Start with what each one cools
The two are not rivals for the same job. A window or wall unit cools the room it sits in. A central system cools every room through ducts. To compare them fairly, count how many room units you would need to get the same rooms as cool, and compare that group with the central system.
If you only cool one or two rooms, room units will almost always run less. If you would need a unit in most rooms, the comparison gets closer, and the answer depends on the efficiency of each and the hours they run.
The numbers that decide the running cost
- Power while running. Capacity in Btu per hour divided by the efficiency rating gives watts. The calculator adds the units together.
- Hours of use. Room units run only when someone is in the room. A central system tends to run longer because it cools the whole house.
- Efficiency rating. CEER or EER for room units and SEER2 for central systems. They come from different tests, so use them as a rough guide to compare a window unit with a central system.
- Price per kWh. Use the rate on your bill. The calculator starts from the state average.
How the number of units changes the power
Power adds up with each unit. The table shows the electric power of window units at a rating of 11 and the cost of 120 days at 6 hours a day, against one 36,000 Btu/h central system at a rating of 16 running 8 hours a day, at the U.S. average price of 16.5 cents per kWh (2024). The sizes, ratings and hours are assumed for this illustration.
| System | Power while running | Cost per 120 days |
|---|---|---|
| 1 window unit (8,000 Btu/h each) | 0.73 kW | $86 |
| 2 window units (8,000 Btu/h each) | 1.45 kW | $173 |
| 3 window units (8,000 Btu/h each) | 2.18 kW | $259 |
| 4 window units (8,000 Btu/h each) | 2.91 kW | $345 |
| 1 central system (36,000 Btu/h, rating 16, 8 hours) | 2.25 kW | $356 |
Look at the price as well
Running cost is only half of the choice. Suppose three window units cost $1,800 installed and a central system costs $7,500 (assumed prices). In the example above the window units cost $259 to run and the central system $356, so the window units save $97 a season. The extra $5,700 is not paid back by running costs, because the dearer system does not cost less to run. Equipment life, repairs, comfort and noise are outside this comparison.
What the calculator does not tell you
- Whether the room units are the right size for each room. Use the AC BTU calculator.
- How well ducts are sealed. Leaky ducts waste cooling and are not in the rating.
- Taxes, fixed charges and demand charges on your bill.
- Rebates and tax credits.