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More PV Generation

Our next step was to add more PV generation.

With only 2kW DC solar, it took all the daylight on a sunny day to run our modest daytime loads and recover 50% of the energy in the batteries.

Several days of snow was enough to run our batteries down below the threshold where our backup system came on, and it took several more days to recover enough state of charge in the main battery bank for the main system to come back on. We discovered that our battery management system relied upon the batteries being brought back to full every few days, and our 2 kW DC of PV just wasn't enough to do it without a generator.

The other reason we needed more generation was to support additional loads. With fumes from our propane-burning stove, the buildup of CO2 from 2 humans and a cat breathing and talking, and radon seeping through the slab, we needed to add HRV, radon fan, and electric cooking appliances to our daily load analysis.

With 175 mph wind gusts possible, we hired a structural engineer to design the ground mount for our new solar array. Mounting the new array on the ground allowed us to optimize its daily production in the winter by tilting the modules to 55 degrees. No more snow removal needed!


Using a solar pathfinder, I checked the hours the foothill and trees to the west of the ground mount site would reduce our production.


The sky to the SE is wide open, and the mornings tend to be sunnier here as well, so we decided to rotate the array to face SSE instead of due S.

Construction of the array involved digging the foundation with a backhoe, lining it with recycled wood, adding rebar reinforcement, building the frame, and filling the form with concrete.

The pipes pulled out of the fittings in the first 60 mph wind gusts and the installers had to come back and tighten the bolts, this time with a torque wrench!

With the ground mount contributing an additional 6 kW of power, it now only takes about 2 hours to bring the batteries all the way to full, even on a cloudy day. Once the batteries are full, the solar modules are open-circuited since there is nowhere for the energy to go. We have an additional 10 kWh+ per day that we could be using to heat water with a heat pump or charge our plug-in car, if we could get it up the driveway...

Even after a snowstorm, it only takes one 20 kWh day to recover. Most days we are generating around half of what we could, if we had more loads to feed.

For the technically inclined, here's the line diagram for our system, with the ground mount array at the upper right, charging our lithium-ion batteries at the lower right, along with the newer one of the rooftop arrays, via 2 100A charge controllers. The older rooftop array (upper right) is keeping our backup lead acid batteries full via the charge controller on our backup inverter. The main 240V inverter feeds our main breaker panel, with one 120V leg of the panel fed through the 120V inverter, pulling energy from the main Li-ion battery bank. If the Li-ion batteries fall below the backup threshold, the 240V inverter shuts down, and the 120V inverter feeds one leg of the panel, pulling energy from the lead acid backup batteries, while the Li-ion batteries recover. Note there is additional capacity for expansion as we continue electrifying our loads -- we could add another ground mount array instead of the small rooftop array, move it to the backup position, and retire the older rooftop array.



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