To determine if using a dishwasher actually saves water, you’ll need to pay attention to your water usage habits, check the efficiency data from your appliances, and possibly do some mild math.
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How much water does a dishwasher use?
Each dishwasher takes a different amount of water to complete a full load of cleaning. Newer models have drastically improved water efficiency, thanks to both increased demand for resource efficiency and new technologies.
For ballpark estimation, older and less efficient dishwasher models may use 15 gallons (68 liters) of water for a load of dishes. 3.5 gallons (about 13 liters) is a common usage amount for modern full-sized home dishwashers. Compact counter-top models use closer to a single gallon or about five liters for a full wash cycle.
If you want to get actual numbers, find your water meter, record the value, and run a load of dishes without using water anywhere else in the home. This takes more effort, but you might identify a problem with your dishwasher or elsewhere in the plumbing.
How much energy does a dishwasher use?
Dishwashers in the US, EU, and several other countries come with energy guide ratings that give an estimation of the long-term energy costs for typical usage. These are based on typical usage for easy-to-use numbers, but they aren’t broken down further.
The total amount used will range between 1.2 and 2.4 kWh per load. Heating the water accounts for a significant portion of the energy used, but not all of it.
Energy used by a dishwasher to heat water
Dishwashers use electricity and hot water. While they mostly rely on a water heater to bring the liquid to a working temperature, they do sometimes heat the water up for heavy-duty wash cycles.
Water heaters can generate heat from a variety of power sources, including natural gas and electricity. When properly working, a heater brings the water up to between 120° F (about 49° C) and 140° F (60° C). The temperature of the standing water in your pipes depends on the climate of the area and the insulation, if any, that the water source has. Water from a well tends to have the same temperature year-round, and home insulation can warm up the water closest to the water heater.
The British Thermal Unit (BTU) is based on the amount of energy it takes to heat up one pound of water by 1°, and it always requires the same amount of energy. Variance in power costs come from the efficiency of the water heater and the type of energy used. There’s a handy formula:
The amount of water in gallons times 8.33 gallons/pound times 1 BTU per pound times the difference between the final and starting temperatures in Fahrenheit.
So, if the load uses 3.5 gallons of hot water with the standing temperature at 55° F and the thermostat set for 130° F, then you need about 1,250 BTUs to heat the water up 75°. Typical electrical water heaters have efficiency ratings that float just above 90%, so the actual BTUs needed would be 1,388 BTUs. With 3,412 BTUs per kilowatt-hour (kWh), that comes out to about 0.4 kWh of electricity to heat the water. The average price of electricity in the U.S. is a little over 12 cents, so it costs about a nickel to heat the water.
You may have a natural gas water heater instead of an electric model. The BTUs come out the same, but natural gas has different measures and costs than electricity. Gas is frequently sold in units called therms that are equal to 100,000 BTUs, and gas heaters are less efficient with ratings around 60%.
The example load would need 2,083 BTUs or a fiftieth of a therm of natural gas. The average price of one therm is around $1.20, so it costs just 2.5 cents on average to heat the water.
How much water does hand-washing dishes use?
The amount of water used while hand-washing dishes varies with the flow rate of the faucet and how long the faucet runs.
In the United Kingdom, kitchen taps are required by law to have a flow rate between four to six liters per minute, or roughly 1 to 1.5 gallons per minute. Faucets in the United States have a federal maximum of 2.2 gallons per minute, but many states and municipalities may restrict it further. Older houses may have faucets that use more water by design or by decay.
When hand-washing a large load of dishes, many people fill up one side of the sink with hot water and use the other side for rinsing with a low-to-medium velocity stream. Sink capacity varies, but 1 to 2 gallons is common for dual-sided sinks in a home kitchen.
Then, water is needed to rinse away the suds. Ideally, the sink has a spray nozzle with a trigger, just like a garden hose sprayer. That way, you’re only using water when you’re actually washing instead of throughout the whole process. Some people leave the water running on one of the spray settings, and others let it flow freely the entire time. The amount of water used varies drastically between those three styles.
As with the dishwasher, track the water meter to get the most accurate numbers.
How much energy does hand-washing dishes use?
Plumbing moves water around through the force of water pressure, but it does take energy to heat up water. Like the amount of water that ends up being used, the amount of energy used to hand-wash dishes varies with your washing habits. After about 6 gallons of hot water is used (about 3 minutes of full-tilt water), the energy used goes above the most efficient dishwashers.
Does using a dishwasher save water?
The typical hand-washer will usually end up using more water compared to efficient dishwasher models when washing the same number of dishes. Determining the exact amount of water saved is difficult, as it depends on your habits and the model of the dishwasher.
For those who rarely eat at home and only ever have to wash a couple of dishes at a time, a small amount of water for suds and a spray to rinse might keep the amount of water used to less than a gallon. In cases like these, it’s sensible to stick to hand-washing. Having to rewash dishes also cuts in on the energy and water savings of a dishwasher.
How do I save more water and energy while washing dishes?
First, replace dishwashers that are very old. It can be a small investment, but the reduction in energy and water usage will eventually pay for themselves. Heating up an extra 10 gallons of water increases the water heating costs by over 400%, and it likely has other inefficiencies that result in more power usage.
Load your dishwasher as much as possible to get the most use out of every cycle. If you find yourself running loads that have empty space, consider switching to a smaller model. Alternatively, use more of that collection of coffee mugs instead of sticking to your favorite.
Avoid ‘heavy duty’, ‘heated drying’, and similar options on dishwashers. Sanitized dishes are more important than a little bit of energy savings, but making your dishwasher work harder costs more.
If you continuously run a stream of hot water while hand-washing dishes, even at a low flow rate, the amount of energy used will spike. To get a flow that can rinse away suds in a reasonable amount of time, use the spray setting. Bonus points for sprayers that cut the flow until a trigger press.
For those who start with one side of the sink full of hot water, be sure to create lots of bubbles. Not only are they good for cleaning, but they help trap the heat in the water. The puddle cools at a slower rate, so fewer doses of extra hot water are needed while cleaning bigger loads.
