A typical central air conditioner costs somewhere between $0.20 and $0.60 per hour to run, which works out to a few dollars a day and often $50–$150 a month during peak cooling season. The exact number depends on three things you can actually measure: how big the unit is, how efficient it is, and what you pay for electricity. Once you know those, the cost to run central air conditioning is just arithmetic.
This article walks through the formula, works real examples, and shows where the big savings hide. The same math that turns tonnage into an hourly cost also tells you whether a higher-efficiency unit is worth the upgrade.
The Formula Behind Your Cooling Bill
Every estimate of air conditioning running cost comes down to one chain. You convert the unit’s cooling capacity into electrical power, turn that power into energy used over time, then multiply by your rate:
- Power draw: watts ≈ BTU/hr ÷ SEER2
- Energy used: kWh = kW × hours of runtime
- Cost: dollars = kWh × your $/kWh rate
The first step is the one most people miss. A central AC’s size is rated in tons of cooling, where one ton equals 12,000 BTU/hr. But your electric meter bills you in kilowatt-hours, not BTUs. The bridge between them is the efficiency rating: SEER2 (Seasonal Energy Efficiency Ratio 2, the current standard that replaced plain SEER). SEER2 tells you how many BTUs of cooling the unit delivers per watt-hour of electricity it draws.
That gives a clean shortcut for power draw: watts ≈ BTU/hr ÷ SEER2. A higher SEER2 means fewer watts for the same cooling, which is exactly why efficiency ratings matter to your wallet.
From tonnage to dollars per hour
Worked Example: A 3-Ton Unit
Take the most common residential size: a 3-ton central air conditioner, which is 36,000 BTU/hr. Assume a baseline efficiency of SEER2 14 and a national-ballpark electricity rate of $0.16/kWh.
Step 1 — Power draw: 36,000 BTU/hr ÷ 14 ≈ 2,570 watts = 2.57 kW
Step 2 — Energy per hour: Running flat-out for one hour uses 2.57 kW × 1 hour = 2.57 kWh
Step 3 — Cost per hour: 2.57 kWh × $0.16 ≈ $0.41 per hour
So at full output this unit costs about 41 cents for every hour the compressor runs. That is the AC cost per hour figure people search for — but it is only the starting point, because a central AC almost never runs continuously.
Scaling to Daily and Monthly Cost
The compressor cycles on and off to hold your setpoint, so what matters is runtime, not wall-clock hours. On a moderately hot day a properly sized unit might run a 50% duty cycle — roughly 12 of every 24 hours. On a brutal day it can climb toward 70–80%.
Using our 3-ton example at $0.41/hour:
- Per day (12 hours of runtime): 12 × $0.41 ≈ $4.90/day
- Per month (30 days): 30 × $4.90 ≈ $147/month
On milder days where the unit only runs 6 hours, that same home might see closer to $2.50/day, or about $75 for the month. This is why two identical houses can have very different summer bills: one runs the compressor twice as long as the other. To estimate heating and cooling costs for your own home and rate, plug your numbers into the HVAC Operating Cost Calculator rather than guessing at duty cycle.
Cost Per Hour by Size and Efficiency
The table below shows approximate hourly running cost at a sample rate of $0.16/kWh, assuming the unit is running at full output. Use it to see how both size and SEER2 move the number. (These are illustrative figures; your real cost depends on your actual rate and runtime.)
| Size | BTU/hr | SEER2 13 | SEER2 16 | SEER2 20 |
|---|---|---|---|---|
| 2 ton | 24,000 | $0.30/hr | $0.24/hr | $0.19/hr |
| 3 ton | 36,000 | $0.44/hr | $0.36/hr | $0.29/hr |
| 4 ton | 48,000 | $0.59/hr | $0.48/hr | $0.38/hr |
| 5 ton | 60,000 | $0.74/hr | $0.60/hr | $0.48/hr |
Two patterns stand out. First, hourly cost scales almost linearly with tonnage — a 4-ton unit costs roughly twice what a 2-ton unit does, because it is moving twice the heat. Second, jumping from SEER2 13 to SEER2 20 cuts the hourly cost by about 35% for the same cooling. That efficiency gap is the lever behind every high-SEER2 upgrade pitch.
Approx. $/hour by size and SEER2
What Actually Changes Your Real Cost
The formula gives a clean per-hour number, but your monthly bill swings on factors that change either the watts drawn or the hours run:
- Electricity rate. This is the single biggest variable. At $0.30/kWh (parts of California and the Northeast) the same unit costs nearly double what it does at $0.12/kWh (much of the Midwest and Southeast). Always run the math on your actual rate, including any tiered or time-of-use pricing.
- Climate and runtime. A unit in Phoenix runs vastly more hours per season than one in San Diego, even if both are the same size. Runtime, driven by outdoor temperature and humidity, often matters more than the hourly rate.
- SEER2 rating. Higher efficiency directly lowers the watts per hour, as the table shows. The catch is that the rated SEER2 only holds if the system is clean and the airflow is correct.
- Thermostat setpoint. The bigger the gap between indoor and outdoor temperature, the harder and longer the unit runs. A 72°F setpoint on a 95°F day demands far more runtime than 78°F does.
- Home efficiency. Poor insulation, leaky ducts, and air infiltration all add heat that the AC must remove, stretching runtime. Sealing and insulating cut the cooling load itself.
- Oversizing and short-cycling. An oversized unit cools the air fast, satisfies the thermostat, and shuts off before it can wring out humidity. Those frequent starts are inefficient and leave the home feeling clammy, so you drop the setpoint and run it more — paying twice for bad sizing.
How to Cut Your Cooling Cost
Because the bill is watts × hours, every saving comes from pulling one of those two levers down. The most effective moves:
- Raise the setpoint. Every degree warmer shortens runtime. Going from 72°F to 76°F can meaningfully cut summer cooling cost with little comfort loss, especially paired with ceiling fans.
- Step up SEER2 on replacement. When it is time to replace, higher SEER2 lowers the watts per hour permanently. Whether the premium pays back depends on your runtime and rate — the SEER Savings Calculator does that payback math for you.
- Seal and insulate. Cutting heat gain reduces the load, so the compressor runs fewer hours for the same comfort. Duct sealing alone can recover airflow that was leaking into attics and crawlspaces.
- Keep up maintenance. A dirty filter, fouled coil, or low refrigerant charge all drag the system below its rated SEER2, quietly raising the watts you pay for. Annual service keeps it running at the efficiency you paid for.
- Right-size the system. A correctly sized unit runs longer, steadier cycles that dehumidify well and waste less energy than an oversized one that short-cycles.
Five levers that cut cooling cost
Cooling vs. Heating Running Cost
The cost to run central AC for cooling is only half your year-round HVAC spend. Heating cost runs on its own math — fuel price, efficiency, and the heat your home loses in winter — and the cheapest way to heat often differs from the cheapest way to cool. If you are weighing equipment for both seasons, our breakdown of Heat Pump vs Gas Furnace running cost compares the heating side dollar for dollar, including the break-even electricity price where a heat pump beats gas.
For cooling specifically, the takeaways are simple: know your tonnage, know your SEER2, know your rate, and watch your runtime. Those four numbers explain almost any cooling bill.
Use the Free Calculator
HVAC Operating Cost Calculator — get your exact answer in seconds.
Enter your system size, SEER2 rating, electricity rate, and estimated runtime, and the calculator returns your cooling cost per hour, day, and month. To see whether a higher-efficiency unit pays for itself, run the SEER Savings Calculator.
FAQ
How much does it cost to run central air per hour?
For a typical 3-ton (36,000 BTU/hr) unit at SEER2 14 and a $0.16/kWh rate, running at full output costs about $0.41 per hour. Smaller or higher-SEER2 units cost less; larger or less efficient ones cost more. The formula is watts ≈ BTU/hr ÷ SEER2, divided by 1,000 for kW, then multiplied by your electricity rate.
How much does AC add to my electric bill per month?
For that same 3-ton unit running about 12 hours a day at $0.16/kWh, expect roughly $120–$150 a month during peak summer. In milder weather with less runtime it may be closer to $60–$80. Your number scales with your local rate, how many hours the compressor actually runs, and the unit’s efficiency.
Is it cheaper to leave AC on all day?
Usually no. Letting the indoor temperature drift up a few degrees while you are out, then cooling back down when you return, generally uses less total energy than holding a cold setpoint all day — because runtime is what drives cost, and a warmer house loses heat to the outdoors more slowly. A programmable or smart thermostat automates this. The exception is extreme heat and humidity, where letting the house get very warm can make the recovery run long and strain a marginal system.
Does a higher SEER2 really save money?
Yes, on the running cost — a SEER2 20 unit draws roughly 30–35% fewer watts than a SEER2 13 unit for the same cooling, as the table above shows. Whether that justifies the higher purchase price depends on how many hours you run it and what you pay per kWh. Heavy cooling climates with high electricity rates pay back a high-SEER2 upgrade fastest; the SEER Savings Calculator estimates the payback for your situation.
Why is my AC bill higher than the formula suggests?
The formula assumes the unit hits its rated SEER2 and runs only as much as needed. In practice a dirty filter or coil, leaky ducts, low refrigerant, an oversized short-cycling unit, or a very low thermostat setpoint all push real cost above the textbook number. Start with maintenance and your setpoint, then check sizing and duct sealing if the bill stays high.