Solar Electric Production in Rain or Shine

What is the difference between a sunny day and a cloudy day?  Up to four times the amount of solar electric power!!

Here is a comparison of two days in August with very different weather.  On August 13, 2013 there were clear blue skies all day.  Two days later on August 15, there were dark clouds and rain all day.  Although the day length was almost identical, the solar production was dramatically different, as this graph below shows.

Solar power rain or shine

Side-by-side comparison of power production for sunny day and dark cloudy day

The clear-sky sunny day produced 21.4 kWh of power, with peak production of almost 2800 watts.  The rainy day with thick dark clouds 2 days later produced only 5.1 kWh — about one quarter of the power — with peak production of 1100 watts.  Not every rainy day is as dark as it was on the 15th and even then there was decent power production, but clearly clear skies makes a big difference.

Solar Electric Production on the Dark Side of the Year

Having a solar electric system is great during the sunny summertime, of course!  But what about during the dark rainy winter season?  Here’s a look at some of the data on our electric production so far, with a focus on the first half of the dark side of the year when the nights are longer than the days.

A Dark Quarter Data Sample

91-Day Data Sample (9/22 – 12/21): Equinox thru Winter Solstice

In brief, it looks like we’ll get 22% of our annual electric production during the darker half of the year. Between the Autumnal Equinox (September 22) and the Winter Solstice (December 21), our Enphase Energy reports show a total energy production of 477 kWh. That means a total dark-half production of 954 kWh — assuming the period between December 22 and the Vernal Equinox (March 21) is similar.  In a previous post, I estimated a total annual production of 5265 kWh (based on a 30-day sample around the Autumnal Equinox).  Logically, the sunnier half of the year should produce around 4311 kWh, or 78% of the annual production.

Enphase Energy Report 9/22-12/21

Electric production 9/22 – 12/21

Within the 3-month dark-half data sample there was a peak day production on Sept 27 of 15.1 kWh and low day on Dec 19 of 0.33 kWh.  The average daily electric production was 5.24 kWh — about enough to power a single 1970’s-era refridgerator (or 4 newer Energy Star models).  In contrast, the expected sunny-side average daily production will be almost 24 kWh.

In conclusion, don’t expect the electric meter to run backwards much during the off season.  But it’s still producing something usable — even during those dark rainy winter days.

Solar Electric Production Exceeds Expectations!

As a brand-new owner of a solar electric system, of course I want to know how it is producing — whether it is on target for producing 90% of the expected electrical use of the house.  The good news is that it’s on track for 100%!  I say this with only 6 weeks of data to look at.  What makes this estimate seem reasonable is the fact that the Autumnal Equinox is within those 6 weeks.

One of the biggest factors which affects the electrical productivity of a solar PV system is the length of day.  The longer the time of daylight, the more energy that is produced. In Seattle day length varies from a maximum of 16 hours in the summer to the minimum of 8 hours in the winter.  Midway through the year at the start of Spring and Fall, the day and night is about equal, at 12 hours of each.

You can never really know exactly how much electricity a solar PV system will produce in a year until it has produced it.  And it may vary year by year because of other factors, like weather and dust and debris accumulation on the panels.  However, in an attempt to predict how our new system is doing, I have taken a sample to extrapolate.  I chose Sept 7 through Oct 6, a 30-day period with the Autumnal Equinox in the middle of the time period.  The graph below shows the length of day and night on the 21st day of each month and the sample range between the red lines.

Amount of daylight for "average" month

30-Day Period (Sept 7 thru Oct 6): Averaging approximately 12 hours of daylight each day

During this 30-day sample period, the system produced a total of 433 kWh.  The best day produced 19.2 kWh, the worst day only 3.9 kWh, the average was 14.4 kWh.  Assuming this period is an “average” month for daylight, by multiplying this value by 12.16 (to get the 365 days of the year), it would seem to indicate the annual electric production potential.  This would mean annual production of 5,265 kWh.  That’s 100%!!!

Looking at one year of utility bills, the annual electrical use was 5,242 kWh.  The solar PV system was originally designed for an annual target of producing 4,536 kWh, or 90% of electrical use. But the installers unexpectedly upgraded a few panels to higher efficiency panels.  And could it be that September being historically the driest/sunniest month of the year in Seattle it’s not an “average” month for weather?

Call me an opptimist, but it’s fun to think it may turn out that the house electricity is 100% solar!  Net Zero Energy, here we come!

 

 

Oh, the joy of seeing the electric meter run backwards!

There are 3 major benefits to having a grid-tied solar electric production system, rather than being totally “off the grid”:

  1. You don’t have to purchase and maintain a set of batteries (save money!).
  2. You don’t have to find a place in your house to put all those batteries.  (save space!)
  3. You get the joy of watching your electric meter run backwards (joy!).

“Net Metering” is an energy swapping deal with the electric utiltiy company.  They take your extra electricity when you don’t need it, and they give it back to you when you do need it.  When your solar PV system is producing more electricity than you are currently using, the extra energy goes onto the utility power grid.  The electric utility company then sells it to your neighbors.  Then when you are using more energy in your home than the solar PV system produces, the extra energy you need is pulled from the from the grid.  The electricity you buy from the electric utility company at that point is free — until you run out of your solar production credits.

You’ll tend to produce extra during the daytime, and draw on your credit at night.  Overall you’ll be accumulating credit during the summer, and drawing down your credit through the winter.  In Seattle the days are so long during the summer that it produces enough during those few months to power a house through the rest of the year — even if the rest of those months are dark and rainy.

I have personally found the joy of seeing the electric meter run backwards so satisfying, that I have noticeably changed my electric use habits almost overnight.  I am much more conscious of my energy use.  I am much more likely to turn the lights off when I don’t really need them — when I can see by the light coming through the windows.  Our system was designed to meet 90% of the expected annual electrical use of the house.  I’m finding myself semi-consciously motivated to have this 90% system cover 100%.  It’s a bit irrational, but even when I’m somewhere else I find myself turning lights off!

Experience the joy for yourself!  Watch how it looks when the electric meter runs backward.  Click below to see a 25-second video:

Net Metering – meter running backwards!

Net Metering

Click to see the video of the meter running backwards!

 

 

 

 

Today’s Solar PV Economics: Buy 1, Get 2 Free!

… and maybe get another 2 or more for free after that!

Once upon a time, it took at least 20 years to get your investment back on the purchase of a solar photovoltaic (PV) system.  The cost of the solar PV systems has been coming down, and state and federal incentives have been increasing to the point that at least in the state of Washington the cost can be as low as an 8-year return on investment.  That makes it a buy-1-get-2-free deal!  In other words: Buy 8 years of electricity, and get 16 years of electricity for FREE!  And that’s just during the 25-year warranty period.  There’s every reason to beleive the equipment should last another 16 years or more after that with minimal maintenance.  So that would mean getting another 16 years of electricity (or more) for free after that!

Solar panels on west-facing roof

Solar panels on west-facing roof

After talking with several different solar electric installers, and carefully considering the options, we chose Oregon-manufactured SolarWorld solar panels with microinverters by California-based Enphase Energy, installed by West Seattle Natural Energy.  While this grid-tied system combination does not take full advantage of the made-in-Washington renewable energy production incentives, it more than makes up for it in low initial cost, length of equipment warranty, and elegance of the installation.  A big selling feature was not requiring any equipment to be installed inside the house — which might constrain future remodel possibilities.  But we especially love the technology which constantly monitors and reports on the production of the system.  We can take a peak at how our PV system is doing through the internet from anywhere in the world.

See for yourself!  Click here to view the Solar PV system performance for the “Green My Bungalow” house.  There are 6 solar panels on the south side of the roof, and 10 panels on the west side.  (Apparently west-facing roofs are almost as good as south-facing roofs for solar electric production, because the vast majority of sunshine in northern areas is during the summer when the sun is high in the sky and the long days mean lots of western sunshine in the afternoon and evening.)  The Enphase Enlighten website shows graphs from daily to lifetime electricity production, and sends email notices to us when there is a problem with the functioning of the system.  I’ll be watching carefully, as I learn more about the day-to-day practical realities of owning a solar-electic system.