8,000 running, 10,000 surge, 20% headroom
- Running ≈ 8.0 kW
- Surge ≈ 10,000 × 1.2 ÷ 1000 = 12 kW
construction calculator
Home Generator Sizing Calculator
Estimate generator size (kW) needed from running and surge watts with optional surge headroom.
Results
- Recommended running size (kW)
- 8.00
- Recommended surge capacity (kW)
- 12.00
How to use this calculator
- Enter continuous running watts of the loads you plan to power.
- Enter surge/startup watts (highest starting load).
- Set extra headroom percentage for added surge margin.
- Review recommended running and surge kW.
Inputs explained
- Running watts
- Sum of continuous loads (fridge, lights, furnace, etc.).
- Surge watts
- Highest startup load (e.g., HVAC compressor, well pump).
- Headroom
- Extra margin above known surge to cover variability.
How it works
Running kW = Running watts ÷ 1000.
Surge kW = max(Running, Surge) × (1 + headroom) ÷ 1000.
Formula
Running kW = Running watts ÷ 1000 Surge kW = max(Running, Surge) × (1 + Headroom) ÷ 1000
When to use it
- Sizing a standby or portable generator for home backup.
- Checking if a planned generator can handle startup spikes.
- Comparing generator sizes before purchase.
Tips & cautions
- List essential circuits and use nameplate/running/surge ratings for accuracy.
- Consider soft-start kits to lower surge if needed.
- Check fuel type and transfer switch requirements separately.
- Simplified; does not model power factor or motor inrush beyond the provided surge.
- Assumes single surge event; overlapping surges may need more headroom.
- Not a substitute for electrician load calculations; verify before installation.
Worked examples
6,000 running, 7,500 surge, 15% headroom
- Running = 6.0 kW
- Surge ≈ 7,500 × 1.15 ÷ 1000 ≈ 8.6 kW
Deep dive
Estimate generator size by entering running watts, surge watts, and headroom to get recommended kW for home backup.
Use it to choose a standby or portable generator that covers your essential loads.
FAQs
- Does this cover whole-house loads?
- It’s a simplified essential-load view. Whole-house sizing may require more detailed load analysis.
- What about 240V loads?
- Include their wattage in running/surge sums. Check generator specs for 120/240V output.
- Is power factor considered?
- Not explicitly. This is watts-based. Motors with low PF may need more headroom.
- Is this install guidance?
- No. Consult an electrician for transfer switches and code compliance.
- Fuel/runtime considerations?
- Not modeled. Size fuel supply separately based on desired runtime.
Related calculators
Simplified sizing aid. Verify loads, surges, wiring, and code compliance with a licensed electrician before purchasing/installing a generator.