Energy Costs

On a recent power bill ( 2011 Sept 23 ) from Portland General Electric, power cost in cents per KWHr:

Natural gas from Northwest Natural Gas is $ 1.11756 per therm (2011 Sept 21) .

A therm is 100,000 BTU. A BTU is 1055.05585 Joules, the energy needed to heat a pound of water 1 degree Fahrenheit. So a therm is 105.505585 Megajoules, or 29.3071 kilowatt hours. So natural gas costs 3.813 cents per kilowatt hour equivalent. Electricity is 2.61 times higher. An electric heat pump can produce perhaps 3 to 4 times as much heat per kilowatt-hour, so that might be cheaper. A hypothetical gas fired heat pump might also be more efficient.

Bathwater

Our bathtub typically gets filled with 5 cubic feet of water. A cubic foot of water is 62 pounds, so that is 310 pounds of water. Assume an inlet temperature of 50F, a bath temperature of 100F, and a gas water heater efficiency of 80%. That is 19375 BTUs, or 0.19375 therms. 21.7 cents worth of gas. In winter, most of that heat helps heat the house. On the hottest days in summer, it adds to the air conditioning load.

We also pay $4.10 per 100 cubic feet for water in the West Slope Water District (2011 Sept 21), so 5 cubic feet of water costs 20.5 cents. So, a bath costs 42 cents.

A car costs about 50 cents per mile to operate. The nearest public swimming pool is 1.7 miles away, and costs $3 for 55+ folk. So if my wife and I drive to go swimming, it costs $3.85 each. If we use the same bathwater, it costs 21 cents each.

Movies

I rarely go to movies - no captioning for the hard-of-hearing, too noisy, too expensive, too much trouble for mostly stupifying content. The movie (with captioning) will probably be a buck from Redbox 6 months later (a 20 minute round trip walk). How much energy do movies deliver?

A lot of kilowatts go into a movie theater, but all you really get from them is the optical energy that hits your retinas and the sound energy that hits your eardrums.

White screen illumination energy is about 17 lumens per square foot. Assume about 5 lumens per square foot hitting your eyes, or 54 microlumens per mm² . A watt is 780 lumens, so that is 0.069 microwatts per mm². If your pupil diameter is 6mm in the darkened theater, that is 56 mm² for two pupils, or 3.9 microwatts.

Assume a very loud movie, 95dB, a power level of 3 mW per square meter or 0.003 microwatts per mm². Assuming 8mm ear canals, that is 0.3 microwatts.

Assume a 90 minute or 1.5 hour movie. Total energy, 6.3 microwatt-hours. The movie costs around $8, or $1.27 per microwatt hour. We are not counting previews/advertisements here, which deliver more energy, but presumably cause you to spend more money later.

Thus, movies cost about 1.3 billion dollars per kilowatt hour, about 10 billion times more expensive than electricity. It is no surprise that many "progressive" movie stars advocate very expensive forms of energy production - compared to their product, gerbils on treadmills are an incredible bargain.

I prefer a long soak in a bathtub, with a good book from the library.