AC BTU Calculator

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, and up to 34,000 BTU for 2,500 sq ft. Adjust for sun, people and kitchens.

144 sq ft falls in ENERGY STAR's 100 to 150 sq ft row, so 5,000 BTU. Add 10 percent if it is very sunny and 600 BTU for each person over two.

No. An oversized unit cools the room fast and shuts off before it removes much moisture, so the room feels cold and clammy. Buy the smallest unit at or above your calculated BTU. Inverter units handle a little oversizing better because they slow down instead of shutting off.

Divide BTU by the CEER rating for average watts. An 8,000 BTU unit at CEER 15 averages about 533 W, about 9 cents an hour at the US average of 17.3 cents per kWh, or about $22 a month at 8 hours a day.

Usually not. Most window units above about 15,000 BTU need a 230V outlet, like the Midea 18,000 BTU and LG 23,500 BTU units here. Units up to 15,000 BTU in our database run on a standard 115V outlet.

A window unit costs a few hundred dollars and installs in minutes. A DIY mini-split costs about $1,500 to $1,900, mounts through the wall, runs quieter and works as a heat pump in winter. If you want one unit to both cool and heat a room for years, the mini-split earns back the difference.

About 450 to 550 sq ft on ENERGY STAR's chart, assuming an 8 ft ceiling and normal sun. A very sunny room adds 10 percent to the load and each regular occupant over two adds 600 BTU, so the same unit covers less floor. The 12,000 to 12,200 BTU window units in this tool (LG, Midea and GE Profile, CEER 14.7 to 15) all run on a standard 115V outlet.

Divide BTU by SEER2 for average watts. The MRCOOL DIY 12K (12,000 BTU, SEER2 17.5) averages about 686 W, about 12 cents an hour at 17.3 cents per kWh, or about $29 a month at 8 hours a day. A CEER 15 window unit of the same size averages 800 W, about $34 a month.

Add 4,000 BTU to the ENERGY STAR number for the room's floor area, because cooking adds heat. A 200 sq ft kitchen is 6,000 BTU on the chart, so it needs about 10,000 BTU before any sun or occupancy adjustment.

No. The Midea Duo portable is rated 12,000 BTU SACC at CEER 10.8, so it averages about 1,111 W, about 19 cents an hour or $47 a month at 8 hours a day and 17.3 cents per kWh. A 12,000 BTU window unit at CEER 15 averages about 800 W and $34 a month. A portable makes sense when a window unit will not fit.