your monitoring site so see if it shows the production per panel, thinking maybe one of them wasn't producing properly, but couldn't see a breakdown per module. Does your monitoring system not track the modules individually? Thanks so much, in advance. By the way, I think I might be a neighbor of yours since I live in Sammamish. Steve Anderson

    1. Steve, I'm so glad you found this information useful. On a password-protected site I'm able to get module-level information. All 14 modules are operational with no shade or blockage between 9am-3pm. I don't know the answer as to why the panels operate below the 240 W rating. Even on the sunniest of days, they rarely go above 200 W each. Your neighbors in Fall City, Suanne & John

    2. Awesome. Thanks for all the information.

    3. I found out why your 240 watt panels produce only 200 watts of electricity. It turns out the 240 watt panels generate up to 240 watts of DC electricity and then convert it to AC electricity via the Blue Frog micro-inverters. In this conversion process, you lose up to 39 watts. There is also some minor line loss between the panels and the inverters. Probably less than 1.5 percent. Once converted to AC, there is almost no line loss the rest of the way to the production meter. It might be inaccurate to call the conversion process from DC to AC a true "line loss." It might be more like comparing apples to oranges. In any case, it's nice to see your array producing. You seem to get a 2/3's harvest even on a cloudy day. That's very impressive. I'm considering a pole mounted array for my place, but do have an unshaded south facing roof with a little room for some panels. Happy New Year.

    4. And Happy New Year to you! And thank you for the information!