The defensible way to size a home backup generator is to total the running watts of the appliances and circuits you expect to use at the same time, add the highest single starting-watt demand from a motor-driven load, and then apply a planning margin. Cummins describes a rough method that multiplies that result by at least 1.25. The calculation is a first estimate—not a substitute for a site-specific electrical load analysis, especially for a permanently installed standby system.
Your final choice depends on the loads you want covered, motor-starting requirements, generator output at the specified fuel and operating conditions, transfer equipment, and local installation requirements. Floor area alone cannot produce a reliable size.
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What generator sizing actually measures
Generator capacity is normally stated in watts (W) or kilowatts (kW), where 1 kW equals 1,000 W. A generator’s running or rated output is the continuous power it can provide. Its starting capability covers the brief extra power that some motor-driven appliances need while their motors begin turning.
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The objective is not to add every appliance’s theoretical surge. It is to model the loads that can realistically operate together and make sure the generator can handle the largest likely startup while already supplying the running load.
Step 1: Choose the outage coverage you actually need
Write down what must remain operational during an outage before looking at generator models. A smaller generator with a deliberate load plan is often more practical than trying to power every circuit in the house.
Essential-circuit coverage
An essential-load plan may include refrigeration, selected lighting, internet equipment, a furnace control, a sump pump, a well pump, medical equipment and a few receptacles. Electric ranges, clothes dryers, electric water heaters, pool equipment and central air conditioning are commonly excluded unless the calculated capacity and connection equipment support them.
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A whole-home plan attempts to leave most or all circuits available. It must account for large resistive loads such as electric heat and water heating as well as the starting demand of motors. “Whole-home” does not mean every appliance can necessarily start at once; the design still needs a realistic load schedule and, where appropriate, load management.
Make a written load schedule
Separate loads into three groups so the calculation reflects how the household will operate:
- Must-run loads: equipment that should operate during the outage, such as a refrigerator, sump pump or critical medical device.
- Convenience loads: lighting, outlets, televisions, computers and small appliances that may be used when capacity allows.
- High-draw or deferrable loads: electric heat, cooking appliances, dryers, water heaters, EV charging, pool pumps and workshop equipment.
Decide which loads can be turned off before a large motor starts. That assumption should be explicit in the load schedule rather than left to chance.
Step 2: Collect running and starting watt data
Use the appliance nameplate, installation documentation or manufacturer specifications whenever possible. Generic internet estimates can be useful for a preliminary conversation, but they can be wrong for your particular model, voltage, age and operating condition.
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| Information to record | Where to find it | Why it matters |
|---|---|---|
| Running watts or amperage | Appliance nameplate, manual or manufacturer specification | Determines the continuous load total |
| Starting watts, locked-rotor data or inrush current | Motor nameplate, technical data or installer measurement | Determines the short startup requirement |
| Voltage and phase | Nameplate and panel documentation | Confirms electrical compatibility |
| Whether the load can run simultaneously | Your outage operating plan | Prevents unrealistic over-sizing or under-sizing |
| Fuel and rated-output conditions | Generator manufacturer specifications | Output can vary by fuel and stated operating conditions |
If a nameplate lists amps rather than watts, do not casually convert it without confirming the voltage and whether the rating is for a single-phase or other configuration. An electrician can convert and verify the values as part of a load analysis.
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Prioritize motor loads
Identify the largest motor or compressor first. Central air-conditioning compressors, deep-well pumps, sump pumps, refrigerators and heat pumps are frequent sources of startup demand. A pump that runs modestly after starting may still determine the generator size if its startup requirement is the highest in the home.
Step 3: Add the simultaneous running watts
Add the running watts for every load that your plan allows to operate at the same time. Do not add appliances that will be deliberately switched off or loads that cannot operate concurrently under the plan.
Illustrative running-load worksheet
The following numbers are an example of the method, not a recommendation for any particular home:
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|---|---|---|---|
| Refrigerator | 700 W | 1,200 W extra | Yes |
| Sump pump | 1,000 W | 2,000 W extra | Yes |
| Lighting and selected outlets | 800 W | None stated | Yes |
| Furnace blower | 600 W | 900 W extra | Yes |
| Electric range | 4,000 W | None stated | No; excluded from outage plan |
For the included loads, the running total is 700 + 1,000 + 800 + 600 = 3,100 W. The largest listed additional starting demand is the sump pump’s 2,000 W.
Step 4: Add the largest starting demand and apply a planning margin
Cummins’ rough approach is to add the single highest starting-watt figure to the running total, then multiply the result by at least 1.25. In the example:
- Running total: 3,100 W.
- Highest additional starting demand: 2,000 W.
- Subtotal for startup planning: 3,100 + 2,000 = 5,100 W.
- Cummins’ planning margin: 5,100 × 1.25 = 6,375 W, or 6.375 kW.
This is a planning estimate attributed to Cummins, not an electrical code requirement and not a bespoke engineering determination. Cummins states that the multiplier provides a margin of safety and allows for potential future needs. A professional may use different assumptions after checking the actual equipment and starting sequence.
Why the highest single start is used in a rough estimate
Motors do not necessarily start at the same instant. A refrigerator may start while a pump is running, but the refrigerator, pump and air-conditioner compressors are unlikely to experience their maximum inrush simultaneously in normal operation. Adding every appliance’s starting figure can produce an unnecessarily large estimate if it assumes an impossible sequence.
Conversely, a home with automatic controls, multiple pumps, several compressors or a frequent chance of simultaneous starts may need a more detailed calculation. Ask the installer to model the actual sequence rather than relying on a simplified addition.
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Step 5: Compare the estimate with the generator’s real output
Compare your calculated requirement with the generator’s rated/running output, not only a marketing peak or surge number. The generator must also be able to meet the relevant motor-start requirement while supplying the other loads that are already operating.
- Check the continuous output at the fuel and operating conditions specified by the manufacturer.
- Verify the generator’s starting capability and any published motor-start guidance.
- Confirm voltage, phase, frequency and connection configuration match the home’s equipment.
- Leave practical headroom rather than planning to operate continuously at the nameplate limit.
- Have a qualified professional review the result for a standby installation or a complex motor load.
If the estimate is close to the generator’s limit, do not solve the problem by assuming a short-term surge rating will carry the entire house. Use a smaller, clearly defined load plan; discuss automatic load management; or ask whether a compatible soft-start device is appropriate for a specific motor. A soft-start or load manager may reduce starting demand, but actual savings depend on the equipment and installation and are not guaranteed.
Portable generator or permanently installed standby?
| Decision | Portable generator | Permanently installed standby generator |
|---|---|---|
| Typical connection | Outdoor receptacles or an inlet connected through compatible transfer equipment | Dedicated transfer equipment and a permanent fuel and electrical installation |
| Load strategy | Often focused on selected essential circuits | Can be designed for essential circuits or broader coverage after a site survey |
| Installation considerations | Correct inlet, transfer switch, grounding and operating procedure | Panel configuration, automatic controls, clearances, fuel supply, permits and code compliance |
| Operating involvement | Usually requires placement, startup, refueling and monitoring by the homeowner | Can start automatically, but still requires maintenance and professional commissioning |
| What determines size | Loads that can be connected at one time and the portable unit’s rated output | Site-specific load analysis, transfer equipment and the selected coverage strategy |
Cummins identifies gasoline- or propane-powered 9,500-watt dual-fuel portable generators as an example of a home or small-business backup category. That category example does not establish that 9,500 W is suitable for your household. Calculate your loads first.
Do not size from square footage alone
House size can be a rough discussion point, but it cannot replace an appliance inventory. Two homes with the same floor area may have very different requirements because one uses gas heat and cooking while the other has electric resistance heat, a well pump, a large heat pump, an electric water heater and an EV charger.
Generac gives illustrative guidance that a 1,550-square-foot home might fall in a 10–18 kW range depending on coverage, while a 2,000-square-foot home might use 20 or 22 kW for whole-home coverage. These are manufacturer examples, not universal sizing tables or population statistics. Treat them as prompts to examine the load plan, not as a shortcut to a purchase.
Fuel, output conditions and outage duration
Confirm the generator’s rated output for the fuel you intend to use. A dual-fuel unit may have different published outputs on gasoline and propane, and a standby system’s performance depends on its specified fuel supply and installation conditions.
Capacity is only one part of resilience. Estimate how long an outage could last, verify that the fuel supply can support the intended run time, and follow the owner’s manual for storage, refueling, ventilation and maintenance. Never assume that a stated tank capacity guarantees a particular runtime under your household’s load; runtime changes with load and operating conditions.
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Never connect a generator directly to a building’s wiring or use an improvised backfeed connection. Generac explains that a transfer switch isolates the generator from utility power. For a portable generator, use a compatible inlet and transfer switch installed and serviced by qualified personnel.
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- Have the electrician identify the service or subpanel arrangement and the circuits that will be backed up.
- Confirm that the transfer equipment is compatible with the generator, voltage, phase and panel configuration.
- Obtain required permits and inspections before operation.
- Install and operate the generator outdoors in the location specified by the owner’s manual and current public-safety guidance.
- Keep exhaust away from windows, doors and other openings according to the exact manual and applicable safety rules, and use carbon-monoxide alarms inside the home.
- Test the transfer procedure and shutdown process before an outage.
Clearances, grounding, bonding, neutral switching, inlet ratings and fuel-line requirements vary by equipment and jurisdiction. A transfer-switch page or generic sizing worksheet cannot answer those site-specific questions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When load management or a soft start may help
If a central air conditioner or pump creates a large startup spike, an installer may propose staged starting, an automatic load-management controller or a compatible soft-start device. These approaches can allow selected loads to share a smaller generator, but they introduce compatibility, control and installation questions.
- Identify the exact motor or compressor model.
- Confirm the proposed device is approved for that equipment and voltage.
- Ask for measured or manufacturer-supported starting requirements before and after installation.
- Define what happens when two managed loads request power at the same time.
- Do not count an unverified reduction as guaranteed generator capacity.
Common sizing mistakes and how to correct them
Using only running watts
Symptom: The generator runs lights and electronics but trips when a pump or compressor starts.
Correction: Obtain the motor’s starting requirement and check the generator’s starting capability, not just its continuous rating.
Adding every surge together
Symptom: The calculated generator is far larger than the household’s realistic operating plan.
Correction: Document which loads can run together and model the most demanding plausible start sequence. Use a professional load analysis when simultaneous starts are uncertain.
Choosing by floor area
Symptom: A unit marketed for a similarly sized house cannot support the home’s electric heat, well pump or air conditioning.
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Correction: Build the appliance-by-appliance schedule and use square-footage examples only as broad manufacturer illustrations.
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Confusing peak watts with continuous output
Symptom: The brochure’s larger peak number looks adequate, but the generator overloads during normal operation.
Correction: Compare the required continuous load with the rated/running output and separately verify motor-start performance.
Ignoring fuel-specific ratings
Symptom: The generator’s output is lower than expected after changing fuel.
Correction: Read the manufacturer’s rating for the actual fuel and operating condition you will use.
Connecting without transfer equipment
Symptom: A homeowner considers a cord or improvised connection to energize household circuits.
Correction: Stop and have qualified personnel install compatible transfer equipment. Direct connection can energize utility lines and create a fatal hazard.
What a professional load assessment should include
For a standby system, ask for a documented site survey and load analysis rather than a recommendation based only on house size. The assessment should address:
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- Service rating, panel configuration and available spaces.
- Essential and optional circuits to be transferred.
- Running and starting requirements for pumps, compressors and other motors.
- Electric heat, water heating, cooking, laundry, EV charging and other high-draw equipment.
- Transfer-switch type, neutral and grounding arrangement, and compatibility with the generator.
- Fuel storage or supply, location, clearances, exhaust routing and maintenance access.
- Permits, inspections and local code requirements.
- Any load-management sequence needed to stay within the generator’s rated output.
For a portable generator that will only supply cord-connected equipment, the same arithmetic can help organize priorities. If it will connect to the home’s wiring, use compatible connection equipment and qualified installation.
A printable sizing checklist
- List every circuit or appliance you want during an outage.
- Mark each item as must-run, convenience or deferrable.
- Record running watts, starting watts, voltage and phase from the nameplate or manufacturer documentation.
- Identify the largest motor-start requirement.
- Add running watts for the loads that can operate simultaneously.
- Add the highest single starting-watt figure for a rough planning estimate.
- Multiply that subtotal by at least 1.25 if using Cummins’ rough method, and label the result as an estimate.
- Compare the result with the generator’s rated/running output and starting capability for the intended fuel.
- Check whether load management or a soft-start approach is needed, without assuming unverified savings.
- Arrange a professional site survey and load analysis before selecting and installing a standby system.
- Use a compatible transfer switch and qualified installation; never backfeed building wiring.
- Follow the specific owner’s manual and current public-safety guidance for placement, carbon-monoxide protection, fueling and operation.
Bottom line
Start with the appliances and circuits, not the size of the house or a generator’s peak-watt headline. Total the simultaneous running watts, add the largest realistic motor-start demand, and apply a clearly labeled planning margin such as Cummins’ 1.25 multiplier. Then verify the result against the generator’s rated output, fuel-specific specifications and transfer equipment. A professional load analysis is the right final step for a permanently installed standby generator or any home with substantial motor, heating or whole-home loads.
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