Air Conditioner Running Cost Calculator
Enter your unit's size and efficiency. The calculator handles duty cycle, which is why a 3-ton unit doesn't cost anywhere near what its nameplate wattage suggests.
Rates and assumptions reviewed · see every source
How this is calculated
SEER is cooling output per unit of electricity, so power draw follows directly:
watts = BTU/h capacity ÷ SEER
kWh = (watts ÷ 1000) × hours × days × duty cycle
A 36,000 BTU unit at SEER 14 draws about 2,571W while the compressor runs — but it does not run continuously, which is what duty cycle captures.
Duty cycle: why nameplate wattage misleads
Air conditioners cycle. On a mild day a correctly sized unit might run 40% of the time; in a heatwave, 85%. That range more than doubles the cost for the same equipment and thermostat setting.
| Conditions | Duty cycle | Season cost |
|---|---|---|
| Mild summer, well insulated | 40% | $181 |
| Typical summer | 60% | $272 |
| Hot climate | 75% | $340 |
| Extreme heat / undersized unit | 90% | $407 |
36,000 BTU, SEER 14, 8 h/day, 120 days, $0.1834/kWh.
Does a SEER upgrade pay for itself?
Only if you cool enough hours. The saving is proportional to runtime, so the same upgrade can be excellent in Phoenix and pointless in Portland.
| Upgrade | Season saving | Cost | Payback |
|---|---|---|---|
| SEER 10 → 14 | $129 | ~$1,500 | 12 yrs |
| SEER 14 → 18 | $72 | ~$2,000 | 28 yrs |
| SEER 14 → 18, hot climate | $215 | ~$2,000 | 9 yrs |
Practical read: upgrade SEER when you are replacing a unit anyway, not as a standalone project. Replacing a working SEER 14 unit with a SEER 20 rarely repays before the new unit needs replacing itself.
What actually cuts cooling bills
- Raise the thermostat. Each degree saves 3–5%. Free.
- Seal duct leaks. Typical homes lose 20–30% of conditioned air through ducts. Often the highest-return fix available.
- Change the filter. A clogged filter makes the unit run longer for the same cooling.
- Shade windows. Solar gain through glass is a large share of cooling load.
- Use fans. Moving air makes a room feel 4°F cooler for a fraction of the electricity.
Sizing: bigger is not better
The common instinct is to oversize. It backfires. An oversized unit cools the air quickly, hits the thermostat setpoint and shuts off before it has removed much humidity — so the room feels cold and clammy rather than comfortable. It also short-cycles, which wears the compressor and is less efficient than steady running.
Rough guidance, for a reasonably insulated room with 8-foot ceilings:
| Room size | Cooling capacity | Typical draw at SEER 14 |
|---|---|---|
| 150–250 sq ft | 6,000 BTU | 430 W |
| 250–350 sq ft | 8,000 BTU | 570 W |
| 350–450 sq ft | 10,000 BTU | 715 W |
| 450–550 sq ft | 12,000 BTU (1 ton) | 860 W |
| Whole home, 1,500 sq ft | 24,000–30,000 BTU | 1,700–2,100 W |
| Whole home, 2,500 sq ft | 36,000–42,000 BTU | 2,600–3,000 W |
Add roughly 10% for a very sunny room, 10% for a kitchen, and 600 BTU for each additional regular occupant.
Window units versus central air
A window unit is far cheaper to buy and lets you cool only the room you occupy. Central air is more efficient per unit of cooling but conditions the whole house whether or not you are in it.
The arithmetic mirrors the space heater case: cooling one bedroom overnight with a 10,000 BTU window unit costs roughly 12.5¢ an hour. Running central air to cool the whole house for the same eight hours costs perhaps 45¢ an hour. If you genuinely only need one room cool, the window unit wins comfortably despite its worse efficiency rating — because efficiency applied to cooling you do not need is wasted.
Why a heatwave costs so much more
Cooling load scales with the temperature difference between inside and out. Holding 72°F when it is 85°F outside is a 13-degree lift. Holding the same 72°F when it is 100°F is a 28-degree lift — well over twice the work, and the compressor also runs less efficiently at high outdoor temperatures.
The practical consequence: your July bill is not simply a bit higher than June's. A sustained heatwave can double cooling costs relative to a normal summer month, which is why bills so often arrive as a shock rather than a gradual climb.
Related
- Appliance running cost
- Heat pump vs furnace — a heat pump cools too
- Estimate your whole bill
Common questions
What is SEER and does a higher number pay for itself?
SEER is cooling output in BTU divided by electricity input in watt-hours across a season. Higher is more efficient. Going from SEER 13 to SEER 20 cuts running cost by about 35%, but the upgrade often costs $2,000–4,000. In a hot climate running 1,500 hours a year that can repay in 5–8 years; in a mild climate running 300 hours it may never repay. Cooling hours decide it.
Is it cheaper to leave the AC on all day or turn it off?
Turning it off is cheaper, contrary to a persistent myth. An air conditioner does not work harder to cool a hot house than it does maintaining a cool one — it simply runs longer. Heat flows into your home in proportion to the indoor-outdoor temperature difference, so letting the house warm while you are out reduces total heat gain. A programmable thermostat that cools before you return captures the saving without the discomfort.
How much does each degree cost me?
Roughly 3–5% of cooling cost per degree Fahrenheit. Moving from 72°F to 76°F typically cuts cooling costs 12–20% — often $80–150 across a summer. It is the cheapest efficiency measure available, requiring no purchase at all.
Why is my bill higher than this estimate?
Usually duty cycle. In a serious heatwave a unit can run 80–90% of the time rather than the 50–60% typical of an average day. Undersized units, poor insulation, leaky ducts and dirty filters all push runtime up. Try raising the duty cycle to see the heatwave scenario.