What Size Generator Do I Need for Central Air Conditioning?

Why Central AC Is Hard to Size for a Generator

An air conditioner has two different electrical problems:

1. Running Load

The power needed while the compressor and fan are operating normally.

2. Starting Load

The much higher short-duration power needed when the compressor starts.

A generator can have enough continuous kW to run an AC after startup and still fail to start it.

That is why:

running watts alone are not enough.

Running Watts vs Starting Watts

Suppose an AC system needs:

3,500W running

but:

8,000–10,000W equivalent starting demand

for a short period.

A generator rated at:

5 kW continuous

may look adequate from running load alone.

But it may trip, bog down or fail to start the compressor.

That does not necessarily mean you need a 10 kW continuous generator.

It means you need enough:

motor-start capability

for the startup event.

What Is LRA?

LRA means:

Locked Rotor Amps.

It is commonly shown on the outdoor condenser/compressor nameplate.

LRA represents the high current associated with the compressor at locked-rotor/start conditions.

It is not the same as normal running current.

For generator sizing, LRA is one of the most useful numbers on a conventional compressor system.

What Is RLA?

RLA usually means:

Rated Load Amps

on the compressor nameplate.

It helps describe compressor running load.

You may also see:

• FLA
• MCA
• MOCP

depending on the equipment.

These numbers solve different electrical questions.

For generator sizing, the most useful data usually includes:

• supply voltage
• compressor running current
• LRA/start requirement
• fan load

Where Do I Find the AC Electrical Specifications?

Look at the outdoor condenser nameplate.

You're looking for information such as:

• voltage
• phase
• compressor RLA
• compressor LRA
• fan amps
• MCA
• model number

Do not use only:

• tonnage
• breaker size

if better data is available.

The model number can also be used to pull the manufacturer specification.

Breaker Size Is Not AC Running Load

This is another common mistake.

If your central AC is on a:

40A breaker

that does not mean it continuously consumes:

40A × 240V = 9.6 kW

during normal operation.

The breaker is protective equipment.

It is not a wattmeter.

Use the actual appliance electrical data.

AC Tonnage Does Not Equal Electrical Demand

“Tons” describe cooling capacity.

One ton of cooling is:

12,000 BTU/hour

of cooling capacity.

So:

• 2 ton = 24,000 BTU/hr
• 3 ton = 36,000 BTU/hr
• 4 ton = 48,000 BTU/hr
• 5 ton = 60,000 BTU/hr

That tells you cooling capacity.

It does not directly tell you electrical input.

Two 4-ton systems can have different efficiency and compressor technology and therefore different electrical requirements.

What Size Generator for a 2-Ton Central AC?

A 2-ton system may be relatively easy for a generator compared with larger units.

But the correct answer still depends on:

• voltage
• running current
• LRA
• other household loads

For AC-only backup, you might find the load fits within a modest generator.

For whole-house backup, add:

• refrigerator
• furnace/air handler
• lights
• pumps
• other essential loads

before deciding.

What Size Generator for a 3-Ton Central AC?

A 3-ton central AC commonly moves into the range where:

compressor starting demand matters much more than simple running watts.

A generator may comfortably run the system once started but still struggle at compressor startup.

The exact LRA tells you more than the 3-ton label.

If the generator is marginal, a correctly selected soft start can change the economics substantially.

What Size Generator for a 4-Ton Central AC?

At 4 tons, I would definitely want the exact:

• model
• LRA
• RLA
• voltage

before recommending generator capacity.

This is where generic online tables become dangerous because the difference between:

• older single-stage compressor
• newer high-efficiency compressor
• variable-speed inverter system

can be substantial.

What Size Generator for a 5-Ton Central AC?

A 5-ton conventional central AC can create a significant motor-start event.

This does not automatically mean you need:

20–25 kW

of continuous generator capacity.

But it often means you need:

• strong surge/motor-start capability
• a suitably sized standby generator
• or a soft start
• plus enough capacity for the rest of the house

For a 5-ton unit, I would not buy a generator until I had the exact condenser electrical data.

Example AC Running-Watt Calculation

Suppose the condenser and air handler together draw approximately:

15 amps at 240V

under normal operation.

Simple power estimate:

15A × 240V = 3,600W

or:

3.6 kW

That gives a rough running-load estimate.

But this still does not tell you startup demand.

You need the compressor's start characteristics.

Example LRA Calculation

Suppose the compressor nameplate shows:

LRA = 75A

at:

240V

A naive multiplication gives:

75 × 240 = 18,000VA

or:

18 kVA

That does not necessarily mean the generator needs 18 kW of continuous capacity.

The startup event is short and motor power factor/voltage dip behaviour matters.

This is why generator manufacturers often publish:

• motor-start kVA
• surge capability

separately from continuous kW.

Generator Motor-Starting Capability Matters

A generator can be rated:

18 kW continuous

but still have enough short-duration motor-starting capability to start a compressor whose instantaneous electrical demand is much higher.

That is why the correct comparison is:

AC starting requirement vs generator motor-start capability

not simply:

AC LRA watts vs generator continuous watts.

Kohler Motor-Starting Example

Current Kohler/Rehlko residential products publish motor-start capability separately.

Existing project research includes:

Kohler 20RCA

Approximately:

41 kVA peak motor starting

Kohler 26RCA

Approximately:

39 kVA peak motor starting

Those are separate from the generators' normal continuous kW ratings.

That is exactly the kind of specification central-AC sizing needs.

Briggs & Stratton Motor-Starting Example

Current Briggs & Stratton residential standby equipment also publishes strong motor-start specifications.

Existing project research includes represented:

• 13 kW: 37 kVA
• 22 kW: 45 kVA
• 26 kW: 65.5 kVA

depending on model/test conditions.

Again:

continuous generator kW and motor-start capability are not the same measurement.

What Is a Soft Start for an Air Conditioner?

A soft-start device reduces the electrical stress associated with compressor startup.

It can reduce:

• starting current
• voltage sag
• generator startup burden

That can make it easier for:

• generator
• inverter
• battery system

to start the compressor.

For generator sizing, this can be a major tool.

Soft Start vs Hard Start Kit

These are not the same thing.

Hard Start Kit

Typically changes compressor-start characteristics using additional starting components.

It is often intended to help a compressor start under difficult conditions.

Soft Start

Electronically controls the starting event to reduce peak current.

For generator/battery applications, a genuine soft-start system is usually the more relevant concept when the goal is reducing startup demand.

Have HVAC/electrical professionals select the appropriate equipment.

Can a Soft Start Let Me Use a Smaller Generator?

Potentially, yes.

If the generator's limiting problem is:

compressor startup

rather than:

continuous household demand

a soft start can materially change the required generator class.

Example:

Your whole house might run at:

12 kW

once everything is operating.

But the AC startup event pushes the generator beyond its motor-start capability.

Reducing that startup event could let a 14–18 kW-class system perform a job that otherwise required larger equipment.

That can be far cheaper than buying several additional kW of generator.

A Soft Start Does Not Reduce Normal AC Running Load Much

This is important.

A soft start mainly solves:

startup demand.

It does not magically turn a:

4 kW running AC

into a:

2 kW running AC.

So if the generator is too small for the ongoing load, a soft start does not fix that.

Central AC + Whole-House Load

You should never size the generator for the AC alone unless the AC is literally the only load.

During an outage, other loads might include:

• refrigerator
• freezer
• well pump
• furnace/air handler
• lights
• outlets
• sump pump
• microwave

The correct calculation is:

whole-house simultaneous running load + AC starting event

with sensible load management.

Use our generator size calculator.

Example: AC + Household Load

Suppose:

Base household load

5 kW

Central AC running load

3.5 kW

Total continuous:

8.5 kW

Now suppose AC compressor startup adds another:

5 kW equivalent short-duration demand

The generator needs to:

• carry 8.5 kW continuously
• survive roughly 13.5 kW during the startup event

That does not automatically mean you need a 14 kW continuous generator.

You need to compare the startup event with the generator's actual surge/motor-start capability.

Two Central AC Units

This changes the sizing problem dramatically.

You generally do not want:

both compressors starting at exactly the same time.

A good system can:

• sequence loads
• delay the second unit
• temporarily shed one AC

This can allow a smaller generator to serve a house with multiple HVAC systems.

Load Management for Multiple AC Units

Standby-generator systems may use:

• load-management modules
• HVAC priority controls
• thermostat control
• generator controller logic

to prevent simultaneous high-demand loads.

This can be much cheaper than sizing the generator for:

every compressor starting at once.

Generac Load Management

Generac residential standby systems can integrate load-management equipment for high-demand appliances.

The practical use case is:

• AC #1 starts
• AC #2 waits
• water heater remains shed

until generator capacity is available.

This is a better system design than blindly buying the largest possible generator.

Champion fleX / aXis Load Management

Champion's current standby systems use fleX/aXis architectures specifically designed around managed whole-house loads.

That makes Champion relevant where:

• multiple AC systems
• high-demand appliances

need sequencing rather than unrestricted simultaneous operation.

Briggs Symphony Choice

Briggs & Stratton's Symphony Choice system can manage large loads including:

• AC
• electric heat
• other high-wattage appliances

Again:

load management can be cheaper than generator oversizing.

Portable Generator for Central AC

A portable generator can run central AC if it has:

• correct 120/240V output
• enough running watts
• enough surge/motor-start capacity
• safe panel connection

This is much more realistic with a:

large 7–12+ kW portable

than a small 120V inverter generator.

30A Portable Generator and Central AC

A 30A 240V connection has a theoretical maximum of:

7.2 kW

That can be enough for some central AC systems plus limited household loads.

But the generator still needs sufficient motor-start capability.

And the actual generator may produce less than 7.2 kW continuously.

50A Portable Generator and Central AC

A 50A 240V connection has a theoretical capacity of:

12 kW

That gives substantially more room.

Large home-backup portables with 50A output are therefore more suitable for:

• central AC
• well pump
• broader household loads

than smaller 30A-only machines.

See our portable generator guide.

Portable Generator + Soft Start

This is often a strong value architecture.

For example:

large 120/240V portable + approved interlock + AC soft start

can provide serious whole-home manual backup without standby-generator economics.

But the exact AC and generator need to be matched.

Portable Generator + Interlock

An interlock allows a large portable generator to feed the existing household panel safely when correctly installed.

That means the AC remains connected through its normal breaker.

You then manually manage other loads.

See our generator interlock kit guide.

Standby Generator for Central AC

Standby is the easier solution.

The generator can be permanently designed around:

• AC starting
• ATS
• load management
• automatic sequencing

If automatic HVAC backup is important, this usually wins.

14 kW Generator and Central AC

Can a 14 kW generator run central AC?

Potentially, yes.

Especially when:

• AC running load is moderate
• motor-start capability is adequate
• soft start is installed
• other loads are managed

A 14 kW generator can be plenty for one house and inadequate for another.

18 kW Generator and Central AC

This is a common useful residential class.

An 18 kW standby can often support:

• central AC
• refrigeration
• lighting
• pumps

with suitable sizing/load management.

But again:

check the fuel-specific output.

An “18 kW” generator may produce less on natural gas.

20 kW Generator and Central AC

Around 20 kW gives a lot of room for typical residential backup.

But it is still possible to exceed capacity with:

• multiple AC units
• electric water heating
• large well pumps
• electric cooking

The load calculation still matters.

22 kW Generator and Central AC

A 22 kW-class standby is one of the most common whole-house choices.

Current examples:

Generac Next Generation 22 kW

• 21 kW NG
• 22 kW LP

Briggs PowerProtect 22 kW

• 22 kW NG
• 22 kW LP

The Briggs gives 1 kW more NG output in that exact comparison.

But either can potentially run central AC when sized correctly.

26 kW Generator and Central AC

A 26 kW-class system provides substantial residential capacity.

Current examples include:

Generac Next Generation 26

• 24 kW NG
• 26 kW LP

Kohler 26RCA

• 24 kW NG
• 26 kW LP

Champion 26 kW

• roughly 23.4 kW NG
• 26 kW LP

These are large residential systems.

But even here:

two large AC systems + electric heat + water heater + EV charging

can exceed capacity.

Generator Size for a 2-Ton AC: Practical Range

Rather than pretend there is one number, I would investigate:

roughly 5–10+ kW generator systems

depending on:

• AC electrical data
• other loads
• soft start
• portable vs standby

This is a starting range, not a recommendation.

Generator Size for a 3-Ton AC: Practical Range

I would often investigate:

roughly 7.5–14+ kW

depending on compressor start and whole-house load.

A well-designed soft-start setup can push the required generator towards the lower end.

A large LRA + substantial household load can push it much higher.

Generator Size for a 4-Ton AC: Practical Range

I would investigate:

roughly 10–18+ kW

but only after checking the actual nameplate.

At this size, generic tonnage estimates become especially weak.

Generator Size for a 5-Ton AC: Practical Range

Often:

roughly 12–22+ kW

depending heavily on:

• compressor technology
• LRA
• soft start
• other loads

A modern variable-speed 5-ton system and an older single-stage 5-ton compressor can behave very differently.

These broad bands are for orientation only.

Use the actual electrical data before buying.

Why These Tonnage Ranges Are Not Sizing Rules

Because tonnage is cooling capacity.

The generator does not care how many tons are printed on the HVAC brochure.

It sees:

• volts
• amps
• power factor
• starting demand

Use tonnage only as a rough initial clue.

Variable-Speed / Inverter Central AC

Modern inverter-driven HVAC systems can have much different startup behaviour than conventional single-stage compressors.

They may ramp rather than create one large locked-rotor event.

That can make generator starting easier.

But they also contain sophisticated power electronics.

Use the HVAC manufacturer's actual generator compatibility/electrical requirements.

Do not apply conventional LRA assumptions blindly.

Heat Pump Generator Sizing

A heat pump adds another complication.

In cooling mode, it behaves similarly to central AC.

In heating mode, additional loads may include:

• defrost
• auxiliary electric heat
• heat strips

The electric backup heat can be enormous.

A generator sized to run the heat pump compressor may be nowhere near large enough to run:

10–20 kW of auxiliary resistance heat.

Gas Furnace + Central AC Is Much Easier

A home with:

• gas furnace
• central AC

is usually much easier to back up than an all-electric heat-pump home with resistance backup.

The furnace blower may only need a fraction of the power required by electric heating strips.

Fuel architecture affects generator sizing.

AC Compressor Starting and Natural Gas Derating

If a generator loses output on natural gas, that can reduce:

• continuous capacity
• available headroom during AC startup

Example:

A 26 kW generator may be:

26 kW LP

but:

24 kW NG.

If your sizing calculation is already close, the fuel choice can decide whether the AC start works comfortably.

Use the fuel-specific rating.

Can a Generator Be Too Big for the AC?

The generator can certainly be larger than necessary.

The problem is not that a 26 kW generator will somehow “send too much power” into the AC.

The AC only draws what it needs.

The problem is economic:

• higher equipment price
• higher fuel demand
• larger infrastructure

without useful benefit.

Can a Generator Be Too Small?

Yes.

Symptoms can include:

• engine bogging
• voltage dip
• breaker trip
• AC failing to start
• generator shutdown
• load-management shedding

Do not normalise an AC that only starts:

sometimes.

The system should be designed to start it reliably.

Does Central AC Need a Dedicated Generator?

Usually no.

The AC is simply one of the major household loads connected to the generator system.

The whole-house standby or portable backup system can also support:

• refrigeration
• lights
• pumps

as long as capacity allows.

Does Central AC Need a Separate Transfer Switch?

Usually not in a normal whole-house standby architecture.

The AC circuit is part of:

• main electrical panel
• ATS/load-management arrangement

Large systems may use specific controls/load-management modules.

The electrical design is system-specific.

How to Calculate Your AC Generator Requirement

Use this sequence:

Step 1 — Find the AC Nameplate

Record:

• voltage
• RLA
• LRA
• model

Step 2 — Estimate Running Demand

Add:

• compressor
• condenser fan
• air handler/blower

Step 3 — Define Starting Event

Use:

• LRA
• manufacturer motor-start data
• soft-start specifications if installed

Step 4 — Add Other Household Loads

Such as:

• refrigerator
• well pump
• lighting
• sump pump

Step 5 — Apply Load Management

Decide what does not need to run while AC starts.

Step 6 — Compare Generator Specs

Check:

• continuous kW on actual fuel
• motor-start/surge capability
• voltage

Then size with sensible headroom.

Use the Generator Size Calculator

Our generator size calculator uses:

running load + largest additional starting event + configurable headroom

rather than summing every motor surge as if they all start simultaneously.

That is a much more useful first-pass sizing method.

For AC specifically, enter:

• AC running watts
• additional startup watts

from real equipment data where possible.

Don't Sum Every Motor Surge

You do not normally need to add:

• AC start
• refrigerator start
• well pump start
• freezer start

all simultaneously.

The useful design event is usually:

base running load + largest likely starting event

unless system behaviour means multiple motors genuinely start together.

This avoids grotesque oversizing.

Generator Headroom

Do not size right to:

100.0% of rated output

on paper.

Some headroom is useful for:

• load variation
• unexpected appliances
• fuel-specific capacity
• ageing/conditions

But headroom should be sensible.

It is not an excuse to automatically double generator size.

Example Whole-House Sizing

Suppose:

Base household loads

5.5 kW

Central AC running

3.5 kW

Total running

9.0 kW

AC additional starting event

4.5 kW

Short-duration requirement:

13.5 kW

A generator with:

• 10 kW continuous
• weak motor-start capability

may be a poor fit.

A 12–14 kW-class system with strong motor-start capability may work.

Or a soft start could materially reduce the startup event.

This is how the system should be analysed.

Example With Soft Start

Same house:

Base + AC running:

9 kW

Original extra AC start:

4.5 kW

After properly selected soft-start reduction, assume the additional event becomes materially lower.

Now the generator's existing surge capability may be enough without stepping up several kW in continuous size.

That can create substantial installation savings.

The exact reduction must come from the actual soft-start/AC combination.

Do not invent one universal percentage.

Can a Soft Start Damage the AC?

Properly selected and installed equipment should be compatible with the HVAC system.

But not every soft-start product is appropriate for every compressor.

Use:

• HVAC model
• manufacturer requirements
• qualified installation

rather than buying by tonnage alone.