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.

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!