6,000 W essential + 2,000 W extra + 3,000 W surge, 20% headroom
- Running = 8,000 W
- Total with surge = 11,000 W
- Recommended ≈ 13.2 kW
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Home Generator Size Calculator
Estimate generator size in kW using running load, motor surge, and headroom.
Results
- Running watts
- 8000.00
- Surge watts
- 3000.00
- Recommended watts
- 13200.00
- Recommended generator size (kW)
- 13.20
How to use this calculator
- List your essential running load and any additional circuits you want powered.
- Enter the largest motor surge (e.g., AC, well pump) to cover startup draw.
- Choose a headroom % to avoid maxing the unit.
- See the recommended generator size in watts and kW.
Inputs explained
- Essential running load
- Critical circuits you must power (fridge, lights, furnace, etc.).
- Additional running load
- Non-critical loads you may include (TV, small appliances, outlets).
- Largest motor surge
- Highest startup surge (HVAC, well pump, compressor) in watts.
- Headroom (%)
- Extra capacity to reduce overload risk and allow future loads.
How it works
Running watts = essential + additional loads you plan to power.
Add the largest motor surge (AC, well pump, etc.) to cover startup draw.
Apply headroom to avoid maxing the generator; recommended kW = (running + surge) × (1 + headroom) ÷ 1,000.
Formula
Running = Essential + Additional Total with surge = Running + Surge Recommended kW = (Total with surge × (1 + headroom)) ÷ 1000
When to use it
- Sizing a standby generator for whole-home or essential circuits.
- Checking if a portable unit can handle fridge + furnace + a small window AC.
- Estimating size before installing a transfer switch or interlock.
Tips & cautions
- Round up to a commercially available size; consider future loads like EV chargers or heat pumps.
- Account for 240V loads and starting currents from compressors and well pumps.
- Have a licensed electrician confirm load calcs and transfer equipment sizing.
- Does not model load sequencing/management modules or soft starters.
- Uses simple running + surge math; real duty cycles and simultaneous loads may differ.
Worked examples
4,500 W running + 2,500 W surge, 25% headroom
- Recommended ≈ 8.75 kW
- Round up to a common size (e.g., 9–10 kW)
Deep dive
Use this home generator size calculator to translate your running loads and motor surges into a recommended kW rating. Add headroom so the unit isn’t maxed out during peaks or future upgrades.
It’s ideal for standby or portable planning before you buy a generator or install a transfer switch. Enter your loads, surge, and headroom to land on a practical, commercially available size.
FAQs
- How much headroom should I use?
- 15–25% is common for standby generators; round up to the next available size to avoid overloads.
- Do I need to count 240V loads differently?
- Include their wattage in running and surge totals. An electrician should size conductors and transfer equipment.
- What if multiple motors start at once?
- Use the largest likely simultaneous surge. Load management modules can stagger starts to reduce surge needs.
- Can a portable generator power my whole house?
- Usually only essentials. Whole-home standby units are larger and tied into a transfer switch with proper interlocks.
- Does power factor matter?
- This estimate uses nameplate watts. Motors with poor power factor can draw more current; oversizing and professional review help.
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Approximate sizing only. Verify actual load requirements and starting currents; consult an electrician for transfer switch sizing.