Best Window Air Conditioner by Room Size (and Cost to Run)

ENERGY STAR's chart: 100 to 150 sq ft 5,000 BTU, 150 to 250 sq ft 6,000, 250 to 300 sq ft 7,000, 300 to 350 sq ft 8,000, 350 to 400 sq ft 9,000, 400 to 450 sq ft 10,000, 450 to 550 sq ft 12,000, 550 to 700 sq ft 14,000, 700 to 1,000 sq ft 18,000. Adjust 10 percent for shade or sun and add 600 BTU per extra person.

Average watts are BTU divided by CEER. At CEER 15 and 17.3 cents per kWh: 8,000 BTU about 9 cents an hour, 10,000 BTU about 12 cents, 12,000 BTU about 14 cents, 18,000 BTU about 21 cents. At 8 hours a day that is about $22 to $50 a month.

Usually. Inverter units slow down instead of cycling on and off, which makes them quieter and helps them run efficiently. Every ENERGY STAR window unit in our database is an inverter with a CEER of 14.4 to 15.

Every unit up to 15,000 BTU in our database runs on 115V. The 18,000 and 23,500 BTU units need 230V.

168 sq ft falls in ENERGY STAR's 150 to 250 sq ft row, so 6,000 BTU. We do not list a 6,000 or 7,000 BTU unit, so our pick is the Midea U-Shaped 8,000 BTU inverter, which throttles down in a smaller room. It averages about 533 W, about 9 cents an hour at 17.3 cents per kWh.

12,000 BTU, from ENERGY STAR's 450 to 550 sq ft row. Add 10 percent for a very sunny room (13,200 BTU) or 600 BTU for each regular occupant over two. The LG LW1224IVSM, Midea U-Shaped 12,000 and GE Profile ClearView 12,200 all fit and run on a standard 115V outlet.

CEER is the Combined Energy Efficiency Ratio, the DOE efficiency rating for room air conditioners: BTU of cooling per watt-hour of electricity, counting standby and off-mode power. Higher is cheaper to run. Divide BTU by CEER for average watts, so a 10,000 BTU unit at CEER 15 averages about 667 W and one at CEER 11 about 909 W.

A window unit with a gap in the middle, so the window sash closes down into the U between the indoor and outdoor halves. That puts the glass between the room and the outdoor half, where the compressor sits. The Midea U-Shaped 8,000, 10,000 and 12,000 BTU models here are inverter units rated CEER 15.

A window unit installs in minutes and suits renters. A DIY mini-split like the MRCOOL DIY 12K mounts through an exterior wall, heats as well as cools, and at SEER2 17.5 averages about 686 W for 12,000 BTU, about $29 a month at 8 hours a day and 17.3 cents per kWh. A CEER 15 window unit of the same size averages about 800 W, about $34 a month.

Multiply average watts by 24. An 8,000 BTU unit at CEER 15 averages about 533 W, so 12.8 kWh a day, about $2.21 a day or $67 a month at 17.3 cents per kWh. On the hottest days it draws more than that average.