What Size Generator for 150-Amp Service?

What a 150-amp electrical service actually means

A 150-amp service identifies the maximum current permitted by the home’s main service equipment. At 240 volts, its theoretical capacity is:

150 amps × 240 volts = 36,000 volt-amps

That is the service limit—not a generator recommendation and not a measurement of normal household demand.

A home’s connected equipment does not all operate continuously or start at the same time. Generator sizing therefore begins with the equipment that will actually be backed up: selected essential circuits, managed whole-home loads, or unrestricted automatic loads.

Generac’s guide describes three NEC-based approaches for optional standby sizing:

• Existing maximum-demand data under NEC 220.87
• The optional dwelling calculation under NEC 220.82
• Other feeder and service calculations under Article 220

The applicable code edition and local authority determine which method may be used.

Installing a generator does not automatically require upgrading a 150-amp service to 200 amps. The generator, transfer equipment, conductors, overcurrent protection, and backed-up loads must still be designed as a complete system.

For broader methodology, see the home generator sizing guide. If your main breaker is smaller, use the 100-amp generator sizing guide for service-specific planning ranges.

How to size a generator for 150-amp service

1. Decide what the generator will power

List the loads you expect to use during an outage.

Essential loads may include:

• Heating equipment
• Refrigerator and freezer
• Well, sump, or septic pump
• Lighting and selected receptacles
• Internet and communications
• Medical or accessibility equipment

Optional or managed loads may include:

• Central air conditioning
• Electric water heater
• Electric range or dryer
• Pool or spa equipment
• EV charging
• A second HVAC system

Do not assume that “whole home” means every connected appliance must operate simultaneously. Automatic load management can temporarily defer lower-priority equipment.

2. Determine the simultaneous running load

Use actual equipment nameplates whenever possible. Generac provides nominal values for preliminary planning when nameplate information is not available.

Nominal motor-planning values

Generac groups items such as refrigerator compressors, sump pumps, furnace blowers, and garage-door openers in its nominal 0.5 HP category. These are reference values—not substitutes for actual nameplate data.

Nominal non-motor values

Use the applicable load-calculation method rather than simply adding every nominal value at 100%. The NEC optional dwelling method applies demand factors to general loads and treats HVAC, supplemental heat, and EV charging separately.

3. Check motor starting separately

Motor-driven equipment can require substantially more current to start than to run.

Begin with the simultaneous running load, then replace the selected motor’s running contribution with its starting demand:

Starting event = simultaneous running load − motor running load + motor starting load

Do not automatically add the full starting demand of every motor. However, if two significant motors can start together or cycle coincidentally, that combination must also be evaluated.

For central air, use the condenser nameplate and the selected generator’s surge-amp data. Generac’s nominal reference for a 3-ton AC is:

• Approximately 3 kW running
• 15 running amps at 240 V
• 100 locked-rotor amps at 240 V

Locked-rotor amps should be compared with the generator’s published surge capability and acceptable voltage dip. It should not be converted casually into a universal “starting-watt” figure.

A compatible soft starter may reduce compressor starting demand, but the amount varies by equipment and installation. Verify the manufacturer’s data and the measured result rather than assuming a fixed percentage.

4. Reserve continuous-capacity headroom

Generac describes operation at approximately 80% of generator capacity as an optimal planning practice rather than an NEC requirement.

To reserve 20% of the selected continuous capacity as planning headroom:

Continuous-capacity target = calculated running load ÷ 0.80

This margin applies to the continuous running requirement. Do not apply it automatically to the brief motor-starting event.

5. Adjust for fuel and installation conditions

Use the selected model’s published output on the fuel that will actually be used.

Natural-gas output is not reduced by one universal percentage. In Generac’s 2025 tables:

• A 22 kW Guardian is listed at 22 kW on LP and 19.5 kW on natural gas.
• A 22 kW Next Gen unit is listed at 22 kW on LP and 21 kW on natural gas.

Other models show smaller, larger, or no difference.

For the air-cooled units in the Generac guide, the published planning adjustments are:

• Temperature: 1% per 10°F above 60°F
• Altitude: 3.5% per 1,000 feet above sea level

Liquid-cooled and diesel models use different thresholds and percentages. Always use the specification for the actual model.

6. Confirm fuel delivery and transfer equipment

The fuel system must support the generator at full load while also serving other connected gas appliances.

For example, Generac lists the 22 kW Guardian at approximately 327,000 BTU per hour on natural gas at full load. The meter, regulator, pressure, pipe diameter, developed length, fittings, and other appliance demand all affect whether the fuel system is adequate.

Transfer equipment, inlet capacity, conductors, overcurrent protection, and neutral configuration must also match the installation.

Worked sizing example

Using Generac’s nominal planning values where nameplate data are unavailable, consider this selected-load running subtotal:

Continuous-capacity target for this subtotal:

4.5 kW ÷ 0.80 = 5.625 kW

This is a continuous-load planning example, not a finished generator size. Each credible motor-start event still has to fit the selected generator’s published capability.

Horsepower alone does not establish a well pump’s running or starting requirement. Use the pump or controller’s actual voltage, applicable running current, and defined starting current. The Well Pump Starting Watts guide explains the manufacturer-specific workflow. The final model must also satisfy its published:

• Fuel-specific running output
• Surge-amp capability
• Acceptable voltage dip
• Environmental derating
• Fuel-supply requirements

If significant motors can start together, that coincidental event also needs to be tested.

Typical planning ranges by load profile

These are SizeMyGenerator planning bands—not Generac model recommendations and not substitutes for an NEC-compliant load calculation.

A home may fall outside these bands. Actual nameplate data and operating priorities control the result.

Does square footage determine generator size?

Not by itself.

Under the optional dwelling calculation described in Generac’s guide, general lighting and receptacles contribute 3 VA per square foot.

For a 2,000-square-foot home:

2,000 sq. ft. × 3 VA = 6,000 VA

That is only one component. Small-appliance circuits, laundry, permanently connected appliances, motors, HVAC, electric heat, water heating, and EV charging must also be considered.

Two homes with the same square footage and the same 150-amp service can therefore require very different generators.

What changes the required generator size?

Gas versus electric appliances

A gas furnace, boiler, water heater, or range may require electricity only for controls, ignition, pumps, or a blower.

Electric resistance heat, water heating, ranges, dryers, and heat-pump auxiliary elements add large continuous loads. Heating and appliance fuel often matters more than whether the service is rated 150 or 200 amps.

Central air and other motor loads

Central AC, well pumps, sump pumps, septic equipment, and pool pumps may determine the required starting capability.

Use:

• Running-load amps
• Locked-rotor amps
• Voltage
• Motor type
• Manufacturer nameplate data

A soft starter or variable-speed controller may help, but it must be compatible with the equipment and verified in the actual application.

For pump-specific guidance, see the well-pump starting-watts guide.

Heat pumps with supplemental electric heat

Generac’s outline of the NEC optional calculation includes:

• 100% of the heat-pump compressor load
• 65% of supplemental central electric heat

The compressor may be omitted from that calculation only when controls prevent it from operating simultaneously with supplemental heat.

Do not disable auxiliary or emergency heat without an equipment-specific control design. Defrost operation, freeze protection, comfort, and equipment safety must be addressed by a qualified HVAC and electrical professional.

Load management

Load management can temporarily defer lower-priority loads such as:

• Electric water heater
• Dryer
• Range
• Second AC system
• Pool equipment
• EV charger

This reduces the load permitted to operate simultaneously. It does not increase generator output or eliminate the need to verify motor starting.

Generac’s guide references NEC 702.4(A)(2)(b) and energy-management provisions under 750.30 for automatically managed loads.

Future loads

Include realistic future changes such as:

• Converting gas appliances to electric
• Adding an EV charger
• Installing a heat pump
• Adding a pool or hot tub
• Expanding the home
• Adding workshop equipment

Use the equipment’s expected operating input—not merely the breaker rating.

Portable versus standby generators for 150-amp service

A portable generator’s usable output is limited by the lowest-rated relevant component and by the load arrangement—not by the connection label alone.

For a 120/240 V connection, the arithmetic ceilings are:

• 30 A × 240 V = 7,200 VA total across the two legs.
• 50 A × 240 V = 12,000 VA total across the two legs.

Those products are not promises of usable generator output and do not mean 7,200 or 12,000 VA is available on one 120 V leg. At the connection rating, a 120 V leg is limited by that leg’s amperage; the exact generator may impose a lower continuous, receptacle, breaker, or per-leg limit. The generator, inlet, breaker, cord, conductors, and transfer equipment must be evaluated as one connection path by the electrician.

Compare connection methods in Transfer Switch vs. Interlock.

Common mistakes when sizing for 150-amp service

1. Sizing directly from 150 amps × 240 volts. The 36 kVA service limit is not the expected generator load.
2. Adding the full starting demand on top of a total that already includes the motor’s running load. Replace the motor’s running contribution with its starting demand.
3. Assuming only one motor can ever start. Evaluate credible coincidental starts when controls or operating patterns allow them.
4. Applying continuous headroom to a momentary starting event. Check continuous capacity and motor starting separately.
5. Using the LP rating when the generator will run on natural gas. Compare the actual model’s published ratings.
6. Assuming a 22 kW generator will run every 150-amp home. All-electric homes, multiple HVAC systems, and supplemental heat can exceed that capacity.
7. Ignoring fuel delivery. The meter, regulator, pipe, pressure, and other gas appliances must be included.
8. Using generic AC “starting watts” instead of nameplate LRA and generator surge data.
9. Disabling supplemental heat without accounting for defrost and equipment controls.
10. Failing to document managed-load priorities.
11. Forgetting future electrification or EV charging.
12. Mismatching portable-generator neutral and transfer-equipment bonding. A licensed electrician should verify the specific generator and connection method.

Frequently asked questions

Related detail: Well Pump Starting Watts and generator sizing for 200-amp service.

How to choose your final generator size

1. Choose the loads

Decide whether the generator will serve:

• Selected essential circuits
• Managed whole-home loads
• Every automatically connected load

2. Calculate the continuous requirement

Use nameplate data or an approved NEC-based calculation method.

3. Evaluate motor starting

Test the largest credible starting event and any significant coincidental starts.

4. Compare actual generator ratings

Check:

• Natural-gas or propane running output
• Surge amps
• Allowable voltage dip
• Temperature and elevation adjustments
• Fuel consumption and supply requirements

5. Document load management

Identify exactly which loads will be deferred, in what order, and under what conditions.

6. Obtain a professional design

A qualified electrician or generator professional should verify the load calculation, transfer equipment, fuel system, conductors, grounding and bonding, permits, and local requirements.

What size generator for 150-amp service?

Many 150-amp homes fall within a 10–22 kW standby planning range, not the panel’s 36 kVA theoretical capacity. Selected essential circuits may require less; heat pumps with supplemental heat, all-electric appliances, several motors, or unrestricted whole-home operation may require more.

The correct choice is the smallest generator whose published capacity on the intended fuel supports:

• The calculated simultaneous running load
• Continuous-capacity headroom
• The largest credible motor-starting event
• Environmental derating
• The documented load-management sequence

Use the standby sizing calculator to build a standby planning estimate. Do not treat that result as portable-generator compatibility; portable selection requires the separate model-specific checks described above.