Puget Sound Energy began recording data for the solar panel array at my house in mid-October last fall. The race was on: at what point would I be selling the utility clean power from my panels faster than I would be buying carbon-based, “dirty” power from them?

Predictably, through the short, dark days of fall and winter, we needed to buy more power from PSE than we were selling back to them. Round about February, though, as I read the numbers on my meter I began to notice a change.

The number of kilowatt hours returned to the grid was beginning to increase more rapidly than the number of kilowatt hours supplied by the grid.

March and April confirmed the trend: the “returned” measure was chasing down the “supplied” number, slowly but surely.

At last, on Day 250 of having solar panels (June 23), we have achieved net-zero! This means:

  • The dirty power we bought from PSE over the winter has now been offset by an equal amount of clean power our panels have produced and pushed out to the grid.
  • The panels have produced power in excess of what was pushed out to the grid, enabling us to meet our household needs with renewable solar power.

Of course, it is true that the calendar will inexorably move towards a less favorable solar season, again. It is possible that “net zero” is a temporary victory. What will be particularly revealing I think is how the numbers stack up exactly one year in. That will give us an annual baseline measure against which to measure future performance as well.

Just a quick entry today, with performance data from the first four weeks of my rooftop solar array. The news is both disappointing and hopeful.

During this time span, we used an average of 12.3 kWh per day, which is more than normal for this household. More on that in a moment. To meet that daily demand, just a tick more than half (6.2 kWh) came from solar power production. 6.1 kWh had to be purchased from the utility.

Over time, I think we’ll see the percentage of my electricity needs offset by solar go way up, from roughly 50% to closer to 100% or more. Here’s what I think happened:

  • Household demand was up due to wrapping up a remodeling project from the summer. Lots of power tools plugged in in the garage, and at one point a high-powered fan blowing 24/7. (Don’t ask…it had to be done.)
  • Solar production was weak for days at a time, as we got soaked with much higher than normal rainfall in October. What I’ve observed so far is that the array is of course highly productive on sunny days, surprisingly productive with the indirect light of partly-cloudy days, but scarcely productive at all on those wall-to-wall rainy days that the Pacific Northwest can sometimes have.

As we head in to the darkest weeks of the year now, it’s possible the array will offset even less of my overall electricity needs. But I knew this going in. Spring and summer of 2015 will be the big test.