What a room air conditioner costs to run
Across 447 certified models ENERGY STAR currently lists in this category, the median uses 531.9 kWh/yr. The cheapest certified model to run costs about $49/year; the most expensive costs about $394/year, more than 8x the spread.
Efficient models versus the rest
Of the 447 certified room air conditioners tracked here, 229 use meaningfully less energy than the category average and 107 use meaningfully more, the rest are within about 8% of it either way. The most efficient certified model found is the Brothers 102372 at 260.4 kWh/yr; the least efficient is the Friedrich KCVL36B30A at 2,096.3 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 room air conditioner costs about $65/year to run in North Dakota (12.19¢/kWh, the cheapest state rate this site tracks) and about $244/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 room air conditioner's service life at 10 years. At that span, a median-efficiency model costs about $1,000 in electricity over its lifetime; the least efficient certified model costs about $3,941, a $2,941 gap from the choice of model alone. See the guide to when replacing one actually pays off.
Shopping for a new one
If the room air conditioner you own is old enough to be well past ENERGY STAR's current efficiency baseline, a current certified model like the ones above can cost meaningfully less to run.
Compare current ENERGY STAR certified room air conditioners before buying.