Window air conditioners and central AC systems both cost roughly the same per BTU delivered, but the right choice depends on apartment-vs-house, number of rooms cooled simultaneously, and how often you actually run the system. A 8,000 BTU window unit costs $0.08 per hour to run. A 36,000 BTU central system costs $0.32 per hour. Per BTU the math is essentially identical. Here is the apartment-vs-home break-even and the SEER rating math.
Per-BTU cost is essentially identical
A window AC unit at SEER 14 uses roughly 600W to deliver 8,000 BTU per hour. That is 0.6 kWh per hour, or $0.10 at 16 cents per kWh.
A central AC unit at SEER 14 uses roughly 3,000W to deliver 36,000 BTU per hour. That is 3.0 kWh per hour, or $0.48 per hour. Per-BTU delivered the math is essentially identical: roughly $0.0000125 per BTU-hour.
Apartment vs home break-even
Apartments and small homes (under 1,200 sq ft) typically only need to cool 1 to 3 rooms simultaneously. A 12,000 BTU window unit handles one room well; a portable AC handles two adjacent rooms.
Larger homes need to cool 4+ rooms simultaneously. Stacking 4 window units pulls more total wattage than a single right-sized central AC and is dramatically less convenient. Central AC wins at 4+ rooms; window AC wins at 1 to 3.
SEER rating compounds the math
SEER (Seasonal Energy Efficiency Ratio) measures cooling output per watt of input across a typical cooling season. SEER 14 is the federal minimum. SEER 22 is the top of the ENERGY STAR range.
Each step up in SEER cuts kWh per BTU by roughly 6 to 9 percent. A SEER 22 central AC uses 36 percent less electricity than a SEER 14 unit. Payback on the SEER upgrade runs 5 to 9 years depending on climate zone.
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