AC Wattage Calculator
This calculator finds AC electrical power in two ways: enter voltage, current, and power factor to get watts from the electrical formula, or enter your air-conditioner's BTU capacity and EER efficiency rating to get its running wattage and electricity cost. Switch between single-phase and three-phase for the electrical method, or choose the BTU/EER tab for air-conditioner sizing. All results update instantly as you type.
Formula
Worked example
A 12,000 BTU/hr window AC with an EER of 10 uses 12,000 / 10 = 1,200 W. Running 8 hours a day at $0.13/kWh costs 1.2 kW x 8 hr x $0.13 = $1.25 per day, $37.44 per month. For the electrical method: a single-phase 240 V circuit drawing 6 A at PF 0.95 gives 0.95 x 240 x 6 = 1,368 W.
Two ways to calculate AC wattage
There are two practical situations where you need to know AC wattage. The first is when you have an air-conditioner and want to know its running power and electricity cost: you divide the unit's BTU/hr cooling capacity by its EER (Energy Efficiency Ratio) to get watts. The second is when you have an electrical measurement and need the real power of a circuit: you multiply voltage by current and then by the power factor. This calculator handles both with a single mode switch, so you can use whichever inputs you actually have available.
The BTU and EER method
Air-conditioner capacity is rated in BTU per hour (or tons, where 1 ton = 12,000 BTU/hr). The EER tells you how many BTUs of cooling you get per watt of electricity. Dividing capacity by EER gives running watts: a 12,000 BTU unit at EER 10 uses 1,200 W. A higher EER means less power for the same cooling, so a unit with EER 14 doing the same job would only draw 857 W, saving nearly 30% on running costs. SEER (Seasonal Energy Efficiency Ratio) is the annual average version of EER; EER is the instantaneous value used in this calculator. Typical EER values range from 8 for older window units to 20+ for modern inverter mini-splits.
The electrical formula: single-phase and three-phase
For direct electrical measurement, real power in watts is the product of voltage (V), current (A), and power factor (PF). In a single-phase AC circuit: Watts = PF x V x I. In a balanced three-phase circuit using line-to-line voltage: Watts = sqrt(3) x PF x V x I, where sqrt(3) is approximately 1.7321. Using line-to-neutral voltage instead: Watts = 3 x PF x V x I. The power factor accounts for the phase shift between voltage and current waveforms, which is common in inductive loads like AC compressor motors (typical PF 0.85-0.97). A power factor of 1.0 means purely resistive load; real motors are always below 1.
Understanding your electricity cost estimate
The running-cost outputs assume a fixed electricity rate and steady run time every day. In practice, an AC cycles on and off, so it only draws rated watts while the compressor is running, not all of the time it is switched on. If your unit runs on a 50% duty cycle (compressor on half the time), your actual consumption is roughly half of what a 100% duty factor would suggest. The estimates here are therefore a ceiling for a given run-time figure. The US national average residential electricity rate used as the default is approximately $0.13 per kWh based on EIA 2024 data; your local rate may be higher or lower.
Typical air-conditioner wattage by BTU and type
| Type | BTU/hr | Running watts (approx.) | Tons of cooling |
|---|---|---|---|
| Window | 5,000 | 385-550 | 0.42 |
| Window | 8,000 | 640-950 | 0.67 |
| Window | 10,000 | 800-1,170 | 0.83 |
| Window | 12,000 | 1,000-1,200 | 1.00 |
| Window | 15,000 | 1,360-1,875 | 1.25 |
| Portable | 8,000 | 940-1,200 | 0.67 |
| Portable | 10,000 | 1,180-1,450 | 0.83 |
| Portable | 12,000 | 1,410-1,650 | 1.00 |
| Mini-split | 9,000 | 750-900 | 0.75 |
| Mini-split | 12,000 | 1,000-1,200 | 1.00 |
| Mini-split | 24,000 | 2,000-2,400 | 2.00 |
| Central | 24,000 (2 ton) | 1,520-1,960 | 2.00 |
| Central | 36,000 (3 ton) | 2,300-2,950 | 3.00 |
| Central | 48,000 (4 ton) | 3,050-3,900 | 4.00 |
| Central | 60,000 (5 ton) | 3,800-4,900 | 5.00 |
Values are approximate running watts at EER 10 (window/portable) and EER 12 (mini-split). Startup watts are typically 2-3x running watts.
Frequently asked questions
How do I find the BTU rating and EER of my air conditioner?
Both values appear on a yellow EnergyGuide label attached to the unit, and on the specification sticker on the rear or side panel. The BTU/hr rating is the cooling capacity and is usually a round number like 5,000, 8,000, 10,000, or 12,000. The EER is the energy efficiency ratio and is typically between 8 and 18 for window units. If you cannot find the label, the model number on the front panel can be looked up on the manufacturer's website or the AHRI (Air-Conditioning, Heating, and Refrigeration Institute) directory.
What is the difference between EER and SEER?
EER (Energy Efficiency Ratio) is the ratio of cooling output (BTU/hr) to power input (watts) at a single, fixed operating condition: 95 degrees Fahrenheit outdoor, 80 degrees indoor, and 50% relative humidity. SEER (Seasonal Energy Efficiency Ratio) averages performance across a full cooling season with varying temperatures and is therefore lower than EER for the same unit. For this wattage calculator, use EER if you have it; if you only have SEER, a rough conversion is EER is approximately 0.875 times SEER.
How many watts does a 1-ton, 2-ton, or 3-ton central AC use?
One ton of cooling equals 12,000 BTU/hr. At a typical EER of 10, a 1-ton unit uses about 1,200 W, a 2-ton uses about 2,400 W, and a 3-ton uses about 3,600 W. Modern high-efficiency central systems with EER 14+ can reduce these figures by 30% or more. Enter your specific BTU/hr and EER in this calculator for an accurate result.
What is power factor and why does it matter for AC wattage?
Power factor is the ratio of real power (watts, which does actual work) to apparent power (volt-amperes, which is what the utility delivers). AC compressor motors are inductive, meaning voltage and current are not perfectly in phase; a typical compressor motor has a power factor of 0.85 to 0.97. If you ignore power factor and multiply volts by amps only, you get apparent power in VA, which overstates actual wattage. The real power in watts is VA times the power factor. For home energy calculations, most manufacturers already factor this in when stating running wattage on the nameplate.
Why does the three-phase formula use sqrt(3) for line-to-line voltage but 3 for line-to-neutral?
In a balanced three-phase system the line-to-line voltage is sqrt(3) times the line-to-neutral voltage. The total real power is the sum of three single-phase legs, each contributing PF x V_LN x I. Adding three identical legs gives 3 x PF x V_LN x I. Substituting V_LN = V_LL / sqrt(3) into that yields sqrt(3) x PF x V_LL x I. Both formulas are algebraically identical; which one you use depends on which voltage you can measure more easily.
How can I reduce my air-conditioner electricity bill?
The biggest levers are raising the thermostat setpoint (each degree Fahrenheit reduces compressor run time by roughly 2-3%), improving insulation and sealing air leaks, keeping the condenser coils clean (dirty coils raise wattage by 5-15%), and replacing an old low-EER unit with a high-efficiency one when it is due for renewal. Using a programmable or smart thermostat to reduce run time during unoccupied hours also makes a large difference over a full cooling season.