Does a Dishwasher Use a Lot of Electricity?

A wash cycle splits its energy into three parts: heating the water, running the pump and controls, and drying.

Heating the water takes the largest share

Water heating accounts for roughly 60 to 70% of the energy in a cycle. The machine fills with cold water from the house supply and heats only the few gallons it needs, which is why it beats a tap left running.

The pump is a small, steady draw

The circulation pump draws only about 160 watts, and the drain pump about 35 watts. That is the load running for most of the cycle, and it is modest.

The heating element is the big intermittent draw

The element switches on and off to bring the water up to temperature and, if you use it, to dry the dishes. Heated dry is the second-largest energy user in the cycle. A dishwasher is not a 1,400-watt appliance running flat out for two hours; it is a small pump running continuously plus a heater cycling on and off.

The nameplate rating on a dishwasher, typically 1,200 to 1,800 watts and about 10 to 12 amps at 120 volts, is the most the machine can draw at any instant, with the heater on. It is not the average draw over a cycle.

Spread 1.2 kWh over a two-hour normal cycle and the average draw is about 600 watts. Multiplying the nameplate wattage by hours of run time gives a number several times too high, because the peak is brief and the heater is off much of the time.

The peak still matters for wiring. A dishwasher needs a dedicated 15-amp circuit, or 20 amps if it shares the circuit with a disposer, because of that short burst of draw. Its energy use stays modest because the burst does not last.

A new ENERGY STAR certified dishwasher uses less than half the energy of washing the same dishes by hand, and saves roughly 8,400 gallons of water a year and 230 hours of the household's time. Hand washing a full 12-place-setting load has been measured at roughly 2.5 kWh and 27 gallons, against about 1 kWh and 4 gallons for the machine.

The advantage depends on running full loads, scraping rather than pre-rinsing under a running tap, and how your water is heated. If the water heater runs on natural gas, hand washing can come out cheaper per load than an electric dishwasher, because gas heat costs less per unit of energy. Running costs depend on your utility rates and on whether your water is heated by electricity or gas.

For a handful of dishes, hand washing in a basin wins on both energy and water. The machine pays off when it is full.

Several habits and settings move the per-load figure up or down.

• Cycle choice. Eco and auto cycles run lower water temperatures and save roughly 20 to 30%. A heavy-duty cycle with a sanitize option costs more.
• Heated dry. Turning it off saves about 15 to 20% of the cycle's energy. Air dry, or drying by condensation or a heat pump, uses no heating element at all.
• Water heater setpoint. 120°F is the efficient setting. A higher setpoint means the dishwasher's heater has less work to do but the water heater has more, and the net is worse.
• Inlet water temperature and load size. Colder incoming water and partial loads both raise the energy per dish.
• Age of the machine. A 1990s dishwasher used roughly 3.5 kWh a cycle, two to three times a current model, which is the real argument for replacing an old one.
• Standby draw. An idle machine draws a watt or two, roughly 15 to 20 kWh a year. Real, but small.
• Time of use. On a time-of-use rate, the delay-start feature can move the load to a cheaper window.

Countertop and compact machines use less per cycle, roughly 0.4 to 0.7 kWh, because they heat less water and run shorter cycles. An ENERGY STAR compact is capped at 155 kWh a year, about 0.7 kWh a cycle.

Per place setting they can be worse. A countertop unit holds four to six place settings against twelve or more in a full-size machine, so the energy per dish can be higher. ENERGY STAR notes that compact models may use more energy than standard models over time because of their smaller capacity.

Portable and 18-inch slim machines sit in between: a smaller tub and less water to heat, but still a full wash cycle, so their per-load figure tracks their capacity. A compact machine is the efficient choice for a one- or two-person household that would otherwise run a full-size machine half empty. A full-size machine run full is the efficient choice for a family.

Running full loads is the single biggest lever. Everything else is smaller but adds up.

• Scrape, don't rinse. Pre-rinsing under a running tap can wipe out the machine's advantage.
• Use the eco or auto cycle and skip heated dry. Together they cut a meaningful slice off the annual bill in a typical home.
• Set the water heater to 120°F.
• Use delay start if you are on a time-of-use rate.
• When replacing, choose an ENERGY STAR certified model and check the yellow EnergyGuide label's annual kWh figure.

Find the annual kWh on the yellow EnergyGuide label or the spec sheet. Divide it by the number of cycles you actually run in a year to get kWh per load. Multiply that by the cents per kWh on your electric bill.

With round numbers: 240 kWh a year divided by 200 cycles is 1.2 kWh a load, and multiplying that by your own rate gives the cost per load. The label's figure assumes heated dry and a standard cycle, so real use with eco and no-heat dry comes in lower.