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 recommendation. 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. Advanced mode screens industrial frames when you already have kW from a factory load study. For the full load → generator workflow, use the 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.
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.
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.
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.