Image credit: Getty Images/David De Lossy
Dishwashers vs. hand washing: Numbers and assumptions
Here's where we got the numbers:
The average -- non-Energy Star -- dishwasher uses 6 gallons of water per cycle, and average kitchen faucet flow is 2 gallons per minute, thanks to the Department of Energy. The average Energy Star-qualified dishwasher uses 4 gallons per cycle.
"Energy Factor" is the number that results from dividing estimated loads per year (215 -- Energy Star estimates about 4 loads per week) divided by the annual energy usage (kWh/year).
The National Appliance Energy Conservation Act (NAECA) dictates minimum standards for energy consumption in dishwashers. All standard-sized dishwashers must have an Energy Factor of at least 0.46. Energy Star-rated dishwashers must exceed the NAECA standard by at least 25 percent.
The standard-size Energy Star models that uses the highest amount of energy are the Monogram ZBD07**K10 & ZBD68**K10 and Profile PDW8***J10, PDW87**J10, and PDW9***J10 (all those asterisks are placeholders for non-energy attributes, such as color), which use 342 kWh/year, translating to an Energy Factor of 0.65, or 40 percent better than the standard. The standard-size model that uses the lowest amount of energy is the Asko D5893 at 187 kWh/year, which translates to an Energy Factor of 1.15 or 150 percent above the standard.
A British Thermal Unit (BTU) is the amount of energy needed to raise one pound of water from 60°F to 61°F at sea level (thanks, Wikipedia), and a gallon of water weighs about 8 pounds -- a pint's a pound, the world around, and there are two pints in a quart and four quarts in a gallon. So the equation for heating water looks like this (assuming water comes in to your home at about 60 degrees):
8 pounds per gallon x (final heated temperature minus 60) / heating method efficiency
So, 16 pounds (2 gallons) x 60 degrees / 65 percent (for tank storage gas water heaters) equals 1477 BTUs.
According to the Environmental Protection Agency (EPA), the national average of carbon dioxide emissions is 7.78 x 10-4 metric tons CO2 / kWh, or 0.000778 per kWh. 0.000778 metric tons is roughly equal to 1.715 U.S. pounds, so that's what we used for pounds of CO2 per kWh.
Burning one therm, or 100,000 BTUs, of natural gas releases 11.7 pounds of CO2, according to the EPA, and the average carbon emissions for 1 kWh is 1.715 pounds of CO2, also according to the EPA. And, for what it's worth, 1 therm = 29.307111111 kWh, approximately.