Estimate generator watts for an air conditioner from its documented electrical running input and starting requirement, then add the other loads that will operate with it. Cooling capacity and seasonal efficiency can provide a rough screen, but they cannot establish the exact generator size for a particular unit.
Open the Generator Size Calculator for Air Conditioner.
Cooling capacity is not electrical input
An air conditioner’s Btu/h rating describes how quickly it removes heat under stated conditions. It does not say how many electrical watts the generator must supply. Two units with the same cooling capacity can draw different input power and impose different starting demands.
For generator planning, collect the exact model, supply voltage, phase, electrical input specifications, and manufacturer generator guidance. A central system may also have an indoor fan or air handler on a separate circuit. Check whether the outdoor-unit figure includes that load before adding it.
A room air conditioner may show an electrical input in watts directly. Other equipment may list operating current, minimum circuit ampacity, maximum protective-device ratings, and compressor locked-rotor current. Each of those quantities has a specific purpose; the largest breaker number is not a measurement of running watts.
If you are comparing existing equipment, take a clear photograph of the accessible rating plate and find the corresponding manual. Similar cabinet sizes or matching nominal tonnage do not guarantee matching electrical demand.
Why dividing Btu/h by SEER is only a rough screen
SEER and SEER2 describe seasonal cooling performance across a defined test procedure. They relate cooling delivered over a season to electrical energy used over that season. ENERGY STAR distinguishes seasonal SEER2 from operating-point EER2 in its central-air-conditioner specification.
The embedded calculator divides the entered Btu/h capacity by SEER to create a running-watt estimate. Although the units resemble a watts calculation, a seasonal efficiency ratio does not establish input power at the hot-weather condition when you need backup cooling.
For an illustrative 12,000 Btu/h unit with an entered SEER of 15, the tool begins with 12,000 ÷ 15 = 800 W. That is a screening assumption. It is not a substitute for a documented electrical input of, for example, 1,100 W at a specified condition.
If an applicable EER value is available at the same operating point as a known cooling capacity, capacity divided by EER can help estimate input at that point. Still confirm the rating boundary, fan inclusion, and actual equipment configuration. Do not silently enter an EER number into a field labeled SEER and then describe the result as a verified specification.
Determine the running load first
Use manufacturer input watts when they are available and applicable. For a suitable measurement, record real power during relevant operation rather than the average over hours that include compressor-off periods.
If the available information is only voltage and current, calculate apparent power separately. At 120 V and 10 A, apparent input is 1,200 VA. Real watts equal voltage × current × power factor, so they cannot be identified exactly without power-factor or real-power information.
For a central split system, make an explicit list of outdoor unit, indoor blower, controls, and any accessories supported by the backup circuit. Some documented system inputs already include multiple components. Add only loads outside that boundary.
A variable-speed compressor also deserves a range. Its electrical demand can change as the equipment changes speed and responds to indoor and outdoor conditions. An efficient low-speed operating point may be useful for energy analysis while being inadequate for the maximum-load comparison.
Obtain starting data rather than assuming every compressor is alike
Ask for manufacturer starting current, starting VA, total starting watts, or a generator recommendation. Record the unit with the number. Locked-rotor amperes multiplied by voltage give apparent demand under that condition, not automatically real starting watts.
A compressor’s initial demand and its acceleration period also depend on the actual system and starting arrangement. A soft starter or inverter drive may change that behavior, but the improvement should come from compatible equipment documentation or suitable verification. A broad category called “low surge” does not prove that a selected accessory will produce a particular reduction.
If a measurement is required, have a qualified person use equipment suited to capturing the event. Fluke explains the importance of an appropriate inrush function in its measurement guide. Ordinary running-current observations can miss the brief period you are trying to characterize.
Starting apparent power and real power can both matter. Keep them in separate columns for the generator supplier rather than collapsing every electrical requirement into one watts figure.
An illustrative room-cooling calculation
Suppose an air conditioner has documented running input of 1,100 W and an illustrative total starting demand of 2,600 W. Other loads are 180 W for refrigeration, 60 W for lights, and 40 W for communications, totaling 280 W.
The steady running load is 1,100 + 280 = 1,380 W. During the air-conditioner start, replace its running input with its total starting input: 2,600 + 280 = 2,880 W. Equivalently, add its 1,500 W additional startup requirement to the 1,380 W running total.
| Check | Calculation | Result |
|---|---|---|
| All loads running | 1,100 + 280 | 1,380 W |
| AC additional start | 2,600 − 1,100 | 1,500 W |
| AC starting with background loads | 1,380 + 1,500 | 2,880 W |
| Running comparison plus 20% | 1,380 × 1.20 | 1,656 W |
| Starting comparison plus 20% | 2,880 × 1.20 | 3,456 W |
These figures are original planning examples, not representative manufacturer specifications. They show how to avoid adding both 1,100 W running and the full 2,600 W start as if they were independent loads.
If the refrigerator also starts at the same moment, its additional startup requirement must be included in that overlap scenario. A small running background load can still contain a consequential motor start.
Compare that example with the calculator output
Enter 12,000 Btu/h, SEER 15, a 2× startup factor, 280 W other load, and a 20% margin. The calculator returns [(12,000 ÷ 15) × 2 + 280] × 1.20 ÷ 1,000 = 2.256 kW, displayed to two decimals as 2.26 kW.
That is materially below the example’s documented-demand comparison of 3.456 kW. The difference comes from two assumptions: the tool’s 800 W running estimate and the selected startup multiplier. Increasing a generic margin does not establish the correct values for either assumption.
Use the tool to explore how capacity, assumed efficiency, and other loads influence a preliminary range. When manufacturer watts and starting figures are available, calculate from them directly outside the tool. Its current input form does not accept verified running watts as a replacement for the Btu/SEER estimate.
The result is in kW. Multiply by 1,000 if comparing it with a generator specified in watts, and keep the continuous and short-duration generator ratings distinct.
Check supply voltage and the rest of the system
A central air conditioner that needs a 240 V supply cannot be assessed by total watts alone. Confirm the generator’s voltage, phase, connection arrangement, continuous current capability, and appropriate transfer equipment for fixed wiring.
For a plug-connected room unit, confirm the approved connection and cord requirements in both manuals. A long cable run can reduce voltage at the appliance, so the generator’s output terminal voltage does not describe the entire supply path.
Outdoor generator placement belongs in the plan. CPSC directs portable generators to be used outdoors at least 20 feet from homes with exhaust directed away, as stated in its carbon-monoxide guidance. Do not move the engine indoors to shorten a cable run.
Also check the generator’s rating on the actual fuel and under expected site conditions. A model’s largest advertised number may describe a short-duration capability or a different fuel configuration from the one you intend to use.
Questions to settle before buying
Can I size from the thermostat’s energy report?
Use it for context about energy use, but do not assume its time-averaged estimate captures compressor starting demand. Ask how the report measures or estimates consumption and obtain electrical equipment data separately.
Will a soft starter always let me use a smaller generator?
A compatible device may change the starting event, but generator selection needs the resulting verified demand and recovery behavior. It also leaves the continuous running load in place. Compare a specific approved combination rather than applying an assumed universal percentage reduction.
Which number should a supplier receive?
Send the exact equipment model, running watts or current and power factor, starting data with units, voltage and phase, separate fan loads, other simultaneous appliances, and any planned starting accessory. Ask for a review of the loaded starting event and permitted voltage and frequency response.
Compare a cooling-only and a heating-capable unit
Some equipment that looks like a room air conditioner also includes a heating mode. The electrical demand for that mode can differ from the cooling scenario, especially if resistance heat is present. A generator comparison based on summer cooling does not automatically cover winter heating.
Record the exact mode being supported and any optional heaters or accessories. If the backup plan allows users to select either mode, calculate both using their applicable electrical inputs. Keep the limiting scenario with the operating instructions so a thermostat or mode change does not silently invalidate the load schedule.
References
- ENERGY STAR: SEER2 and EER2 definitions.
- Fluke: inrush-current measurement.
- CPSC: portable-generator operation and placement.