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Don't skimp on the power supply: a 20 kW solar installation reviewed, part one


My house has a TDP

There are a lot of reasons I decided to have a big honkin' solar array installed on my roof. I'll get to the tangible ones shortly—backed by cold, hard data. The main reason that comes to mind, though, is less concrete. It's a feeling, or an instinct; a desire to realize the expectations of my 20th-century childhood as a 21st-century adult. This is something I've wanted even before I plopped down my first solar power plant as an 11-year old SimCity 2000 mayor. My infatuation with solar arrays has grown.

Now, I understand that the emotional investment I have in this venture isn't something everyone is going to share. Still, it's an undeniable factor in my decision-making process. My parents were promised an inevitable fusion-powered, robot-filled, smart-home by science fiction when they were kids. Hanna-Barbera's vision of the future may not have come to pass, but I want my daughters to grow up in that futuristic fantasy, so it's up to me to deliver on that dream.

Beyond being simply a cool toy that I've wanted since I was a kid, or a sound investment I can make now that I'm all grown up, I consider this project to be a personal moral imperative. That may be a ironic phrase to invoke for what should be a by-the-numbers adventure, but I was happy to find that both my wallet and my conscience would be soothed by this endeavor. The math works out, but I'll add this much: I've already left my mark on the planet, and now I want to erase it. I think this is a good start.

And so, it begins

As any adventure should, I began with research. Question number one was, "does solar power even make sense in Michigan?" Following closely behind, there was a second question of "exactly how large of an installation would I need?" There are tons of calculators online for finding those answers, and I used more than a few to get multiple opinions. Ultimately all the numbers really clicked for me once I saw the map below. It breaks down solar irradiance into kWh-per-kW-of-panels-installed, per year.


Image source: solarpowerrocks.com

That image is backed up by the feds and other resources online. You can see that Michigan's 1400 kWh/kW-yr ranking isn't amazing, but it isn't tragic either. Since my goal was to offset 100% of my electrical consumption with solar energy, the math here is pretty easy. All I had to do was add up my annual kWh usage based on my electrical bill, then solve for X. In 2018, our household included an entire extra family for eight months. That lead to an over-the-top electrical consumption of 25,650 kWh for the year. In 2017, our usage was more normal, clocking in at 20,263 kWh. Overkill is underrated, so I used the total from 2018 for my calculations. Also, solar panels lose about 0.5% of their production each year—another reason to aim high.

A solar panel system's capacity is expressed as its peak potential output, or kWp. I divided my consumption of 25,650 kWh by the 1400 kWh/kW-yr from the map and came up with a requirement of an 18.32 kWp system. The calculations aren't quite that easy, though. There are other factors to consider, like the laws of physics. Any system is going to have efficiency losses. Roughly speaking, you're talking about needing an extra 20-25% more capacity to counter those losses. That brings my requirements up to 22.9 kWp for my 2018 numbers and 18.09 kWp if I look back to my 2017 usage. With that, I had my rough estimate.

Of course, there's still more to it than that. There's potential shade to consider, as well as the directions the panels are facing, and the pitch of your roof. Those variables are a bit too specific to be within the scope of this piece, though. Just know that they will come into play when you start fooling with full-fledged solar calculators online. In my case, you'll soon see that shade was not a problem for me. I do however have an east-west facing roof instead of a the idyllic southern facing one. As it turns out, that's not nearly as significant a factor as it used to be with today's modern panels and their prices.

Speaking of problems—Fish, you idiot. The. Sun. Goes. Down. That's where "net metering" comes in, though. Net metering means you can bank credit with your power company when you produce more power than what you're using. Think of it as using the electrical grid as a battery, at least financially. Alternatively, think of it as old-school rollover minutes on your cellphone. You'll have to do your homework to see if it's available where you live, but it made everything a lot easier (and cheaper) for me.

Net metering is critical, because unless you plan on storing all the excess power yourself (more on that later), any solar panel array you invest in is only going to lower your bill while it's actively generating power. You'll still be drawing from the grid when it's dark, cloudy, or when the panels are covered in snow. Without net metering, there would be no point to installing a system capable of producing more power than you use during the day. The bottom line is that with that ability, you can install whatever size system you want without having to worry about what to do with the power you're not using in real time.


Number of minutes between sunrise and sunset for my latitude and longitude. Source: USNO

In my case, the type of net metering at my disposal means that any excess power I produce in a month carries over to the next month as a credit on my bill. I don't get paid cash for producing more power than I need, but the credit stays on my account for 12 months. At that point, any surplus I may still have drops off. It's early days yet for my system, but my hope for this year is that I'll bank up enough credit over the spring, summer, and fall to be able to get through next winter (when the panels are covered with snow) by pulling from my credits. We'll talk about how that's working out so far in a bit, and my intention is to revisit that specific topic six months from now, and again six months after that.