Tuesday, January 5, 2010

Dishwasher/handwashing: It's all in the (hot) water

To hand-wash dishes, or not, which is greener?

There are so many folks out there with half-baked opinions that you might never get to a supportable answer.

That said, here's ours, which contradicts popular counter-intuitive “green” thinking: If you have a solar water heater, and you're an efficient washer, hand-washing has dramatically fewer environmental impacts.

You can waste a lot of water before you overcome the cost of the electricity for the washer's electric motor and the environmental cost of building and shipping that dishwashing machine.

On the other hand, if you're paying a utility to heat your water, either alternative will be an expensive one, and the judgment comes down to how much more hot water is used in your style of hand dishwashing than the dishwasher you select.

All kinds of green gurus are arguing that running a dishwasher has less impact on the environment. Mostly, they justify this position with a deplorable bit of analysis—they consider the amount of water used, and ignore electrical costs and the inherent environmental cost of the existence of the dishwasher.

Here's an example from a Hawai'i newspaper. The average dishwasher uses 8.7 gallons of water per load, while hand washing can use 10 to 20 gallons, depending on whether you leave the water running. Therefore the dishwasher is greener.

Here's an About.com argument that dishwashers are greener than hand washing.

They are, of course, both wrong in almost every scenario. [They're pretty much only right if 1) you're using a very efficient dishwasher, 2) you're a very inefficient hand-washer, and 3) you're using fossil fuels to heat your water.]

The best electric dishwashers can do the job with five gallons, more or less, and the worst with 15 or so, but there's far more to dishwashing than water.

Let's call the detergent use even. And for the purposes of this assessment, let's call water heating even, assuming you have a solar water heater and aren't drawing electrical power to heat water.

In that case, figuring a 1000-watt dishwasher that runs for an hour, you'll pay electric costs of $.30 to $.40 per load, depending on where in Hawai'i you live. And the water costs about 3 cents for 10 gallons. (The electrical cost of pumping the water out of the ground is built into this price.)

Anybody paying attention? You can use two 55-gallon drums full of water hand-washing before it costs as much as the electrical cost alone of one dishwasher load.

That calculation is child's play. It gets trickier for those who don't have a solar water heater, and who are paying the electric utility or the gas company to heat water. From various sources, I've come up with about .2 kilowatt hours to heat a gallon of water from tap cold to dishwashing hot.

A 20-gallon hand-wash then costs 6 cents for the water and $1.20 for electricity to heat it, for $1.26. An efficient 10-gallon hand-wash costs $.63. For the dishwasher, the total costs depends in part on how much of the heated water comes via the household water heater and how much from the washer. Let's say $.30 to $.40 for electricity plus a $.32 hot water bill (5 gallons plus the water cost)--about the same as an efficient hand-wash, and less than an inefficient hand-wash.

But the upshot is that if you use a lot of hot water, and you're using electricity to heat it, it's going to be a costly venture either way. If you're an inefficient hand washer and you're paying the utility to heat your water, the dishwasher might indeed make more sense. (If you use more efficient fuel-based water heating like gas or heat pumps, the calculation shifts back toward hand-washing.)

But if you're using the sun to heat your water, you can be awfully inefficient as a dishwasher before you overcome the significant cost of running the machine.

I like this note from Dave Brook, energy extension agent at the Oregon State University: “Studies showing an advantage to one method usually assume less-efficient practices for the other—such as leaving rinse water flowing continuously when washing by hand. Because there’s no clear advantage of one method over the other, the main benefit of automatic dishwashers is convenience, not energy savings. Many people find dishwashers are a great place to store dirty dishes out of sight before washing them.”

As a University of Bonn study says, “while on the water there is a clear advantage of using a dishwasher, on energy in real life the situation is more complex as distribution and generation losses have to be taken into account as well.”

But once again, all those assessments assume you're paying to heat the water. If you're not, the choice is abundantly clear: wash 'em by hand and your environmental impact is minimal.

© Jan TenBruggencate 2009


Rosette said...

eat sandwhiches and salad less dishes..or bar b cue...or go eat out use banana leaves as plates !!

cbuddenhagen said...

I always wanted to know whether you save money/power/greenhouse gas when you fill your water kettle with hot water versus cold water from the faucet (assuming you are using power from the main grid to heat water in your water heater)?

Jan T said...

I'm not sure how to do that calculation. Certainly it depends somewhat on whether you're further heating your water with gas or electricity. But generally, I think it's unwise to use hot water that's been through the hot water tank coils, where it can sit for long periods, for drinking.