Generator Wattage Calculator
Sum appliance running wattage, add the largest starting increment and compare it with a documented margin for instant kW and kVA—a formula screen, not an OEM SKU picker. Starting increment means starting watts minus that load’s running watts, so the running portion is not counted twice.
Calculator
Quick: appliance load builder (watts). Advanced: known kW + PF for catalog kVA frames.
Appliances to power simultaneously
Calculation Results
Calculation Results
Largest starting increment (added): 1200 W
Power factor: 0.85
Safety margin: 25% (× 1.25 on running load)
Governing load: 3880 W (3.88 kW)
Required apparent power: 4.56 kVA
Screened next portable kW frame: 4 kW (next standard size at or above 3.88 kW—not an OEM model pick)
Standby catalog kVA: use portable kW rating for residential loads under 15 kVA
Screening result — verify the selected frame against the candidate generator's continuous, peak, transient, fuel and environmental derating data.
Engineering disclaimer
Screening estimate only—not a NEC 220 load calculation or manufacturer transient study. Confirm gas meter BTU, panel capacity, altitude derating, and harmonic loads with a licensed electrician or engineer before purchase.
Continue Your Calculation
Screen transfer-switch amp frames next, or size generator kVA with UPS load when bridge minutes matter.
About this calculator
Quick mode sums running watts, adds the largest starting increment, applies PF and a safety margin, then maps to portable kW and catalog kVA. Results are brand-neutral—the commercial module only opens product-class searches and never changes the engineering result. Advanced mode screens industrial frames when you already have kW from a factory load study. Workflow: power calculator hub.
Generator sizes for common scenarios
Illustrative market ranges only—not a load calculation and not an OEM recommendation. Use the calculator with your nameplate appliance list, then apply the selected model’s altitude, temperature, harmonic and fuel curves.
| Scenario | Typical running load | Largest surge (illustrative) | Typical screening range |
|---|---|---|---|
| Camping / tailgate | 1–2 kW | ~1 kW | 2–3 kW |
| RV / travel trailer | 2.5–4 kW | ~2.2 kW (AC) | 4–5.5 kW |
| Home essentials | 3–5 kW | ~2.4 kW | 5–7.5 kW |
| Full house + central AC | 8–15 kW | ~5–8 kW | 12–20 kW |
| Job site (tools) | 3–7 kW | ~3–4.5 kW | 5–10 kW |
| Industrial (known kW) | 50–500+ kW | Study separately | 100–1000 kVA |
Generator sizing formula
Step 1: Sum running watts for all loads that operate at the same time.
Step 2: Required W = max(running W × (1 + margin), running W + largest starting increment W), where starting increment = starting watts − that load’s running watts.
Step 3: Required kW = required W ÷ 1000
Step 4: Required kVA = required kW ÷ power factor
Step 5: Select the next standard portable kW or catalog kVA frame at or above the result.
Brand-neutral generator sizing
This page is an independent sizing screen: no OEM product push, no Generac/Honda/Cat SKU ranking, and no affiliate “recommended model” list. Use the kW/kVA result to compare any manufacturer’s datasheet, then verify transfer switch, fuel, and local code with a qualified electrician.
Rule of thumb (incl. “20/20/20” chatter)
Online “20/20/20” talk for standby sets is usually a loose mnemonic (margin / diversity / future growth)—not an NEC formula and not calculated by this tool. Prefer: sum simultaneous running watts, evaluate the largest starting increment, compare against a documented margin path, then verify transient performance in manufacturer data.
3-phase generator sizing
For three-phase plant or commercial sets, switch to Advanced mode: enter diversified kW and PF, apply margin, map to catalog kVA. Line current after sizing: I = kVA × 1000 ÷ (√3 × V)—use 3 phase power and kVA to amps. Motor inrush and step-loading still need manufacturer transient data.
Worked example — home essentials backup
Loads: refrigerator 200 W, sump pump 800 W, window AC 1200 W, lights 150 W, router 30 W, PC 300 W → 2680 W running. Window AC starting watts are 2400 W, so its added starting increment is 2400 − 1200 = 1200 W. PF = 0.85, margin = 25%.
Margin path: 2680 × 1.25 = 3350 W. Start path: 2680 + 1200 = 3880 W → governs. Required kW = 3.88 kW. kVA = 3.88 ÷ 0.85 ≈ 4.56 kVA → screen a 4 kW or larger set and verify its peak/starting rating.
Worked example — industrial screening (Advanced mode)
80 kW running @ 0.90 PF with 10% margin: base kVA = 80 ÷ 0.9 = 88.89 kVA; required = 88.89 × 1.10 ≈ 97.8 kVA → next catalog frame 100 kVA. Add motor starting and altitude derating before procurement.
Last updated: 2026-07-29. Screening estimate only—not an installation design or product recommendation.
Standard Generator Sizes & Commercial Sizing Guide
After calculating required kW/kVA, match to the next standard catalog frame. Portable generators follow kW ratings; standby and industrial sets are sold by kVA at a nominal power factor (typically 0.8).
Standard Generator kW to kVA Reference Table (PF 0.8)
Arithmetic conversion only: kVA = kW ÷ 0.8. The duty-class column is illustrative market language—not a sizing authority for healthcare, data centers or any facility type.
| Rated kW | kVA @ PF 0.8 | Illustrative duty class (not authority) |
|---|---|---|
| 3.5 kW | 4.4 kVA | RV / camping / small tools |
| 5.5 kW | 6.9 kVA | Home essentials backup |
| 7.5 kW | 9.4 kVA | Large home / well pump + AC |
| 10 kW | 12.5 kVA | Whole-home standby / small office |
| 20 kW | 25 kVA | Light commercial / retail |
| 50 kW | 62.5 kVA | Commercial building / small factory |
| 100 kW | 125 kVA | Industrial / data center / healthcare |
| 200 kW | 250 kVA | Large industrial / municipal |
| 500 kW | 625 kVA | Heavy industrial / prime power |
Commercial & Industrial Generator Sizing Considerations
For standby and prime power applications above 20 kW, sizing is not just a wattage sum. Key factors include:
- Load type and starting: Use nameplate running data plus manufacturer locked-rotor, inrush, or starting-kVA data for motors and compressors. Always identify the largest starting load and its starting method.
- Three-phase imbalance: Check each phase against the selected generator manufacturer's allowable steady-state and transient unbalance limits; this calculator does not impose a universal percentage.
- Step loading: Large motors started sequentially reduce peak demand. If the transfer switch stages loads, the generator can be smaller than a single-step start would require.
- Harmonic distortion: VFDs, UPS systems, and LED lighting are non-linear loads. Use the alternator manufacturer's harmonic/load-step study; no universal kVA multiplier is applied here.
- Altitude and temperature derating: Apply the selected engine-generator model's combined altitude/temperature derating table. The multiplier varies by engine, cooling system, duty and ambient condition.
- Fuel type and runtime: Size on-site fuel and fuel-supply capacity from the selected model's consumption curve, required autonomy, usable tank volume, replenishment plan and applicable code.
Fuel consumption after sizing
Fuel consumption is model-, fuel-, duty- and load-dependent. After selecting a candidate set, use its current OEM fuel-consumption curve at the planned load points; then calculate usable tank volume and autonomy. This page does not infer fuel use from nameplate kW alone.
References
- NFPA 110 — Emergency and Standby Power Systems
- NEC (NFPA 70) — Article 445 generators (via adopted code)
- OSHA electrical safety overview
Related: motor starting current guide, kW to kVA formula, kW to kVA calculator.
Typical appliance watts (running / surge)
Example defaults used by this page’s load builder—replace every row with nameplate or measured watts before procurement. Values are not OEM-certified starting data.
| Appliance | Running (W) | Surge (W) |
|---|---|---|
| Refrigerator | 200 | 600 |
| Window AC (10k BTU) | 1200 | 2400 |
| Central AC (3 ton) | 3500 | 8500 |
| Sump / well pump (1/2 HP) | 800–1000 | 2000–3000 |
| Table saw / air compressor | 1500–1800 | 3600–4500 |
Frequently Asked Questions
What size generator do I need for a house?
For essential circuits (fridge, sump pump, lights, router, one window AC), illustrative screens often land near 5–7.5 kW after surge and this tool’s optional planning margin. Full-house backup with central AC commonly screens near 12–20 kW as a market range—use the load builder with nameplates and confirm with an electrician. Altitude, fuel L/h and OEM derating are not baked into these ranges.
What size generator for a 2000 sq ft house?
Square footage alone does not size a generator—simultaneous watts do. A typical ~2,000 sq ft home on essentials often screens near 5–7.5 kW; whole-house with central AC commonly 12–20 kW as an illustrative market range. Run the load builder above; treat sq-ft rules of thumb as rough only.
What is the 20/20/20 rule for generators?
It is informal standby chatter (margin / diversity / growth), not a code calculation. Use running watts + largest surge + a documented planning margin (this tool offers 20–25% as optional inputs). See rule of thumb notes.
Is this a brand-neutral generator calculator?
Yes. CalcPanel does not sell generators or push OEM SKUs—results are kW/kVA screens you can take to any manufacturer datasheet. See brand-neutral sizing.
How do I size a 3-phase generator?
Use Advanced mode with diversified kW and PF, apply margin, round up to catalog kVA. Then check line amps with kVA to amps / 3 phase power. See 3-phase notes.
Should I use running watts or starting watts?
Both, without counting the same running watts twice. Sum running watts, then add the largest starting increment (starting watts − that load’s running watts). The tool compares this start path with the margin path.
How does power factor affect generator size?
Lower PF means more kVA for the same kW. Use 0.85 for typical mixed home loads; generators are often rated in kVA.
Why do I need a safety margin?
This tool’s optional 20–25% planning margin can cover error, future loads, and brief peaks—it is not a universal engineering rule. Running at 100% continuous rating often causes overheating and nuisance trips.
Can central AC run on a portable generator?
Illustrative market ranges: a 3-ton central AC may demand about 10–12 kW+ peak depending on SEER, starting method and voltage. Many portables top out near 7–8 kW continuous—window AC or a standby set sized from nameplate starting data is usually more realistic. Confirm OEM surge ratings.
How do I size an industrial generator in kVA?
Switch to Advanced mode, enter total kW and PF from your load study, apply margin, and map to catalog kVA (100, 150, 200 kVA…). Validate motor inrush, altitude derating and harmonics separately with OEM data.
How do I estimate fuel use after sizing?
This page does not invent L/h from nameplate kW. After a candidate set is chosen, use that model’s OEM fuel curve and the Generator Fuel & Runtime Planning guide as a secondary screen—not a stamped fuel budget.
Is this calculator enough for final selection?
No. Confirm NEC/local load methods, altitude and temperature derating, harmonics, and manufacturer curves with a qualified engineer or electrician.
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Related Guides
- Generator Fuel Consumption & Runtime Planning — Calculate diesel/gas consumption, runtime, and fuel tank sizing for standby generators
- How to Calculate Factory Load — Step-by-step factory electrical load calculation to determine generator capacity requirements
- Factory Load vs Transformer Size — How factory load profile affects both generator and transformer sizing
- UPS Sizing for Factories — Coordinate generator and UPS sizing for seamless backup power transition