Home Generator Size Calculator

Use this calculator to estimate how much generator capacity your selected home loads may require. It adds the running watts of everything you expect to use at the same time, then accounts for the largest extra starting demand from a motor-driven appliance. The result is a planning estimate - not a code-compliant load calculation, permit approval, or final generator recommendation.

Estimate your generator load

How the generator size calculator works

Generator sizing starts with the loads you actually want to power, not just the square footage of the house. A refrigerator, well pump, furnace blower, air conditioner, electric water heater, range, dryer, or other large load can change the result much more than home size by itself.

1. Add the running watts for every selected load that may be on at the same time. This is the continuous power the generator needs to carry.

2. For each motor-driven load, compare its starting watts with its running watts. The difference is the extra startup demand.

3. Add the largest single extra startup demand to the total running load. That gives the calculator’s estimated peak starting demand.

Planning formula: Estimated peak demand = total running watts + largest single additional starting watts.

This method assumes one major motor starts at a time. If two or more large motors or compressors can start together, the calculator can understate the required starting capacity.

Example: If the selected loads total 4,250 running watts and the largest additional starting demand is 2,350 watts, the estimated peak is 6,600 watts, or 6.6 kW. A final equipment choice still has to meet the generator manufacturer’s continuous-output and motor-starting requirements for the exact installation.

Running watts and starting watts are different

Running watts are the power an appliance or system uses after it is operating. Starting watts describe the higher, short-duration demand some motors and compressors can create when they start.

That distinction matters for refrigerators, pumps, furnace blowers, air conditioners, garage-door openers, and other motor-driven loads. The calculator should use the actual starting value when it is available rather than assuming every motor follows the same multiplier.

Generator size is normally discussed in watts or kilowatts (kW), which measure power. Kilowatt-hours (kWh) measure energy over time and are not the sizing unit used by this generator calculator.

Use actual equipment data when you can

Preset appliance values are useful for a first pass, but they are not specifications for your exact home. Two refrigerators, pumps, HVAC systems, or dryers can have very different power requirements. Before buying equipment, replace planning values with the numbers from the product label, owner manual, or manufacturer documentation whenever possible.

You can safely gather useful information without opening electrical equipment or removing panel covers:

• The appliances and systems you want available during an outage.

• Model numbers and visible equipment labels for major loads.

• Running watts and starting watts when the manufacturer provides them.

• Whether a major load is 120 V or 240 V.

• Whether you want essential-load backup or broader whole-home coverage.

• Photos of accessible equipment labels and the closed electrical panel for discussion with an installer.

If a motor or compressor has no usable starting-watt figure, do not guess with a universal multiplier. Use the manufacturer’s documentation or have the starting requirement verified as part of the system design.

Whole-home backup and essential-load backup can produce very different results

A generator sized for essential loads does not need to carry every electrical load in the house. A homeowner might prioritize refrigeration, a well or sump pump, furnace controls or blower, lighting, internet, and a few receptacles while leaving large discretionary loads off during an outage.

Whole-home backup means the system is designed to serve the home more broadly, but that still does not always mean every large load must run at the same moment. Load-management equipment can sequence or temporarily shed selected loads so the generator is not asked to carry all of them at once. The exact strategy depends on the equipment and system design.

For the broader sizing decision, see What Size Generator Do I Need for My House?. If you are still deciding between generator types, compare Standby Generator vs. Portable Generator.

Why square footage alone is not enough

Two homes with the same floor area can have very different backup-power needs. One may use natural-gas heat and water heating while another uses a heat pump, electric resistance backup heat, an electric water heater, a well pump, or other large electrical loads. A load-based calculator is more useful than a square-foot shortcut because it reflects what you actually plan to power.

The main-breaker or electrical-service rating is not a direct generator-size recommendation either. Service capacity, panel rating, available load, generator capacity, and transfer equipment are related but different parts of the project. A permanent system should be evaluated using the actual home load and the rules that apply to the installation.

What can change the result after the calculator

The calculated load is only one part of selecting a generator. The final system can change because of:

• Multiple motors or compressors that can start at the same time.

• Central air conditioning, heat pumps, electric resistance heat, well pumps, septic or lift pumps, and other large motor loads.

• Verified soft-start equipment or load-management controls that change when or how a large load starts.

• The exact generator model and its published motor-starting capability.

• Fuel choice and the model’s rated output on that fuel.

• Manufacturer derating for altitude, temperature, fuel quality, or other operating conditions.

• Whether the installation is essential-load, selected-circuit, managed whole-home, or unrestricted whole-home backup.

• Transfer equipment, electrical-service configuration, fuel-supply capacity, site constraints, and local permitting or inspection requirements.

Do not treat a calculated 10 kW, 15 kW, or 20 kW load as proof that every generator carrying that nominal label will perform the same way. Verify the exact model’s ratings and installation requirements.

How to interpret your calculator result

Treat the output as a comparison point, not a purchase order. The result should tell you how much continuous running load you selected and the estimated peak created by the largest startup event. When you compare equipment, confirm that the generator can support the continuous load and the required motor starting performance under the intended fuel and operating conditions.

This calculator does not add a hidden 15% or 20% “safety margin.” Extra capacity for future loads can be a reasonable planning choice, but it should be visible and intentional rather than presented as a universal code rule. Oversizing also has cost, fuel, installation, and operating implications, so the final choice should match the home and the backup goal.

If the result is much larger than expected, review the selected loads before simply moving to a larger generator. Some high-demand appliances may not need to run during every outage, and a qualified installer can explain whether load management changes the system options.

Portable generator safety is separate from sizing

What to do after you get an estimate

First, replace any rough preset values with actual equipment data. Then use the result to narrow the conversation with an installer rather than trying to finalize the system from the calculator alone.

If you are considering a permanent system, review the Whole-House Generators: A Homeowner’s Guide, the Whole-House Generator Cost guide, and Who Installs Whole-House Generators?. If the generator will connect to home wiring, also read the Generator Transfer Switches: A Homeowner’s Guide.

Related guides

• What Size Generator Do I Need for My House? — Understand the sizing decision in more detail, including whole-home vs. essential-load goals and starting demand.

• Whole-House Generators: A Homeowner’s Guide — See how standby systems, transfer equipment, fuel, maintenance, and installation fit together.

• Standby Generator vs. Portable Generator — Compare system types before using the estimate to shop for equipment.

• Whole-House Generator Cost — See the equipment, installation, transfer, fuel, electrical, and site factors that drive project cost.

• Who Installs Whole-House Generators? — Understand which trades may be involved and what a useful site evaluation should cover.

• Generator Transfer Switches: A Homeowner’s Guide — Learn how generator power is transferred to home circuits and why backfeed protection matters.

Sources

Source access date: September 24, 2026. Calculator formula and preset values rechecked September 25, 2026.

1. Briggs & Stratton - Choosing a Portable Generator (opens in a new tab). Publication/update: Updated January 13, 2026 | Accessed: September 24, 2026.

2. Honda Power Equipment - What Wattage Generator Do I Need? / Generator Wattage Estimation Guide (opens in a new tab). Publication/update: Live reference; page carries 2026 copyright, no specific update date shown | Accessed: September 24, 2026.

3. U.S. Consumer Product Safety Commission - CPSC Warns of Generator, Carbon Monoxide and Fire Hazards Ahead of Hurricane Season (opens in a new tab). Publication/update: May 27, 2026 | Accessed: September 24, 2026.

4. Electrical Safety Foundation International (ESFI) - Generator Safety (opens in a new tab). Publication/update: Current live page; accessed September 24, 2026 | Accessed: September 24, 2026.

5. Generac Power Systems - 26 kW Guardian Residential Standby Generator Specification Sheet (G007290/G007291) (opens in a new tab). Publication/update: Rev. D, September 19, 2022; accessed September 24, 2026 | Accessed: September 24, 2026.