What a dishwasher costs to run
Across 737 certified models ENERGY STAR currently lists in this category, the median uses 239 kWh/yr. The cheapest certified model to run costs about $15/year; the most expensive costs about $45/year, more than 3x the spread.
Efficient models versus the rest
Of the 737 certified dishwashers tracked here, 61 use meaningfully less energy than the category average and 0 use meaningfully more, the rest are within about 8% of it either way. The most efficient certified model found is the Loch L1126 at 80 kWh/yr; the least efficient is the Amana ADFS2524R** at 240 kWh/yr.
Your state changes this more than you'd think
The figures above use the simple average of all 51 published state electricity rates (18.8¢/kWh). A median-efficiency dishwasher costs about $29/year to run in North Dakota (12.19¢/kWh, the cheapest state rate this site tracks) and about $110/year in Hawaii (45.95¢/kWh, the most expensive), the identical appliance. Every model page on this site lets you pick your own state to see the real number. See the guide to why location matters this much.
Over its lifetime
NAHB's Study of Life Expectancy of Home Components puts a typical dishwasher's service life at 9 years. At that span, a median-efficiency model costs about $404 in electricity over its lifetime; the least efficient certified model costs about $406, a $2 gap from the choice of model alone. See the guide to when replacing one actually pays off.
Shopping for a new one
Comparing your current dishwasher's age and running cost against a current certified model is the whole decision this category boils down to.
Compare current ENERGY STAR certified dishwashers before buying.