Window AC vs Central AC: Yearly Cooling Cost Calculator

How the $85.68 and $367.20 are worked out

Both costs come from the same three numbers: watts, hours, and the electricity rate. A window unit rated at 8,000 BTU with an energy efficiency ratio (EER) of 11.4 draws 8,000 ÷ 11.4, or about 702 watts — the model rounds that to 700 watts, or 0.7 kW. Run it 8 hours a day for 90 days and it runs 720 hours a year: 0.7 × 720 is 504 kWh, and 504 × $0.17 is $85.68 a year. A 3-ton central system draws about 3,000 watts, or 3.0 kW, and running the same 720 hours it uses 3.0 × 720, or 2,160 kWh, and 2,160 × $0.17 is $367.20 a year. The difference is $367.20 − $85.68, or $281.52 a year to the window unit — for one room. The central system is not wasteful; it is cooling the whole house while the window unit cools one room, which is why the room count matters more than the wattage.

The branches: hours, days, rate, and rooms

4 hours a day: halve the hours to 360 and both costs halve — the window unit uses 0.7 × 360, or 252 kWh, costing $42.84, and central uses 3.0 × 360, or 1,080 kWh, costing $183.60. The window unit saves $140.76. 120 days a year: a four-month summer is 960 hours — the window unit uses 672 kWh and costs $114.24, central uses 2,880 kWh and costs $489.60, and the window unit saves $375.36. $0.30 per kWh: the kilowatt-hours do not change, only the price — the window unit costs 504 × $0.30, or $151.20, central costs 2,160 × $0.30, or $648.00, and the window unit saves $496.80. 2 rooms: two window units double the window side to 1,008 kWh and $171.36, while central stays at $367.20 — the window units still save $195.84.

The honest flip: five rooms

Window units win one room at a time, but each extra room adds a whole second air conditioner. At 5 rooms the window side is 5 × 504, or 2,520 kWh, costing $428.40 a year — more than central's $367.20. Central saves $61.20 a year. The exact flip point is where the two costs meet: central's 3,000 watts divided by the window unit's 700 watts is 4.29 rooms, so central is cheaper from 5 rooms up and the window unit is cheaper from 4 rooms down. And the energy bill is only half the story: a window unit costs about $250 to buy, while a central system costs $5,000–7,500 installed. On energy alone, a central system cooling one room never pays for itself — the $281.52 a year it would save by not existing is not a saving at all, because the window unit is the cheaper purchase too. Central is a whole-house and comfort decision: even temperatures in every room, no unit blocking a window, quieter bedrooms, and filtered air. Buy it for that, not for the electricity bill.

Rounding convention, stated plainly: each kilowatt-hour figure is rounded to two decimals, each yearly cost is the rounded kilowatt-hours times the rate, rounded to cents, and the saving is the difference of the rounded yearly costs — so $367.20 minus $85.68 is exactly $281.52. The flip boundary is 3,000 ÷ 700, or 4.29 rooms.

Output · formula · meaning

Who uses this

• The renter cooling one bedroom: runs a window unit 8 hours a day for 90 days and learns it costs $85.68 a year against $367.20 for central — a saving of $281.52, and the unit itself costs about $250, not $5,000–7,500.
• The homeowner cooling five rooms: enters 5 rooms and learns the window units would cost $428.40 a year against $367.20 for central — central saves $61.20, and the flip boundary of 4.29 rooms says central is the right call.
• The long-summer household: cools for 120 days and learns the gap widens to $114.24 against $489.60, a saving of $375.36 for the window unit — the longer the summer, the more the single-room choice matters.
• The high-rate household: pays $0.30 per kWh and learns the gap widens to $151.20 against $648.00, a saving of $496.80 — the dearer electricity gets, the more a window unit earns its keep in one room.

Accuracy & limitations

This is a rated-watts model, not a meter reading. The default wattages are typical values — an 8,000 BTU window unit at EER 11.4 draws about 702 watts, rounded to 700 here, and a 3-ton central system draws about 3,000 watts — but a real unit cycles on and off once the room reaches temperature, and a real central system with a variable-speed compressor draws less at part load, so the model charges full power for every hour. The 8-hours-a-day, 90-days-a-year defaults describe a three-month summer; a mild summer or a household that runs the cooling overnight changes the hours. The model also assumes the window unit and the central system cool to the same temperature, which is rarely true in practice — central keeps the whole house even, while a window unit leaves the hallway and the far rooms warm. It excludes the purchase and installation costs ($250 for a window unit, $5,000–7,500 for central), maintenance, and the value of comfort, quiet, and filtered air. Enter your own appliances' watts from their labels and your own rate from your electricity bill for the most accurate result.