At the end of 2021 I had solar panels installed on my house. The impact on my electric bill was dramatic and immediate, even in winter. With the days getting longer and longer my little power plant is producing more and more energy. The near real-time visibility from the iPhone app that goes along with the solar hardware is pretty fantastic. I’ve spent a bunch of time monitoring the output of my panels. Among folks with new installations I’m pretty sure this is "normal" behavior. It’s kinda fun to root for the sun!
As I've spent time looking at my solar power generation I've gotten more curious about the source of the power I'm using when it isn't coming from the panels. When my panels are generating electricity it gets used first in my house, with excess going out to the grid. At night, on cloudy days, or when then panels are covered with snow, I pull from the grid. During these times the source of the power I consume is comes from whatever is sending power to the grid at that moment.
The grid operator, PJM, makes the overall mix easy to find out on their website. Broadly speaking my grid provided power is 32% nuclear, 36% gas, 19% coal and the remaining 13% is a mix. Wind does make a surprise appearance and seems like a provide a pretty constant 4%.
MW of power by fuel source on April 26, 2022. Source
pjm.com
Living in Washington, DC, I’m able to choose my energy provider and I do. Before installing solar I was paying way above market rates for green power. According to my provider it was 100% hydroelectric. Still, I’m not sure that's how the grid actually works. Seems like I don’t get to pick the source of the actual electrons which are pushed into my home. At best I’m altering the overall mix in the grid. At worst, I’m just giving somebody extra money for something they’d be doing anyhow. I’d like to think it’s the former, but I’m pretty sure it’s the latter.
To do a comparison I picked a week in April, because the weather that time of year is pretty moderate and we don't need heating or cooling. Pepco, the electric utility here in DC, lets you download your usage data going back pretty far, so I pulled data from Pepco for effectively the same week this year and last year.
Hourly use in KWH for April 18-24, 2021 & before solar panels. Retrieved from pepco.com
I didn’t have any real expectations about my daily power usage. That we often used dramatically more power in the evening hours was interesting. Otherwise this data is pretty boring. Saying I use more power when I use more power isn’t very compelling. That starts to change when I’m not just using power, but producing it too.
Hourly use in KWH for April 17-20, 2022 & with solar panels. Excess generation is negative. Retrieved from pepco.com
Now in 2022 my consumption looks different because I’m producing power when the sun is out. Right off the bat it’s clear that I’m generating way more than I use. You can still see those evening peaks on nights where we cook dinner as a family, run the dishwasher or do laundry at night. But those peaks are absolutely dwarfed by power generation most days. For this week my house provided 188.4 kwh to the grid and used 29.1 kwh. So we pushed way more power than we consumed and that is fantastic.
Still this net usage way of thinking is not entirely honest. Despite attempting to buy "green" power, I know that coal & gas are are still being burned to power my fridge and lights. So, I calculated how much. Nuclear, wind and hydro provide 42% of the power on a regular basis so I’ll ignore those. Natural gas is 32% and coal around 19%. There is another 3% of “other” that I’m going to lump (heh) in with coal. The EIA has average emissions for coal at 2.23 lbs per kwh and gas at 0.91 lbs per kwh.
For this one week in April 2022, generating power for my solar equipped home emitted 22.8 lbs of CO2. Comparable to the ~20 lbs of CO2 you get from using a single gallon of gas.