Our Solar PV Journey
Waving goodbye to the contractor leaving my house today left me with a feeling of dread. I know he will need to come back, but there isn’t anything more they could have done on this day. I wander back inside to go look back at my current production and open up a tab to see if the power company updated my usage from 3 days ago. So far, I feel pretty duped and wondering if I made a horrible mistake.
Rewind about one year.
I get on kicks once in a while about reading anything and everything about sustainable energy. Solar, wind, geothermal, space and battery tech. Frequently after reading these articles my browser and phone have caught on and I am now seeing ads every 3 seconds for “Get Solar NOW” and “Geothermal Installers In Michigan.” I will admit I have clicked on a few just to see what sort of click bait I was dealing with. Two times I actually filled out forms to have someone call me. They all go about the same way. Someone will call me and discuss what I want to do and then they want to send out some sales guy to discuss in person the systems they offer. These consultations feel exactly like ordering from Empire Carpet or Closets By Design.
After all was said and done I had a couple of quotes for relatively speaking the same system size. You start to learn a lot about how the industry works at this point. Here are some of the first things we learned:
- The power company will not approve any solar system with a production capacity larger than your current usage. I need to provide the solar installer with a copy of my previous year usage and they build the system around that. In my case we averaged 980kWh per month, so everyone put a system together between 8.2 and 9.5kW with various panel manufacturers and setups.
- The lending practices and costs are all over the place. I was actually charged more money to go through their preferred lenders with “the best” interest rates. One company said if I wanted to pay cash, they would discount the whole system by just over $6,300!
- If you do get a solar loan through solar specific lenders, some of them REQUIRE you do take the tax credit and directly apply it to the loan once you receive it after filing your tax returns.
- None of the 7 total quotes I received provided any guarantee of panel production or system production. In fact, most of the contracts specifically state that they are not responsible for the output of the system even though they are the ones that size it and put together the interconnection agreements with your utility.
System prices (before any incentives) between $29,000 and $45,000. Now that I had hard numbers in front of me, it was math time.
I know, at some point, I want to be completely off the grid. It’s basically a bucket list thing for me. Silly, right? To do that, like a good engineer, I have to design the system for worst case with some safety factor. This turned out to be a lot more complex than I first thought.
The worst case solar production in Michigan is Dec-Feb. We recently had a real stretch of 6 full days without actually seeing the sun. Ignoring that for a moment; I went back to my electric provider and looked up my average and highest electrical usage for each month. With a 980kWh average per month that puts me at around 32–33kWh a day. We did have a spike at 50kWh around Christmas when every light in the house was on, music and heat on full blast. If you aren’t gasping yet, I’d need 242kWh of total capacity for a week (assuming average for 6 of those days and 50 for one day). With an average of a 9kW system with one sunny day the system should produce about 20kWh on the sunny day and probably one quarter that for the cloudy days (at best?). That brings the backup capacity requirement to about 192kWh!
Again, this is worst case in the winter without seeing the sun for 6 days for complete off grid living. This also means no electric cars and we are using natural gas for heating.
Ok, so obviously a 9kW system isn’t big enough to go off grid. Where is the natural balance line? If we assume it scales up directly and go 3x the size we end up right about the break even point for usage vs. production, but that isn’t how this actually works of course.
Winter only has production ramping up for about 4 hours a day so basically in 4 hours you need to recharge the battery bank for whatever you used previously, while still supplying power to the house. Without exact data we have to simplify it down to averages. 32kWh average per day with 4 hours of production which means 20 hours of the day the system is running on battery. Divide that out and you have around 27kWh of battery backup needed at a minimum assuming you can charge the bank back up in 4 hours and supply power to the house. Every day you’d need to charge at least 27kWh in 4 hrs, which is approximately 6.75kW per hour. Seems very doable with a large enough solar setup.
Back to the battery backup though. You can’t discharge more than 50% and you need a FOS. Maybe you don’t consider a full 1.5 for FOS and just figure in half battery capacity (batteries don’t like being drained by more than half) the system would need about a 54kWh battery backup. With a decent FOS it would mean about a 100kWh setup.
Now we have a comfortable setup that isn’t completely off grid, but basically off grid for electric. I’m still driving regular gasoline cars. But does it work in practice? All I can do now is run some basic simulations hand calculating (and using Excel) how one week would go worst case. Lets look at that. Start your Monday and its cloudy. Battery bank is full and stays full all day until about 4pm. Solar output drops and you are running off battery now. Fast forward to Tuesday morning. You’ve used about 27kWh and bank is down to 73kWh. Its cloudy again and you produce a total of 20kWh (again this is best case) and you are back up to 93kWh. In the best case scenario you stay above board for the rest of the week.
If you live in the Midwest you know it snows. Snow on solar panels really cuts down on power production. Depending on how you figure the runs/sims you either make it work or the lights aren’t going to be on very much. Again, this is still using natural gas and we aren’t driving hybrids or full electrics.
Wrapping this all up I need 3 times the size of the system originally quoted and I need a 100kWh battery backup which is basically a Tesla Model S. The highest capacity Tesla Power Wall available is a 13.5kWh and they appear to cost somewhere in the $12,000 range for 1. Basically, I’d need 8 power walls for a nifty $96,000 and three 9kW systems for $29,000 each for a grand total of $183,000. At that price point I’m sure you could get a discount and maybe knock it down to a cool $150k or so. Still, at the end of it all this seems as though there is a lot missing from our understanding of what it will take for the renewable energy phase to be actually sustainable.
Knowing that technology is advancing at a good clip we decided to break this all up into spaced up additions to the house. With the 26% tax credit still available we jumped on a 9kW (actually 8.89) system for a grand installed total of $33,994 in October of 2020 with the promise to be online before the end of the year and solidify one of the last remaining spots in NetMetering.
After signing the paperwork I had the feeling this was either the best or the worst decision I have ever made.