What Size Generator for 100-Amp Service?

A 100-amp panel does not require a 24 kW generator just because 100 amps multiplied by 240 volts equals 24,000 volt-amperes. The panel rating is a ceiling. The generator is sized to the load that will actually be connected and allowed to operate during an outage.

Two homes with the same 100-amp service can need very different generators. A gas-heated home may fit 10–14 kW; another with central A/C, electric appliances, and a well pump may need 22–24 kW or load management.

Why a 100-Amp Panel Does Not Equal a 24 kW Generator

A 100-amp, 120/240-volt service has a theoretical capacity of:

100 amps × 240 volts = 24,000 VA, or 24 kVA

For a unity-power-factor load, 24 kVA is approximately 24 kW. But that number describes the service’s maximum capacity, not the home’s normal demand.

Generator sizing depends on four separate checks:

1. Calculated or measured load: how much load the generator must carry at one time.
2. Motor starting: whether it can start the central A/C, well pump, sump pump, or other large motor.
3. Fuel-specific output: many gaseous generators produce less power on natural gas than on propane.
4. Site conditions: altitude, high outdoor temperature, and an inadequate fuel supply can reduce available output.

Takeaway: The service rating sets the upper boundary. The connected loads and starting requirements set the generator size.

How Do You Calculate Generator Size for 100-Amp Service?

The Two Main Calculation Methods

For an optional standby system, NEC Article 702 requires the source to carry the loads that will be connected automatically unless an approved energy-management system limits simultaneous demand. For an existing home, two commonly used approaches are measured demand under NEC 220.87 and the optional dwelling calculation under NEC 220.82. See the home generator sizing guide for broader sizing methodology.

Method 1: Use a Year of Measured Demand

NEC 220.87 may allow an existing service to be evaluated using actual maximum-demand data. Generac summarizes the method as:

• Use one contiguous year of maximum-demand data.
• Multiply the recorded maximum demand by 125%.
• Add any new or future loads.
• Confirm that the generator can start the largest motor loads.
• Verify that the local authority having jurisdiction accepts the method and the data.

Measured demand reflects how the home actually operates through both heating and cooling seasons.

Method 2: Use the Optional Dwelling Calculation

NEC 220.82 applies to qualifying 120/240-volt dwelling feeders or services rated 100 amps or more.

Step 1 — Identify the backed-up loads. Decide whether the generator will serve essential circuits, a managed whole panel, or all automatically connected loads without shedding.

Step 2 — Add the general loads. Include:

• General lighting and receptacles at 3 VA per square foot
• At least two small-appliance circuits at 1,500 VA each
• The laundry circuit at 1,500 VA
• Fastened-in-place and dedicated appliances at nameplate rating
• Permanently connected motors at nameplate rating

Step 3 — Apply the demand factor. Count 100% of the first 10,000 VA and 40% of the remainder.

Step 4 — Add the applicable heating or cooling load. Use the largest applicable selection under 220.82(C):

• A/C and cooling equipment at 100%
• Heat pumps without supplemental heat at 100%
• Heat-pump compressors at 100% plus 65% of supplemental central electric heat, unless controls prevent simultaneous operation
• Up to three separately controlled electric-heating units at 65%
• Four or more separately controlled electric-heating units at 40%
• Electric thermal storage at 100%

Step 5 — Add EV charging at 100% unless an approved control system limits the load.

Step 6 — Check motor starting separately. A load calculation does not prove that the generator can start a compressor or pump. Compare actual locked-rotor or starting-current data with the generator’s published surge capability and allowable voltage dip.

Step 7 — Apply model-specific margin and derating. Generac recommends selecting a residential generator approximately 10% to 20% above the calculated load as an optimal practice. That margin is manufacturer guidance, not an NEC requirement.

Worked Example: A 1,600-Square-Foot Home

Consider a 1,600-square-foot home with gas heat, gas water heating, a 3-ton central A/C, an electric range, an electric dryer, a dishwasher, a disposal, a microwave, a refrigerator, and a sump pump.

Using Generac’s nominal reference values where a nameplate is unavailable:

Apply the demand factor:

• First 10,000 VA at 100% = 10,000 VA
• Remaining 16,800 VA at 40% = 6,720 VA
• Calculated general load = 16,720 VA

Generac’s nominal reference lists a 3-ton A/C at about 3,000 VA running load:

16,720 + 3,000 = 19,720 VA, or about 19.7 kVA

That result is below the 100-amp service ceiling, but it is too close to the natural-gas output of some nominal 22 kW models to leave Generac’s preferred margin. Depending on the model series and fuel, a nominal 24 or 26 kW generator may be the better unrestricted choice.

Now assume automatic load management prevents the range and dryer from operating on generator power. The calculated load drops to approximately 14.3 kVA, making an 18 kW-class generator a realistic option—provided it also passes the A/C starting check.

Takeaway: Load management does not erase appliances. It limits which loads may operate together, reducing the maximum simultaneous demand the generator must carry.

What Size Generator Do Different 100-Amp Homes Need?

Gas Heat and Gas Water Heating

Gas heat and gas water heating remove two major electrical loads, but electric cooking, a dryer, central A/C, or a well pump may remain.

A 5,000–8,500-watt portable is usually an essential-circuit generator for refrigeration, the furnace blower, sump pump, lights, internet, and selected receptacles.

A 10–14 kW standby may cover a low-load gas home when central A/C and large electric appliances are absent, managed, or intentionally excluded. Do not label that range “whole house” unless the calculation and controls support the claim.

Window or Portable Air Conditioners

A room A/C may draw roughly 1,000 to 1,500 watts while running, but the compressor adds a starting surge. Multiple units can create a meaningful coincident load after an outage.

A 7,500–10,000-watt portable can often serve household essentials plus one or two room units. A 14–18 kW standby is a practical planning range for broader coverage.

Use the actual nameplate and avoid assuming that all units with the same BTU rating draw the same current.

Central Air Conditioning

Central A/C often controls the generator’s starting requirement. Generac’s nominal residential motor table lists:

Actual condenser nameplate data take precedence.

Generac’s 2025 air-cooled tables list maximum 240-volt surge capability of approximately 102–110 amps for 14 kW models, 150 amps for 18 kW models, 210 amps for 22 kW models, and 220–230 amps for 24 kW models.

That is why the earlier claim that a 5-ton compressor is “near the limit” of every air-cooled generator was incorrect. A nominal 145-LRA compressor is near the published surge capability of an 18 kW model, but well below the listed surge capability of the 22 and 24 kW models. Other loads operating during the start still matter.

For a 100-amp home with central A/C, 18–24 kW standby is the normal planning range. A large portable may work for selected loads and a smaller A/C system when a compatible soft starter has been verified, but it should not be promised from wattage alone.

Takeaway: Size central A/C from both its running load and its starting current. Do not convert locked-rotor amps into a simple “starting watts” number and assume the result is exact.

Well Pumps and Other Large Motors

Generac’s nominal motor reference lists a 2 HP well or septic lift pump at about 2 kW running and 6 kW starting. A smaller pump may still produce a large momentary load.

A 7,500–12,000-watt portable may handle a well pump plus selected essentials; 14–22 kW standby is a common planning range. Use the pump nameplate or control-box data, and review the well-pump starting-watts guide for a closer look at pump surge.

Electric Water Heating and Electric Heat

Generac’s nominal residential references include:

• Electric water heater: about 4.5 kW while heating
• Electric dryer: about 5.5 kW
• Electric range: about 8 kW
• Electric resistance heat: about 12 kW per 1,000 square feet
• Heat-pump elements: about 7 kW per 1,000 square feet

These are reference values, not substitutes for nameplates.

Electric water heating, resistance heat, or heat-pump strips can consume most of a generator’s output. The practical solution is often to manage or lock out those loads rather than buy enough capacity to run everything simultaneously.

Why Natural Gas Output Changes the Recommendation

The number in a generator’s model name may not be its natural-gas output.

Generac’s 2025 guide lists these examples:

A home near 20 kVA may fit one 22 kW model on propane but have little margin—or insufficient output—on natural gas.

Also verify:

• Temperature and altitude derating
• Natural-gas meter capacity
• Pipe size, length, and pressure
• Simultaneous demand from furnaces, boilers, water heaters, and other gas appliances
• Propane tank capacity and cold-weather vaporization

For the air-cooled models in Generac’s guide, the published planning adjustments are 1% per 10°F above 60°F and 3.5% per 1,000 feet above sea level. The selected product’s current specification sheet controls.

Common Mistakes When Sizing a Generator for 100-Amp Service

1. Buying 24 kW simply because the panel is 100 amps.
2. Calling a portable generator “whole house” without identifying the loads that must remain off.
3. Checking running watts but ignoring central-A/C or well-pump starting current.
4. Assuming a fixed soft-starter reduction without verifying the actual equipment.
5. Using the propane rating when the generator will operate on natural gas.
6. Treating managed loads as though they no longer exist.
7. Ignoring altitude, temperature, gas pressure, pipe sizing, or LP vaporization.
8. Backfeeding through a receptacle instead of using listed transfer equipment.

Operate a portable outdoors, away from doors, windows, and vents. Use a listed manual transfer switch or correctly installed interlock and inlet—never a male-to-male cord or receptacle backfeed. See how a transfer switch compares with an interlock before planning the connection.

Frequently Asked Questions

Next Steps

1. Use the generator sizing calculator to organize the backed-up loads.
2. Compare portable and standby generators for the coverage and operating style you need.
3. Review transfer switches and interlocks.
4. Consider load-management modules for the range, dryer, water heater, A/C, EV charger, or heat strips.
5. Have a qualified electrician verify the calculation, motor starting, transfer equipment, fuel supply, and locally adopted code before installation. Review the site’s educational-use and professional-verification limitations.

For a larger electrical service, see the 150-amp generator sizing guide and 200-amp generator sizing guide.

The panel tells you the ceiling. The load calculation, motor starts, fuel rating, and management strategy tell you the generator size.