Most dishwashers use around 1800 watts of electricity per hour. This number is not written in stone, however, and the wattage can vary between 1200 and 2400 watts on average.
Some dishwashers have an Energy Saver cycle that can use as little as 500 watts per hour, though that will depend on the model of dishwasher in question. Using the dishwasher's drying cycle will also greatly increase the amount of power it uses.
While the above will provide a relatively accurate picture of general energy usage, the most exact way to figure out a given dishwasher's energy needs is to look at the label or stamp on the dishwasher itself. Like the majority of kitchen appliances, most dishwashers will have their maximum wattage stamped or labeled in an unobtrusive place, usually on the back of the appliance.