The short answer
Modern ENERGY STAR dishwashers use 1.2-1.5 kWh and 3-5 gallons of hot water per load — about $0.30-$0.50 total. Handwashing the same dishes uses 8-15 gallons of hot water and similar energy to heat it. The dishwasher wins on water, energy, and labor (saves 71 hours/year for typical household).
Counterintuitively, a modern dishwasher uses less energy and water than handwashing the same dishes. A typical ENERGY STAR dishwasher uses 1.2-1.5 kWh and 3-5 gallons of hot water per load. Handwashing the same dishes uses 8-15 gallons of hot water plus the time + soap.
Per-load cost — modern ENERGY STAR dishwasher
Energy: 1.2-1.5 kWh per load. At 17¢/kWh, $0.20-$0.25/load. Annual cost at 285 loads/yr: $57-$71.
Water: 3-5 gallons per load. At average US water rate of $0.005/gallon, water cost is negligible.
Detergent: $0.10-$0.20/load. Total per-load cost: $0.30-$0.50.
Handwashing cost comparison
Hot water use: typical handwashing of one dishwasher load worth uses 8-15 gallons. Heating that water takes 0.6-1.5 kWh (electric water heater) or 0.1-0.2 therms (gas water heater).
Total energy + water cost: $0.25-$0.50/load — similar to dishwasher.
But the time cost is different. Handwashing 15 minutes vs dishwasher 0 minutes (load + start, walk away). Over 285 loads/year, handwashing burns 71 hours of human time.
Dishwasher settings that maximize efficiency
Skip the heated dry cycle. Air-dry instead — saves about 15% of dishwasher energy use per cycle.
Run full loads only. A half-full dishwasher uses the same water + energy as a full one.
Use eco mode. ENERGY STAR-certified dishwashers have an eco mode that uses 20-30% less energy. Pre-rinse only what is necessary.
Recap
Bottom line
A modern ENERGY STAR dishwasher uses less energy and water than handwashing the same dishes — a counterintuitive result that comes down to engineering. The dishwasher uses 1.2 to 1.5 kWh and 3 to 5 gallons of hot water per load while handwashing uses 8 to 15 gallons of hot water plus the time to scrub. For a typical household running 285 cycles per year, the dishwasher saves roughly 71 hours of human time and modest amounts of water and energy.
For maximum savings, run only full loads, skip the heated dry cycle, use eco mode where available, and replace dishwashers older than 10 to 12 years with ENERGY STAR models. The water-heater-temperature-savings guide covers the related move of dropping water heater setpoint to 120F, which compounds the dishwasher energy savings.
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