|April 6th, 2014|
We pay for most things directly as we use them: to park at a meter you put in quarters; to rent an apartment you send a check each month. For some things, however, our usage is metered and we pay after the fact: electricity, gas, water. In these cases we're often not well calibrated for marginal cost, which means we're not in a good position to make tradeoffs. Can I leave the fan on all night? Should I worry about letting this frozen food thaw in the fridge before I put it in the oven? How much should I be trying to take shorter showers? Colder ones?
I thought it would be useful to compile some rough costs of these pay-later activities:
|Gas Oven||one hour||$0.16 |
|Gas Dryer||one load||$0.35 |
|Front Loading Washer, Warm||one load||$0.39 |
|Cold Shower||ten minutes||$0.57 |
|Hot Shower||ten minutes||$0.76 |
|Dishwasher||one load||$0.31 |
|Microwave||two minutes||$0.01 |
|Box Fan||overnight||$0.17 |
This is based on a marginal cost of electricity of $0.21/kWh, natural gas of $1.40/therm, and water of $0.017/gallon which includes the sewer charge.  These come from our current bills in Medford MA; it's probably cheaper in lower cost of living states.
While these are approximately in line with what I'd thought, I do see two changes I should make:
- Be willing to use the dryer when line drying is inconvenient. (Previously I never used the dryer, for cost reasons.)
- A long cold shower (in summer) is actually more expensive than a short hot one.
 Gas Oven, 0.112 therms per hour.
 Gas Dryer, 0.21kWh to spin the drum and 0.22 therms to heat the clothes.
 Cold Shower, 3 gallons per minute, 30 gallons total, $0.57 for the water.
 Hot Shower, 0.00014 therms per gallon per degree F. Ground water at around 55F, shower water at around 100F, so a 45F rise. Shower flow of about 3 gallons per minute. So over ten minutes that's thirty gallons each heated 45F. So $0.57 for the water  and $0.19 to heat it.
 Dishwasher, 4 gallons of hot water and 1kWh to run the machine. That's $0.10 for the water and heating  and $0.21 for the electricity.
 Microwave, 0.024 kWh per minute.
 Box Fan,100W.
 This is much higher than I'd thought, 10x the cost of desalination.
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