Figures for estimating and for checking a quote or a generator. The nameplate and the manufacturer's electrical data come first. Breakers and wire are sized from the nameplate, and a licensed electrician does the work.

Maintained by BuildSolver · Updated October 16, 2026

Calculator · HVAC circuits

AC amps and watts calculator

An air conditioner's EER2 rating already holds its power draw. A 36,000 BTU/h unit at 11.7 EER2, the federal Southwest minimum for a split AC of that size rated under 15.2 SEER2, uses 3,077 W at the 95°F rating point. At a power factor of 1 that is 12.8 A at 240 V and 14.8 A at 208 V, and a real motor's power factor below 1 raises it. The page also turns a current reading into watts and the nameplate LRA into starting kVA, for checking a generator. It does not size the circuit. For that, use the AC breaker size calculator and the AC wire size calculator, which work from the nameplate MCA and MOP.

Look up the unit's AHRI certificate for the cooling capacity and the EER2 from the same line, or read the LRA off the nameplate.
Choose the voltage and phase the unit runs on, and enter a power factor if the manufacturer or a power meter gives one. For starting kVA, choose the voltage the nameplate gives the LRA at.
The result shows the running watts and the current they mean, the watts behind a current reading, or the starting kVA, which is the voltage times the LRA divided by 1,000, with the voltage times 1.732 on three phase.

Running watts at the EER2 rating point, watts from amps and starting kVA. No power factor is guessed. The nameplate governs, and the circuit is sized from it.

What to work out

Voltage and phase

The nameplate prints the phase as 1 Ph or 3 Ph. For a 208/230 V unit, pick the voltage the building really has for running watts and amps, since the current at 208 V is higher for the same watts. For starting kVA, pick the voltage the nameplate gives the LRA at.

Rated cooling capacity, BTU/h

The capacity printed next to the EER2 on the unit's AHRI certificate, so the two come from the same test. A 3 ton unit is 36,000 BTU/h only by name.

EER2

EER2, not SEER2. SEER2 is a season average and usually gives too few watts for a hot afternoon. 11.7 is the example here, the federal Southwest minimum for a split AC under 45,000 BTU/h rated under 15.2 SEER2. A rating under the newer M2 test is called EER and goes here the same way.

Power factor (optional)

A fraction such as 0.85, not 85. From the manufacturer's data or a power meter. Leave it blank if you do not know it, and the page shows the bound it can prove instead of guessing.

Example result for the default inputs

36,000 BTU/h at 11.7 EER2, 240 V single-phase. These are the page example values, 11.7 EER2 being the federal Southwest minimum for a split AC under 45,000 BTU/h rated under 15.2 SEER2. Replace them with the capacity and EER2 on the unit's AHRI certificate.

Running power at 95°F outdoors

3,077 W

Current at 240 V, power factor 1

12.8 A

Calculation details
Power	36,000 BTU/h / 11.7 EER2 = 3,077 W, or 3.08 kW
Current	3,077 W / 240 V = 12.8 A at a power factor of 1. A motor load usually has a power factor below 1, which raises the current. Enter a power factor from the manufacturer's data or a power meter to work it.
Against a clamp meter	The watts are the whole system at the rating point, indoor fan included. A clamp on the outdoor unit reads less when the indoor fan runs on the furnace circuit, on a cooler day or with an inverter below full speed, and more on a hotter day.
Energy	3.08 kWh for each hour at full load at the 95°F rating point. The SEER2 energy cost calculator works out a whole season.
Breaker and wire	Not from these numbers. The nameplate MCA sets the wire and the nameplate maximum breaker (MOP) sets the breaker. The AC breaker size and AC wire size calculators on this site work from those.

For estimation purposes only. Consult a licensed engineer for final designs.

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How the watts and amps are worked

An air conditioner's efficiency rating already holds its power draw. EER2 is the cooling delivered per watt of electric input, measured at 95°F outdoors, so dividing the rated capacity by the EER2 gives the watts at that point. Turning watts into amps needs the voltage, the phase and the power factor.

Running power: W = rated cooling capacity (BTU/h) / EER2EER2 is in BTU per watt-hour and comes from the federal A test, 95°F outdoors with 80°F dry bulb and 67°F wet bulb air entering the indoor coil (10 CFR 430 Appendix M1). 36,000 BTU/h at 11.7 EER2 is 3,077 W. The figure covers the whole system at that point, indoor fan included.
Current, single phase: A = W / (V x PF)With no power factor entered the page divides by the voltage alone, which gives the current at a power factor of 1, 12.8 A at 240 V for the example. At a power factor of 0.9 it would be 14.2 A. It is the whole system with the indoor fan, so a clamp on the outdoor unit alone can read less.
Current, three phase: A = W / (1.732 x V x PF)V is the line-to-line voltage and A the average of the three legs, as in DOE's motor fact sheet. The same watts on three phase draw less current per leg.
Watts from amps: W = V x A x PF, or 1.732 x V x A x PF on three phaseVolts times amps alone is the apparent power in VA. It equals the watts only at a power factor of 1, so without a power factor the page gives it as an upper bound. DOE's appliance example, 120 V x 3 A, is 360 VA.
Starting kVA: kVA = V x LRA / 1,000, or 1.732 x V x LRA / 1,000 on three phaseLRA is the locked-rotor current on the nameplate, the draw in the first moment of a start. An LRA of 77 A at 240 V, an illustration and not the figure of any model, is 18.48 kVA.

Running watts at the federal minimum EER2

The EER2 values below are the federal Southwest minimums for split-system air conditioners, used as examples. A split AC installed there must rate at least the figure for its size and SEER2, and its own EER2 is on its AHRI certificate. The current is worked at a power factor of 1, and a real motor's power factor below 1 raises it. It is the whole system with the indoor fan.

Running watts and the current at a power factor of 1, at 240 V and 208 V, for six cases
Case	EER2	Watts	At 240 V, PF 1, A	At 208 V, PF 1, A
24,000 BTU/h, SEER2 under 15.2	11.7	2,051	8.5	9.9
36,000 BTU/h, SEER2 under 15.2	11.7	3,077	12.8	14.8
36,000 BTU/h, SEER2 15.2 or more	9.8	3,673	15.3	17.7
48,000 BTU/h, SEER2 under 15.2	11.2	4,286	17.9	20.6
60,000 BTU/h, SEER2 under 15.2	11.2	5,357	22.3	25.8
60,000 BTU/h, SEER2 15.2 or more	9.8	6,122	25.5	29.4

The lower EER2 rows are the minimum for units rated 15.2 SEER2 or more, which are allowed a lower EER2, so a higher seasonal rating does not by itself mean fewer watts on a hot afternoon. For a start, an LRA of 77 A at 240 V, an illustration and not the figure of any model, is 18.48 kVA. On three phase the voltage is multiplied by 1.732, so a 10 A reading at 460 V is 7,967 VA.

Standards and data sources

10 CFR 430.23(m) and 10 CFR 430 Subpart B, Appendix M1 - EER2 as the efficiency from the A test, in BTU per watt-hour, with the capacity from the same test. The A test runs at 95°F outdoors and 80°F dry bulb, 67°F wet bulb indoors (Table 5 of Appendix M1 as published before the January 2025 amendment, which moved the conditions into AHRI 210/240-2024).
10 CFR 430.2, definition of a central air conditioner - single-phase, air-cooled and rated below 65,000 BTU/h, the residential equipment EER2 applies to.
10 CFR 431.97(i), Table 19, and Appendix F1 to 10 CFR 431 Subpart F - three-phase air-cooled commercial package air conditioners and heat pumps below 65,000 BTU/h, tested to AHRI 210/240-2023, with minimums in SEER2 from January 1, 2025. Single package vertical units have EER minimums under 431.97.
10 CFR 430.32(c)(6), regional standards - the Southwest EER2 minimums for split-system air conditioners, 11.7 under 45,000 BTU/h, 11.2 at 45,000 BTU/h and up, and 9.8 for either size at a SEER2 of 15.2 or more. Used as examples, not as typical ratings.
ENERGY STAR Certified Air-Source Heat Pumps, data.energystar.gov - the spread of certified EER2 and SEER2 below 65,000 BTU/h, read on October 12, 2026: EER2 from 7.3 to 21.45 with a median of 12.0, SEER2 from 15.2 to 35.1. Above 14.5 almost only mini and multi splits rate, so a higher EER2 gets a note for ducted units, and one above 16 a warning that it may be a SEER2.
U.S. Department of Energy, Determining Electric Motor Load and Efficiency - the three-phase input power equation, kW = V x I x PF x 1.732 / 1,000 with line-to-line voltage, and its worked example of 469.7 V, 37 A and a power factor of 0.763.
U.S. Department of Energy, Energy Saver, Estimating Appliance and Home Electronic Energy Use - the volts times amps estimate of an appliance's wattage, 120 V x 3 A = 360 W, which this page labels as apparent power. DOE's live page address returned an error on October 12, 2026, so the text was read from an archived copy of July 2026.

Limitations of this calculator

The nameplate and the manufacturer's data come first. The figures here are worked from a rating or a reading for estimating, and they do not replace the unit's published electrical data.
Running watts are at the rating point, 95°F outdoors with 80°F air at 67°F wet bulb entering the indoor coil. On a hotter day the unit draws more, and with a dirty coil or the wrong charge the draw changes too. A power meter or a clamp meter on the running unit tells you what it really uses.
Capacity and EER2 must come from the same line of the same rating, usually the AHRI certificate for the outdoor unit and coil or air handler together. A nominal 3 ton label is not the certified capacity.
The indoor fan is part of EER2. When the fan sits in a furnace on its own circuit, part of these watts is on that circuit, and the current shown is worked as if everything ran on the one voltage you picked.
EER2 under 10 CFR Part 430 is the federal rating for single-phase air conditioners and heat pumps below 65,000 BTU/h. A three-phase unit below that size is commercial equipment under Part 431, and its own efficiency at 95°F, EER2 or EER, goes in the same way. This page stops below 65,000 BTU/h. When the federal test moves from Appendix M1 to Appendix M2, the rating is called EER and is used here the same way.
Variable-speed and inverter units are rated at full speed for EER2 and can run below it much of the time. Some print no LRA, and without one the page cannot work the start. Ask the manufacturer for the starting current.
Heat pump heating is not covered. EER2 is a cooling rating, and the heating draw depends on the outdoor temperature in the manufacturer's data.
The power factor is your input. The page has no typical figure for a compressor and does not invent one. Without it, the current from watts is worked at a power factor of 1, and a real one below 1 raises it, while the watts from amps are an upper bound.
No code sets a generator size for an air conditioner. The page gives the running watts and the starting kVA to compare with a generator's own ratings. A licensed electrician installs the generator connection and the transfer equipment.
Breakers and wire are not sized from watts. The nameplate MCA and maximum breaker set them, under NEC 110.3(B) and Article 440, and the work is done by a licensed electrician under the local authority having jurisdiction.

Common mistakes with AC watts and amps

Putting SEER2 in place of EER2. SEER2 is a season average over outdoor temperatures from 67°F to 102°F, most of them cooler than the 95°F rating point. Dividing by it usually gives too few watts for the hot afternoon a generator or a meter check is about.
Reading volts times amps as watts. Volts times amps is the apparent power. With a power factor below 1 the real watts are lower. DOE's 120 V x 3 A = 360 W is an estimate of that kind, fine for a resistance heater and an upper bound for a motor.
Taking the MCA as the running current. MCA is the sum of the loads plus 25 percent of the highest one, usually the compressor, a figure for sizing the wire. RLA is a rating as well. Neither is a reading of what the unit draws.
Forgetting the start. A generator that carries the running watts can still stall on the start. The example LRA of 77 A at 240 V is 18.48 kVA for the first moment of the start, and the generator has to carry that on top of everything else running.
Picking single phase for a three-phase unit. Three phase multiplies by 1.732. A 10 A reading on 460 V three phase is 7,967 VA, and treating it as single phase would understate it by about 42 percent.
Using 240 V when the building has 208 V. The same 3,077 W draws 14.8 A at 208 V against 12.8 A at 240 V, both at a power factor of 1.

Common questions about AC watts and amps

How many watts does a 3 ton AC use?

Divide the rated cooling capacity by the EER2 on the unit's AHRI certificate. A 36,000 BTU/h air conditioner at 11.7 EER2, the federal Southwest minimum for a split AC under 45,000 BTU/h rated under 15.2 SEER2, uses 3,077 W at the 95°F rating point. At 9.8 EER2, the minimum for a unit rated 15.2 SEER2 or more, it is 3,673 W. A unit with a higher EER2 uses fewer watts for the same cooling.

How many amps does a 3 ton AC draw?

Divide the watts by the voltage and the power factor. The example 3,077 W at 240 V is 12.8 A at a power factor of 1, and 14.2 A at 0.9. At 208 V it is 14.8 A at a power factor of 1. Those watts are the whole system with the indoor fan, so a clamp on the outdoor unit reads less when the fan runs on the furnace circuit. The nameplate RLA and MCA are ratings for the circuit, not a reading. A clamp meter on the running unit gives the real current.

How many amps does an AC pull on startup?

The locked-rotor amps, LRA, printed on the nameplate next to the compressor. It lasts for the first moment of the start. As kVA it is volts times LRA divided by 1,000, so an LRA of 77 A at 240 V, an illustration and not the figure of any model, is 18.48 kVA. Work it at the voltage the LRA is rated at, since the plate gives no LRA for another supply voltage.

What size generator do I need for a 3 ton AC?

No code sets one. Work the running watts and the starting kVA for the unit, add everything else the generator will carry, and compare them with the generator's running and starting ratings in its spec sheet. A soft starter lowers the starting current by an amount its maker publishes. A licensed electrician connects the generator through transfer equipment.

Why are watts not just volts times amps?

Volts times amps is the apparent power, in VA. A motor load draws part of its current without doing work, and the power factor, a fraction of 1 or less, says how much of the VA is real watts. This page uses volts times amps as an upper bound unless you enter a power factor.

Should I use SEER2 or EER2 to find the watts?

EER2. It is measured at one point, 95°F outdoors, which is close to a hot afternoon, and on a hotter day the unit draws more still. SEER2 averages a season with most hours cooler, so it usually gives too few watts for a hot afternoon. For yearly energy and cost, SEER2 is the right number, and the SEER2 energy cost calculator works it.

How do I work watts on three phase?

Multiply the line-to-line voltage by the current and by 1.732, then by the power factor. DOE's motor fact sheet works 469.7 V, 37 A and a power factor of 0.763 to 22.9 kW this way. A 10 A reading on 460 V three phase is 7,967 VA.

Can I size the breaker or the wire from these watts?

No. The nameplate MCA sets the wire and the nameplate maximum breaker sets the breaker. Use the AC breaker size calculator and the AC wire size calculator, which work from those numbers. A licensed electrician installs the circuit.

Used on the same job

Get the house load in chat

The chat does not work out watts, amps or generators. It runs the free whole-house Manual J load, which tells you how much cooling the house needs before anyone compares units and their EER2.

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