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  • wildcatrussell

Off-Grid Solar

When we bought our 1200 square foot off-grid house in 2020, it had a small photovoltaic system. The system was designed for a single frugal person back when solar panels were relatively expensive, and consisted of 2.5 kW of solar panels on the roof tilted to capture the winter sun, a 120V Outback inverter, and a new 20 kWh bank of sealed lead-acid batteries.

Actually, why the batteries were new is a story in and of itself. The previous owner had been carefully maintaining his giant lead acid batteries for years past their usual end of life, to the point where the inspector red-tagged them. There were hot spots where some of the batteries were older than the rest, and as they charged, they were emitting clouds of hydrogen sulfide gas, indicating that the electrodes were breaking down. So we asked the seller to replace them with new batteries, and he did, with smaller sealed ones that we could more easily manage.

We went into our first winter with 10 kWh of storage capacity, since lead acid batteries can only be discharged 50%. We found that staying out of the bottom 50% of the battery capacity was tricky, because just understanding where we were involved reading a graph of the battery voltage versus discharge. There were no bars or percent readout like your phone battery. It was more like checking the voltage on rechargeable 1.5 V batteries to judge how charged they are -- not an exact science. Since on a sunny day our 2.4 kW array would generate about 8 kWh of energy, refilling the batteries was pretty quick, but discharging them was discouragingly quick, too. And we discovered that lead acid batteries like to be trickle-charged, even when they are full, so a lot of our generation was just going into "floating" the batteries!

In order to save our batteries, we had to reduce our loads, which involved unplugging the refrigerator and finding alternatives for food storage (more on that in a different article). On several cold nights we found ourselves turning off the main breaker for the house to preserve the batteries, grilling dinner outside in the snow, and then playing board games by lantern light in front of the propane fireplace. Not the modern fuel-free lifestyle we wanted to show was possible in our demonstration project!

We were able to run our electronics to work from home, a few lights and the well pump, but that was about it. Even when the batteries were full, the inverter couldn't handle running multiple loads at once (accidentally turning on the microwave while the well pump was running was enough to flip a breaker), and we wouldn't be able to change to electric cooking, since electric stoves require a 240V system.

We considered connecting to the grid, since there is a power pole on the edge of our property, just 600 ft from the house. But the cost for grid-tying would be $25K, including trenching to the house, meter, etc., not to mention the monthly bills for the rest of our lives. We could buy a lot of solar PV system upgrades for that $25K, we decided, have no power bill, and not participate in the carbon emissions of Xcel Energy's coal-fired power plants!

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