Solar Project: How's It Going?

Our solar project was to put my freezer and a small chest refrigerator on solar energy. We started with the freezer (last January) and added the fridge toward the end of March.

Since getting both appliances off the grid, we’ve gotten two electric bills. We’re now averaging 11 kWh per day. To put that into perspective, back when we used the HVAC for heating and cooling, we averaged about 25 kWh per day. After we quit the HVAC in 2015, our average usage dropped to 18 kWh per day. Until now. 😎 I’m excepting our numbers to go up during summer because we rely on ceiling and box fans for cooling and to keep air circulating in the house. Plus, I use my electric stove more in summer for canning.

The other thing we’ve been curious about is how long our battery bank will last during a string of cloudy days. We’ve had a lot of those lately, which has given us the opportunity to answer that question. The battery bank lasts pretty much as long as I calculated—about two days. But the panels still produce a some electricity on cloudy days, and our charge controller optimizes that, so I can’t simply count cloudy days. I have to keep an eye on the battery bank to make sure we don’t drain it too low.

One method to monitor the battery bank is by the batteries’ state of charge (SOC). Technically, this refers to the specific gravity (i.e. chemical composition) in each battery cell, but bank voltage gives an approximate idea of what’s going on. This varies by battery type and manufacturer, so my figures are pulled from our battery user manual.

• 100% SOC is 12.73 volts
• 90% SOC is 12.62 volts
• 80% SOC is 12.50 volts
• 70% SOC is 12.37 volts
• 60% SOC is 12.24 volts
• 50% SOC is 12.10 volts

The SOC is measured when the batteries are neither being filled nor powering something. So the best time to get the most accurate reading is at night when neither appliance nor cooling fans are running. (The charge controller, inverter, and battery box vent fan all draw their energy from the batteries too.)

Typically, folks say not to let the batteries drain below 50%. However, they will last longer if not drained even that low. I keep my eye out for 12.4, which is about 75% SOC. If the voltage is that low at bedtime when nothing is running, then I turn the inverter off for the night and wait for the batteries to refill the next day. A refrigerator is said to keep things cold for 24 hours with no electricity, and a freezer for 3 days. So as long as I don’t open them, I have a time window to work with. If it’s darkly overcast the next day and the panels aren’t putting out much, I plug the appliances back into the grid. They are on a power strip, so if I need to plug into the grid it’s easy to do. If the grid is down, well, then I have to move on to plan B, just like everybody else.

So far, we’ve had one power outage this spring due to high winds. I was so thankful my freezer and fridge were still running.

Things that help us get the most out of the system.

1. Adjusting the panel array angle to get the most sun.

We’ve been adjusting the array angle every two months. This enables us to take advantage of the sun’s seasonal position in the sky and make the most electricity we can. (How we do that in this post, “Adjusting Our Solar Panels.“) Here it is for June -

Here was the angle for January -

Quite a difference, isn’t it?! Makes me glad we abandoned the idea of installing them on the roof, where they would be pretty much fixed. For quick angle adjustment in the future,  I’ve been marking the channel strut with the number of the month by the slot.

2. Something else that helps is keeping an ice bottle in the fridge. It’s a two-quart juice bottle, and it takes days to defrost, which helps keep the fridge temperature cool.

I know several of you were curious as to how this was working out, so there’s my report. I hope I covered everything you were interested in. If not, just ask. For anyone interested in the whole story, it starts here.