Tuesday, October 23rd, 2007
What’s your biggest drain on the environment? Showering, Air Conditioning, Heating, or Lighting?
Of course the answer depends on a lot of factors about you, your surroundings and how you relate. I’ll share some average for the US and go into some basic ways to calculate this for you. I was quite shocked with the magnitude of the difference when I went through this exercise myself. Please don’t mistake me for saying we shouldn’t do more just because it doesn’t do as much. Every little bit helps. If you are like me, you can’t do as much as you want right now. Hopefully you can use these concepts to prioritize your actions and where best to apply your energy and resources.
For an average US household, the energy utilization is mainly around HVAC (Heating Ventilation and Air Conditioning), Lighting, Cars, Refrigeration. The percentages from Wikipedia are:
32% space heating
13% water heating
12% lighting
11% air conditioning
8% refrigeration
5% electronics (includes computers)
5% wet-clean (mostly clothes dryers)
To calculate your annual cost (both environmental in terms of carbon footprint and economic in terms dollars) follow the steps below.
A house is a complex system of inter connected components. Most of us, myself included, only have gross utility usage from each source to gleam some useful information from. Even with just your gas bill (natural gas, heating oil, or propane) and electric bill you can calculate a reasonable estimate of major energy usage. If you use electricity for heating, hot water, clothes drying and cooling, then this becomes much harder and you may wish to have an expert do a home energy audit. Check with your electric company as they may offer this for free if you are really interested in conserving. Given that gas furnaces/boilers, hot water heaters and clothes dryers are much more efficient than equivalent electric models, you should consider switching if that is an option. Here is how you can calculate how much energy goes into heating, hot water and air conditioning if you have gas for heating, hot water and clothes drying, but use electricity or air conditioning.
To figure out what your gas baseline for heating water, cooking and drying your clothes, take an average over the summer months when you hopefully don’t have your heater turned on. From last year’s gas usage, I picked our three or four smallest gas bills in terms of therms and found that my family uses about 25 therms for heating water, drying clothes and cooking food. One trick, most utility bills include monthly breakouts of your previous year’s usage. Next, figure out your average therms used for the three of four highest months when the boiler/furnace was really needed. I found we used about 180 therms for December through March. Since the load for heating water, drying clothes and cooking is basically the summer time gas usage, we can subtract that from the winter months to estimate the heating load. In my example, that works out to be 155 therms per month.
To figure out how much that works out to be in dollars, calculate the blended rate for a typical month by dividing the total gas bill by therms each month. Mine works out to be around $1.85/therm, so heating costs about $290 per month in the winter. To calculate the carbon footprint, multiply therms by the conversion factor, I found online that 1 therm of natural gas emits about 11.7 pounds of CO2, so I get about 1,800 lbs CO2 per month for heating {corrected - thanks Dennis}.
Now let’s do the same thing for your summer cooling bill (i.e. electricity). Take the lowest electric bills, probably December through March and note the average kWh. For me that is about 700 kWh which makes up my normal monthly electric load for computers, lighting, refrigeration, and other things like the home network and wireless. Now look at the highest electric bills for the year, probably July through September. For me that is about 1025 kWh. The difference, 325 kWh/month is a good estimate for the air conditioning load from the 3 window units I use to cool my house in the summer.
To figure out how much that works out to be in dollars, take the difference times the cost pe
r kWh. You may need to average that or figure in some more complicated formulas for different markets that do time of day and charge more for heavy usage. For me, including the opt-in GreenUp fee, I pay about 18.5 cents per kWh, so the monthly AC bill is about $60. The carbon footprint is also a bit complicated as it depends on what goes into making the electricity you use. EPA has excellent information for various regions in the US. Being in New England, most of our electricity comes from natural gas, so the conversion to CO2 is 0.91 lbs/kWh so about 300 lbs CO2/month for AC. The national average is about 1.36 lbs/kWh, but can easily be over 1.5 lbs/kWh if an area relies heavily on coal.
Given that I live in New England, it is not surprising that our mothly winter heating costs are 4 times more than our summer cooling costs. In terms of carbon dioxide it is even bigger at heating producing 6 times the carbon dioxide. This tells me that if I am going to make investments to save money and the environment by improving my home environment like installing solar panels, replacing our boiler, replacing air conditioning units, etc., I should focus on things that cut my natural gas utilization in the winter to have the biggest financial and environmental return.
{section rewritten to because earlier calculation was off by a factor of ten and the outcome now has both heating costs and carbon dioxide production higher than cooling}
Fortunately there are things that can reduce both my heating and cooling loads simultaneously like adding insulation to the house, replacing windows with better ones, etc. and I hope to focus on those in the near term while planning for something larger like a solar thermal or PV installation or a geothermal heat pump.
on Sunday, December 9th, 2007 at 8:30 pm:
please correct the pounds CO2 per therm from 1.17 to 11.7 for natural gas. The numbers in the link are correct for natural gas but the decimal may have been inadvertently shifted.
on Monday, December 10th, 2007 at 10:35 pm:
Thanks Dennis for finding the error. The outcome makes a lot more sense in that the heating load is more costly and generates more CO2. I’m working on correcting the aritcle.
on Sunday, February 10th, 2008 at 5:30 pm:
It took me a while, but I have finally corrected this article. Thanks to Dennis for noting the moved decimal place.
on Saturday, May 31st, 2008 at 7:06 pm:
Fresh water can only become scarcer and go up in price! I am in Michigan, so I am lucky to be surrounded by it, but it is still getting costly and our freshwater lakes are depleting! I do all I can to conserve water! My newest home water conservation addition is a Hot Water Lobster Instant Hot Water Valve. This thing is great! It saves a lot of water and my entire home has instant hot water! I save a lot of time not having to wait for water to heat up at my taps! I paid $179.95 off the internet for it, which seems quite reasonable! My water savings will recover it’s cost in not too long. It only took my like 10 minutes to install!
You can’t beat that it’s made in the U.S.A., has a 10-year warranty, and comes with a money back guarantee! I surely won’t be returning it! I bought it from:
www.hotwaterlobster.com