A Full Dishwasher Beats Hand Washing on Water if You Skip the Pre-Rinse

A team at the University of Bonn once gave 113 volunteers the same pile of dirty dishes, 12 place settings smeared with spinach, egg yolk and minced meat. Each person was asked to wash up exactly the way they would at home. The water they used swung so widely that the researchers found no single way people wash dishes, even among volunteers from the same country.

A full, modern dishwasher beat nearly all of them on water. If you have one, how you load it and what you do at the sink before it runs decide how big that win is for you.

What a certified dishwasher uses

Your dishwasher doesn’t fill up like a bathtub. It holds a small pool of water in the bottom, heats it and sprays it across the racks again and again, then drains and refills for the rinse.

That reuse is how the machines got so frugal.

Under the ENERGY STAR criteria that took effect on July 19, 2023, a standard-size dishwasher can use no more than 3.2 gallons a cycle and 240 kilowatt-hours a year to earn the label. Compact models, the ones that hold fewer than 8 place settings, are capped at 2.0 gallons a cycle.

ENERGY STAR’s dishwasher page also says a certified machine uses less than half the energy you’d spend washing the same dishes by hand. The same page notes that a dishwasher uses about the same amount of energy and water regardless of the number of dishes inside. That makes the size of your load the setting you control most. The EPA’s WaterSense program says running the dishwasher only when it’s full can cut one load a week and save the average family nearly 320 gallons a year.

Open yours tonight and look at how full the racks are before you press start.

What hand washing really uses

The Bonn volunteers came from seven countries and regions, from Turkey to Ireland. The team, led by Rainer Stamminger, presented the results at a 2003 European conference on appliance energy use. Each person faced 140 pieces of crockery, glasses and cutlery, two sinks with hot and cold taps, and a choice of 22 hand dishwashing liquids. Water meters and a video camera recorded everything they did.

On average, a volunteer used 103 liters, about 27.2 gallons, and took 79 minutes to wash and dry the load. The group averages ran from about 12.2 gallons for the German volunteers to about 44.9 gallons for those from Spain and Portugal. One person went through 447 liters, about 118 gallons, on a load you could fit in a single dishwasher.

Keep in mind that the study dates from 2003 and that people washed under observation in a lab. That set everyday hand washing against machines used exactly as intended.

Gallons to wash 12 place settings. The hand washing figures come from the University of Bonn study, converted from liters; the top bar is the most a standard-size ENERGY STAR dishwasher may use per cycle under the 2023 criteria.

Where the sink water goes

The researchers sorted the volunteers into rough types. The type they called the “carefree” dishwasher left the tap running most of the time, sometimes even while drying. The EPA’s WaterSense program puts a number on that habit, saying a faucet left running for five minutes while you wash dishes can waste 10 gallons.

Cleanliness didn’t favor the sink either, though you might expect careful hand washing to win there. About half the volunteers left dishes the researchers judged not clean enough to put on a dinner table, while the study’s two dishwashers got dishes at least as clean as any volunteer did with far less water.

ENERGY STAR adds that certified machines boost their water to 140 degrees, which it says disinfects better than hand washing and is far hotter than you’d want on your hands.

Next time you wash by hand, notice how long the tap runs while you’re scrubbing a pot with nothing under the stream.

Energy and time, side by side

The study also measured energy and time for the same 12 place settings. Its two dishwashers were new, top-rated machines in 2003, which makes them older than anything you’d buy today.

Method, 12 place settings	Water	Energy	Your time	Best fit
Dishwasher, full load	4.0 to 5.8 gallons	1 to 2 kWh	About 15 minutes to load and unload	Any home that fills a rack every day or two
Hand washing, German volunteers’ average	12.2 gallons	1.3 kWh	76 minutes	Small loads, homes without a machine
Hand washing, all volunteers’ average	27.2 gallons	2.5 kWh	79 minutes	The habit to improve on

Source: Stamminger and colleagues, University of Bonn, 2003. Water converted from 15 to 22 liters, 46 liters and 103 liters.

Most of the energy in hand washing goes into your water heater. The Bonn team counted only the energy needed to warm cold water from 15°C, about 59°F, up to washing temperature. Even on that narrow count, the volunteers averaged 2.5 kWh against 1 to 2 kWh for the machines, which used 4.0 to 5.8 gallons a load. The thrifty German group got down to 1.3 kWh, but it still took them 76 minutes.

When you do wash by hand, one sink of hot water you fill once asks far less of your water heater than a hot tap you leave running.

Your hours at the sink

Time is where you’ll feel the gap most.

A dishwasher program in the study ran 100 to 150 minutes, but a person spent only about 15 of them loading and unloading. If you wash by hand every evening, the gap between 15 minutes and 79 comes to more than an hour you get back each night. ENERGY STAR estimates that a certified dishwasher saves over 230 hours of personal time a year and about $220 a year in utility bills compared with washing by hand.

Time your next sink session once, from the first plate to the last towel, and set it next to those 15 minutes.

Pre-rinsing erases the savings

ENERGY STAR puts its advice in three words, scrape don’t rinse, since rinsing dishes before you load them adds to the total water and energy the job uses.

A 2015 version of ENERGY STAR’s dishwasher page warned that rinsing can use up to 20 gallons before the dishes are even loaded, which is more than six cycles’ worth at the 3.2-gallon cap.

Gouache illustration of a smiling woman in her sixties with an emerald headwrap and a white apron, standing in an open back doorway and scraping leftover salad and a banana peel off a white plate with a fork into a green compost bin with its lid open. Behind her is a sunny lawn under blue sky. — *If dirty dishes will sit overnight, ENERGY STAR suggests the machine’s own rinse setting, which uses a fraction of the water a hand rinse does.*

Who should pick which

Small households. If you live alone or with one other person, a rack can take days to fill, and that tempts you to run it half empty. Wait for a full load and use the rinse setting in between, and wash a greasy pan that can’t wait in a stoppered sink.

Homes without a machine. Hand washing can be lean, and the German volunteers’ 12.2-gallon average proves it, though that’s still nearly four times the 3.2-gallon cap.

WaterSense’s advice is to plug the sink or use a wash basin when you wash by hand. That one habit keeps the tap from running the whole time you’re at the sink.

People with a two-basin sink. Fill one side with hot, soapy water and the other with clean rinse water, then turn the tap off. The Bonn team watched some volunteers use as many as four baths per item, from a soak to a final rinse. A two-basin setup gets you most of that with water that stays put.

The worst choice in any kitchen is the running tap.

Before tonight’s dishes

A lot of dish water never does any washing. It runs down the drain under a tap nobody turned off, or over plates that were headed into the rack anyway, and you can stop both habits without buying anything.

After dinner tonight, scrape each plate into the bin with a fork, load it straight into the rack, and press start only when the rack is full.