# Built In Dishwashers vs. Hand Washing: Which is Greener?

*Image credit: Getty Images/David De Lossy*

## Dishwashers vs. hand washing: Numbers and assumptions

**Water**

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**

"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.

**Carbon dioxide**

According to the Environmental Protection Agency (EPA), the national average of carbon dioxide emissions is 7.78 x 10-4 metric tons CO_{2} / 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 CO_{2} per kWh.

Burning one therm, or 100,000 BTUs, of natural gas releases 11.7 pounds of CO_{2}, according to the EPA, and the average carbon emissions for 1 kWh is 1.715 pounds of CO_{2}, also according to the EPA. And, for what it's worth, 1 therm = 29.307111111 kWh, approximately.

**More 'this vs. that' calculations**

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