Monday, May 25, 2015

Home Energy Use

Several of my friends have been receiving home energy use reports for the last few months, comparing their electricity and natural gas use to the average of their neighbors. I wasn’t selected to participate in this program/study, but I’m glad it has generated so many discussions about energy conservation. Meanwhile, folks are talking more and more about rooftop solar photovoltaic systems, which are now more or less paying for themselves even in Utah where electricity is cheap.

As a numbers guy, I’ve always paid attention to my own utility bills, trying to understand (at least in broad outline) how much energy I was using and how I could use less. And I’ve saved my utility bills for many years, so I can document exactly what I’ve used.

Here’s a plot of my monthly electricity use for the last 16 and a half years, since I bought my house. The vertical scale is in kilowatt-hours per day, plotted for each billing month, so multiply by 30.4 to get the typical monthly use, or divide by 24 to get the average power in kilowatts:
There’s quite a bit of information in this graph:
  • The three highest spikes are from when I had renters or guests (one to three at a time) living in my basement.
  • Soon after the first of these renters moved in, in September 2001, I bought a new refrigerator for the kitchen and moved the old refrigerator into the basement for the renter to use. The old fridge used about 3 kWh/day and the new one uses only 1 kWh/day (as measured with a handy power meter), so when the renter moved out in early 2002 and I unplugged the old fridge, my household electricity use dropped by about 2 kWh/day from what it had been before. (The new fridge cost $650, but it saves me about $70 a year, so it paid for itself in nine years.)
  • There are some pretty reliable seasonal cycles. I use the most electricity in the winter, thanks to the furnace fan, a space heater, an electric blanket, and having more lights on. I also use somewhat more in July and August than in the spring and fall, because the refrigerator works harder then and I use fans to keep cool.
  • Finally, there’s been a gradual increase in my electricity use over the last 13 years. I need to make some measurements to figure out exactly why, but I suppose I’m using the fans and heaters more as I become old and soft, and my laptop computers have gotten greedier for power over time. Also, since the beginning of 2012 I’ve been spending about half of every work week at home, helping to edit the American Journal of Physics.
At present, my electricity use averages just under 4 kWh/day, or about 160 watts. For comparison, the average U.S. household uses about 30 kWh/day, or 12 kWh/day per person. I use less than average because my house has no air conditioning, and because my refrigerator and lights and computer are all pretty efficient. I do cook with electricity, but I hang my clothes (indoors) to dry. And I don’t indulge in power-hungry extravagances like a second refrigerator or freezer or hot tub.

Still, my home electricity use is far from negligible. It’s pretty close to the household average (counting only electrified households) in China and Mexico; it’s nearly twice the total per-capita use (including all commercial and industrial uses) in India; and it’s a hundred times greater than the total per-capita use in some African countries.

If all my electricity came from coal, the resulting CO2 emissions would be about 3000 pounds per year. The actual carbon footprint is less than this by an amount that’s ambiguous, because of the way electricity from natural gas and renewables is mixed into Utah’s grid. I actually pay Rocky Mountain Power an extra $3.90 per month to participate in their Blue Sky program, nominally buying 200 kWh of wind-generated electricity—enough to cover 170% of what I use. For about $1500, after federal and state tax incentives, I could install enough rooftop solar panels to cover my household use, and thereby reduce each of my monthly bills by about $10. Neither wind nor sunshine, however, is always available at the times when I’m using electricity, so neither provides complete freedom from the fossil fuels that dominate Utah’s electrical grid.

Meanwhile, there’s a carbon-emitting elephant in the room that I haven’t yet mentioned: natural gas, which my house uses for space heating and water heating. Here is a plot of my monthly gas use over the last 16 and a half years:
I’ve again plotted my average daily use for each billing month, in millions of BTUs (the gas company’s billing unit, also called decatherms). Along the right side I’ve multiplied by 300 to convert this unit to approximate kilowatt-hours (the more accurate conversion factor would be 293), to facilitate comparison to my electricity use. Notice the following:
  • Nearly all of my natural gas use is in the winter. Water heating in the summer is small by comparison.
  • My 2001-2002 renter produced a significant spike, as we kept the basement warmer than usual. My other renters/guests don’t show up on this graph because they weren’t around in the winter.
  • In December 2003 my old (from 1980 or so) furnace died, and the house was without heat for a week or two before I had a new one installed. The new one is a “condensing” furnace, rated at 92% efficiency because it sends less heat up the chimney. At the same time, I moved the thermostat from the front room to the back of the house, so I could close off the front room and avoid heating it for most of the winter. These changes reduced my gas use by more than 40%. The new furnace has just about paid for itself in the 11 years since it was installed, so it would have been a good investment even if the old one hadn’t died.
  • Any other changes or trends (such as the new storm windows that I got in 2011) are indiscernible due to the weather-caused variations.
  • Even with the new furnace, my average daily gas use is about 0.08 MBTU, or 23 kWh: six times as much energy as I use from electricity.
I use a lot of natural gas because my house, though small, is 80 years old and poorly insulated. But the factor of 6 is somewhat misleading, because when electricity is generated from fossil fuels (or nuclear fuel, for that matter), only about a third of the energy in the fuel is actually converted to electricity. (The rest is given off as waste heat at the power plants, and the second law of thermodynamics says there’s not much we can do about it.) So instead of a factor of 6, we could say that my natural gas use is only about twice the amount of fuel that I cause to be burned for electricity. Perhaps coincidentally, the amount that I pay for natural gas is also close to twice what I pay for electricity (about $20/month on average vs. $10), if you neglect the flat fees that are charged just for being hooked up to these utilities.

Burning one MBTU of natural gas emits 117 pounds of CO2, so my annual CO2 emissions from burning natural gas come to 3370 pounds—unambiguously more than the emissions from my electricity use. Thus, even if I reduce my electricity-related carbon emissions to zero, I shouldn’t feel too proud of myself unless I also reduce gas use. Unfortunately, I may have no good cost-effective ways to do that. One option might be to turn the thermostat down and rely more heavily on electric heating pads and blankets and space heaters—and then invest in a rooftop solar system that’s big enough to offset the electricity used by these appliances.

Before wrapping up this article, I should mention that my overall carbon footprint includes quite a few other contributions besides home energy use. There are significant emissions from driving, from flying, from growing and transporting the food that I eat, and from making the stuff that I buy. Perhaps I’ll detail my estimates of these in a future article. For now I’ll just say that each of these four is very roughly comparable to the footprint of my electricity or natural gas use; no one of them seems to be so large that it makes my home energy use negligible in comparison.

In any case, the graphs above make it pretty clear that the new furnace and new refrigerator were the “low-hanging fruit” for reducing my utility bills and the associated carbon emissions. I hope others can learn from these examples, even as I ponder which fruit to reach for next.

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