The energy sweet spot

posted by Jeff | Tuesday, December 8, 2020, 9:39 PM | comments: 0

One of my hobbies (don't be a hater) is obsessively tracking the way we generate and use electricity. I'm fascinated with the potential of renewables and the way you can easily measure everything. There are few technological, or even economic reasons that we can't get there quickly for a significant portion of our energy consumption. In the mean time, I can do my part at home.

The sweet spot for weather and such, in Central Florida, is surprisingly narrow for us. As it turns out, a large, two-story house is not easy to keep cool (or warm), even with modern construction standards. We weren't in the house for very long when that became obvious. It was always the plan to put solar on it, but we ended up having to wait for about six months because the money for that was tied up in the previous house when the first buyer bailed.

We've got about two years and change of data now, though it was slightly thrown off by an unusually warm winter last year. Some observations:

  • Spring is the best time of year for us because the days are long enough and dry enough that we over-produce many days. You get a lot of "perfect curve" days with no clouds. March to May this year, we were covering more than 85% of our energy usage with solar. Mind you, this was at the start of the pandemic, so we weren't driving our electric cars anywhere.
  • The days are longer in the summer, but because we're in the weather convergence zone, the energy and moisture from the coasts meets up and causes rain most afternoons, so July and August are not our best production months.
  • If we both commute, we "burn" about 200 kWh a month on the cars. I'm remote regardless, so in practice now we'd likely use at most 100 kWh for driving.
  • Our average total monthly consumption is 1,760 kWh, our average generation is 1,090 kWh, which is an average of $141 worth of power each month. I'm not sure if that's good for a 10 kW system, but our roof angles are not entirely ideal.
  • After the federal tax credits, the solar system was about $17k, which puts our "pay back' period, the time required to generate enough electricity to pay for the installation, at about 10 years even. That's disappointing, because the early estimates were more like 8.5 years. Still, with an expected lifespan of 30 years, that's a lot of free power.
  • Heat pumps are surprisingly efficient. Our blowers have traditional electric "toaster" coils, but they're never used. It seems so obvious that you can essentially run the air conditioning heat exchange in reverse. Apparently they work down to 30 degrees Fahrenheit!

The sweet spot is when the weather doesn't require a lot of cooling or heat, and you're generating enough to cover your usage. The spring is pretty good for that, with lots of days where we run the air maybe an hour or two. We can keep the windows open and it's not humid. We get a few days like this in November before it seems like we have to close up and turn the heat on. That's where we are this week.


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