Saturday, 9 Mar, 2019 was our first full day off the grid.I had decided to charge the car from the house batteries the evening before. That consumed 6.2 kWh of battery charge.
In the morning when I woke, the batteries were at 46% charge. Had I made a big mistake charging the car from batteries? Not this time. The batteries resumed charging as the sun rose, and were at 75% by noon.
At 1:20 pm, the batteries reached full charge, and shut off the solar system.
You read that right, the battery controller shut off the solar system. A good thing too.
Why does off-grid solar turn off when house batteries reach full charge?Here’s an important difference between being on-grid and off-grid. When you’re on-grid and your batteries are fully charged, all the extra solar power you generate, beyond what house is using that instant, gets pushed onto the grid where you run a net-metering credit. But what happens to that solar power when you are off-grid and the batteries are fully charged? The battery controller system tells the solar inverters to turn off, so the batteries don’t over-charge. At that point, you and the planet miss out on that extra solar power you could have been generating. You can get back in the game by turning on some appliances and depleting the battery a few percent.
To capture the solar power we were losing because of full batteries, I ran a load of laundry. Turns out the clothes washer doesn’t use much power, so the solar system turned on and off, and on and off, etc. But when the laundry was clean, I threw it in the clothes dryer, started a new load in the washer, and plugged in the car. That did the trick. I ran down the batteries some and I got to hang onto the last rays of sunlight that hit my solar panels before the sun dipped behind the trees.
Here's a visual analysis of the day's solar & battery budget:
(It took me a while to make this figure so I hope you enjoy it)