Generator sizing
Generator Sizing Calculator: Find Your Exact Wattage
Check off appliances —results update instantly Based on NEC 702 (NFPA 70) & manufacturer specs
Based on NEC 702, DOE Energy Use Chart, manufacturer spec sheets
What do you need to power?
Check everything you want to run at the same time during an outage. Results update automatically.
Running watts of selected appliances. Largest starting surge and 25% buffer are added automatically in the result.
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Recommended Generator Size
Starting wattage needed: 3,200 W
Default result based on refrigerator (150W) + furnace fan (800W) + sump pump (400W). Check your appliances on the left for your exact number.
Calculation breakdown
| Selected devices | 3 |
| Total running watts | 1,350 W |
| Largest starting surge | 1,200 W |
| Running watts × 1.25 buffer | 1,688 W |
| Recommended generator | 3,200 W |
Formula: (Running Watts × 1.25) + Largest Starting Surge per NEC 702. Rounded up to nearest standard generator size.
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How This Calculator Works: NEC 702 Generator Sizing Methodology
Our generator sizing calculator applies the electrical engineering principles and code requirements defined in NEC Article 702 (Optional Standby Systems) from NFPA 70. Here is the step-by-step methodology, the code references behind each calculation, and what the results actually mean for your home.
Step 1: Total Running Watts (Continuous Load)
The calculator sums the running (rated) watts of every appliance the user checks off. Running watts is the steady-state power draw once a device is running normally. These values are sourced from manufacturer specification sheets (Generac, Honda, Champion), the DOE Appliance Energy Use Chart, and real-world field measurements. For example, a typical refrigerator draws 150-200 running watts, a furnace blower (1/3 HP) draws 800 watts, and an electric water heater draws 4,500 watts.
Step 2: 25% Continuous-Load Safety Buffer (NEC 210.20(A))
Per NEC 210.20(A) and 215.2(A), continuous loads (operating for 3 hours or more) must be sized at 125% of the actual load. Since generators power homes for extended outages that routinely exceed 3 hours, the calculation multiplies your total running watts by 1.25. This 25% buffer ensures the generator operates within its safe continuous rating rather than at peak capacity, preventing overheating, excessive wear, and voltage drop during sustained operation.
Step 3: Largest Single Starting Surge (Motor Inrush)
Motors (refrigerator compressors, well pumps, AC compressors, furnace blowers) draw 3-7× their running wattage for 2-3 seconds at startup. This is called inrush current or starting surge. The NEC 702 standard requires generators to handle the starting surge of the largest motor that could start while other loads are running. Critically, our calculator adds only the single largest starting surge —not the sum of all surges —because motors do not start simultaneously in the real world. For example, if your largest motor is a 2.5-ton central AC with a 7,000W starting surge, only that 7,000W is added, not your refrigerator's 600W surge on top of it.
The Complete Formula
Recommended Generator Size = (—Running Watts × 1.25) + Largest Single Starting Surge
The result is rounded up to the nearest standard generator size (2,000W, 3,500W, 5,000W, 7,500W, 10,000W, 12,000W, 15,000W, 20,000W, or 22,000W). The product recommendations then match your calculated specification to verified generator models, prioritizing exact-spec matches first.
Limitations and When to Consult a Professional
This calculator provides a code-based estimate using standardized wattage tables. It does not account for altitude derating (generators lose ~3.5% capacity per 1,000 ft above sea level), extreme temperatures, specific manufacturer tolerances, or complex multi-panel installations. For whole-house standby installations with automatic transfer switches, a licensed electrician must perform a site-specific load calculation and ensure NEC 702.5 compliance regarding system capacity and transfer equipment ratings. Always verify generator sizing with a qualified professional before purchase.
Standards and References
- NEC Article 702 —Optional Standby Systems (NFPA 70)
- NEC 210.20(A) —Continuous-Load Overcurrent Protection
- DOE Appliance Energy Use Chart —Running & starting wattage references
- Manufacturer spec sheets —Generac, Honda, Champion, Westinghouse
- ENERGY STAR Emergency Preparedness guidelines
Why Generator Sizing Matters: Undersized vs Oversized
Undersized Generator
- Overload shutdown: Generator trips its breaker when demand exceeds capacity, cutting power to everything
- Motor damage: Insufficient starting current burns out compressor and pump motors
- Voltage sag: Lights dim and electronics malfunction when voltage drops below 110V
- Overheating: Running at 100%+ load continuously destroys the generator engine within months
- Warranty void: Most manufacturers void warranties for sustained overload operation
Oversized Generator
- Wasted money: Each 5,000W step up adds $300-800 to the purchase price
- Wasted fuel: Generators running below 30% load are 20-40% less fuel-efficient
- Wet stacking (diesel): Unburned fuel accumulates in the exhaust, causing carbon buildup and maintenance issues
- Larger footprint: Bigger generators need more storage space and heavier-duty pads
- Unnecessary noise: Larger engines run louder even at low load
The sweet spot per NEC 702: your generator should run at 50-80% of its rated continuous capacity under normal load, with headroom for the largest motor startup. This gives you efficient fuel consumption, normal engine wear, and reliable surge handling —without paying for watts you'll never use.
5 Common Generator Sizing Mistakes
- Adding all starting surges together. Motors don't all start at the exact same instant. Per NEC 702, you only need to cover the single largest starting surge, not the sum of every motor's inrush current. Adding all surges together can nearly double your calculated size unnecessarily.
- Confusing running watts with starting watts on the generator label. A generator sold as "5,000 watts" often means 5,000 starting watts / 4,000 running watts. Always buy by running watts. The starting watts rating is for brief surges only and cannot be sustained.
- Forgetting the 25% continuous-load buffer. NEC 210.20(A) requires loads operating 3+ hours to be sized at 125%. Since outages routinely last 6-48 hours, every load is continuous. Skipping this buffer means your generator runs at 100% capacity, which it cannot sustain.
- Using nameplate watts instead of actual measured watts. The wattage printed on an appliance label is the maximum possible draw, not the typical draw. A nameplate might say 1,500W but actual measured running watts are 800W. Our calculator uses field-measured and manufacturer-published typical values.
- Ignoring altitude derating. Gasoline and propane generators lose approximately 3.5% of their rated output for every 1,000 feet above sea level. A 7,500W generator in Denver (5,280 ft) effectively delivers only ~6,100W. If you live above 3,000 ft, either buy a larger generator or choose one with an altitude kit.
Common scenarios for generator sizing
If your situation matches one of these, jump straight to a detailed guide:
Frequently asked questions
What size generator do I need to run my house?
For essential circuits only (fridge, furnace fan, lights, phone charger): 5,000-7,500 running watts. For most of a 2,000 sq ft home including well pump or central AC: 10,000-15,000 running watts. Whole-house with every circuit: 20,000-25,000 watts.
How do I calculate generator size?
Add the running watts of everything you want to power simultaneously, then add a 25% safety buffer per NEC 210.20(A) for continuous loads. Add the largest single motor starting surge (not the sum of all surges). The formula is: Total = (—Running Watts × 1.25) + Largest Starting Surge. Our calculator above applies this automatically.
What size generator for a 200-amp service?
To cover a 200-amp residential service panel completely, you need a 15,000-25,000 watt (15-25 kW) standby generator with an automatic transfer switch. For essential-circuit coverage only (fridge, HVAC, lights, sump pump), a 10,000-12,000 watt unit is sufficient. The 200A rating is the panel capacity, not your actual load.
What's the difference between running watts and starting watts?
Running watts (rated/continuous watts) is the steady power the generator delivers. Starting watts (surge/max watts) is the brief burst for motor startup —typically 2-3× running watts lasting 2-3 seconds. Always size your generator by running watts plus the largest single motor surge, not the sum of all surges.
Portable or standby generator —which is better?
Portable generators ($500-2,000) are manual-start, gasoline-powered, 3,000-10,000 watts, with 8-12 hour runtime per tank. Standby generators ($5,000-15,000 installed) auto-start during outages, run on natural gas or propane with unlimited runtime, provide 8,000-25,000 watts, and require professional installation with a transfer switch per NEC 702.
What size generator for central AC?
A 3-ton central AC draws 3,500-4,500 running watts with a 9,000-13,500 watt starting surge. You need at least a 10,000-12,000 watt generator for a 3-ton AC plus essentials. A 5-ton unit needs 15,000+ watts. Mini splits are far more efficient on generator power —a 12,000 BTU unit only needs 1,500 watts.
How do I calculate generator size for my whole house?
List every appliance and circuit you want to power simultaneously. Add their running watts. Multiply by 1.25 for the continuous-load buffer. Add the starting surge of the single largest motor (usually central AC or well pump). The total is your minimum generator wattage. For a typical 2,000 sq ft home with central AC, this comes to 12,000-15,000 watts.
What can a 5,000-watt generator run?
A 5,000W generator can run: refrigerator (150W), furnace fan (800W), sump pump (1,000W), lights (300W), phone/laptop chargers (150W), and a microwave (1,000W) —but not all at once without load management. Two 1,500W space heaters would max it out. It cannot run central AC or an electric water heater.
What can a 10,000-watt generator run?
A 10,000W generator can run: most of a 2,000 sq ft home including refrigerator, furnace, well pump, sump pump, lights, TV, computer, microwave, and either a small window AC or one major 240V appliance like an electric range. It can handle a 3-ton central AC if other large loads are managed. It cannot run a whole house with electric water heater and dryer simultaneously.
Do I need a transfer switch for my generator?
Yes —for any generator connected to your home's electrical panel. A transfer switch prevents dangerous backfeed that can electrocute utility workers and is required by NEC 702. Manual transfer switches ($300-500) work for portables. Automatic transfer switches ($1,000-3,000) come with standby generators. Never use a male-to-male suicide cord.
How long will a generator run on a tank of gas?
A typical 5,000W portable with a 6-gallon tank runs 8-12 hours at 50% load (2,500W) and 6-8 hours at full load. A larger 7,500W unit with a 7-gallon tank runs 10-14 hours at 50% load. Standby generators on natural gas have unlimited runtime. On propane, a 500-gallon tank runs a 22 kW standby for 7-10 days at typical household load.
What is the NEC 702 standard for generator sizing?
NEC Article 702 (NFPA 70) governs Optional Standby Systems. Key requirements: (1) the generator and transfer equipment must have sufficient capacity to supply all loads intended to be operated simultaneously, (2) continuous loads must be calculated at 125% per 210.20(A), and (3) motor starting surges must be accommodated. Our calculator applies all three requirements.
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