Generator Sizing Calculator
Pick appliances or enter kW, add the largest motor surge and a 20–25% margin, and get an instant kW and kVA screen—independent of any OEM catalog. Example: home essentials ~3.5 kW running + window AC surge → about 5.5 kW portable. Use Advanced mode for industrial kW + PF catalog frames.
Calculator
Quick: appliance load builder (watts). Advanced: known kW + PF for catalog kVA frames.
Appliances to power simultaneously
About this calculator
Quick mode sums running watts, adds the largest motor surge, applies PF and a safety margin, then maps to portable kW and catalog kVA. Results are brand-neutral—CalcPanel does not sell generators or rank OEM SKUs. Advanced mode screens industrial frames when you already have kW from a factory load study. Workflow: power calculator hub.
Calculation Results
Calculation Results
Largest motor surge (added): 2400 W
Power factor: 0.85
Safety margin: 25% (× 1.25 on running load)
Governing load: 5080 W (5.08 kW)
Required apparent power: 5.98 kVA
Recommended portable generator: 5.5 kW (next standard size above 5.1 kW)
Standby catalog kVA: use portable kW rating for residential loads under 15 kVA
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.
Generator sizes for common scenarios
Typical screening ranges—use the calculator with your actual appliance list rather than relying on generic sizes alone.
| Scenario | Typical running load | Largest surge | Recommended size |
|---|---|---|---|
| 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 motor surge W)
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 + largest motor surge + a documented 20–25% margin, then round up. Treat viral rules as conversation starters only.
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. Largest surge: window AC 2400 W. PF = 0.85, margin = 25%.
Margin path: 2680 × 1.25 = 3350 W. Surge path: 2680 + 2400 = 5080 W → governs. Required kW = 5.08 kW. kVA = 5.08 ÷ 0.85 ≈ 5.98 kVA → pick a 5.5–6.5 kW portable generator with adequate peak 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.
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)
| 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), screen 5–7.5 kW after surge and margin. Full-house backup with central AC often needs 12–20 kW—use the load builder and confirm with an electrician.
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. 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 20–25% margin instead. 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. Sum running watts, then add the largest single motor surge. The tool uses required W = max(running × (1 + margin), running + largest surge).
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?
A 20–25% margin covers error, future loads, and brief peaks. Running at 100% continuous rating causes overheating and nuisance trips.
Can central AC run on a portable generator?
A 3-ton central AC may need 10–12 kW+ peak. Most portables top out around 7–8 kW—window AC or a standby 16–22 kW set is more realistic for whole-house AC.
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 separately.
How do I estimate fuel use after sizing?
See the Generator Fuel & Runtime Planning guide for screening L/h and tank hours once kW/kVA is set.
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.