| Appliance | Running W | Starting W surge per unit |
Qty | Volt | Leg | Row Running | Row Surge Δ | Remove |
|---|---|---|---|---|---|---|---|---|
| Tip: Starting watts are the temporary surge when a motor starts. The calculator uses total running watts plus the single largest surge delta. For split-phase 120/240V, balance 120V loads across legs. | ||||||||
Generator is ideally loaded below this percent continuously.
For kVA estimate. Many alternators assume ~0.8 PF.
Environment & Derating
Derating Multiplier: 1.00 (available fraction of nameplate)
Derated Recommendation: 0 W and 0.00 kVA
We increase the suggested size so your generator still meets demand at altitude/temperature. Nameplate × multiplier ≈ available power.
Fuel & Runtime Estimates (Dual‑Fuel)
Expected Continuous Load: 0% of derated rating
Computed as total running W ÷ derated recommended W.
Gasoline
Est. Gasoline Runtime: — hours
Runtime uses piecewise linear interpolation between the 25/50/75/100% rates.
Propane
Fill: 80%
Est. Propane Runtime: — hours
Vaporization Check
Per‑cylinder capacity @ temp: — lb/hr
Total available with 1 active: — lb/hr
Required @ load: — lb/hr
Results & Balance Summary
Total Running: 0 W
Largest Surge Δ: 0 W
Starting Requirement: 0 W
Recommended Size: 0 W (at target load)
Approx. Rating: 0.00 kVA (@ PF)
Estimated Current: 0.0 A @ 120 V
Leg A Running: 0 W
Leg B Running: 0 W
Imbalance: 0 W (0%)
Leg A Worst Start: 0 W ≈ 0.0 A @ 120 V
Leg B Worst Start: 0 W ≈ 0.0 A @ 120 V
Method: Starting Requirement = Sum of all running watts + single largest surge delta among motor loads. Derated recommendation inflates nameplate to offset altitude/temperature losses.
Transfer Switch Sizing Guidance
Safety factor on continuous amps (typical 125%).
Max per‑leg starting: 0.0 A
Max per‑leg continuous × factor: 0.0 A
Suggested switch: —
For portables, common receptacles are L14‑30 (30A) and 14‑50 (50A). Always follow local code and manufacturer instructions.
Formulas Used
- Row Running = Running W × Qty
- Row Surge Δ = max(Starting W − Running W, 0) for one unit
- Total Running = Σ(Row Running)
- Largest Surge Δ = max(Row Surge Δ) across all rows
- Starting Requirement = Total Running + Largest Surge Δ
- Recommended Size (W) = Starting Requirement ÷ (Desired Load % ÷ 100)
- Derating Multiplier = (1 − Altitude_ft/1000 × AltRate%/100) × (1 − max((Temp°F−60)/10 × TempRate%/100, 0))
- Derated Recommendation (W) = Recommended Size ÷ Derating Multiplier
- Recommended Rating (kVA) ≈ Rating_W ÷ (1000 × PF)
- Current (A) ≈ Starting Requirement ÷ System Voltage
- Runtime (h) = Tank Capacity ÷ Consumption_at_Load
- Split‑Phase: 240 V loads split equally across legs; 120 V loads land on a chosen leg; auto mode alternates units to minimize imbalance.
- Transfer Switch: pick next standard size ≥ max( per‑leg starting A, 1.25 × per‑leg continuous A ).
- Propane chaining: runtime scales with total fuel weight = cylinder_size × count.
- Vaporization Capacity: per‑cylinder ≈ 0.125 × cylinder_lb × f(temp°F); total = per‑cylinder × active_cylinders. Warn if required > total.
Estimates only. Consider simultaneous starts, inverter behavior, fuel quality, altitude, temperature, cylinder vaporization, and manufacturer specs.
How to Use This Calculator
- Add appliances. Enter running watts, starting surge watts per unit, quantity, voltage type, and leg assignment.
- Pick your system voltage, desired continuous load percent, and PF for kVA.
- Use altitude presets or input your own altitude and temperature to see derated recommendations.
- Enter fuel details including cylinder count for propane to estimate runtime.
- Review transfer switch guidance, then export CSV or PDF for records.
Rule of thumb: Keep legs within ~20% of each other for 120/240 V panels; aim for 70–80% continuous loading for longevity and efficiency.
Quick Example Appliances
| Appliance | Running W | Starting W | Volt |
|---|---|---|---|
| Refrigerator (modern) | 180 | 1200 | 120 |
| Window AC 1 Ton | 1000 | 2000 | 120 |
| Well Pump 1/2 HP | 1000 | 2000 | 240 |
| Microwave Oven | 1100 | 1100 | 120 |
| LED Lights (10×) | 60 | 60 | 120 |
| Wi‑Fi Router | 10 | 10 | 120 |
| Laptop (charging) | 60 | 60 | 120 |
| TV 50″ | 120 | 120 | 120 |
Values are typical estimates; check nameplates for exact ratings. For 120/240V, place 240V loads first, then distribute 120V loads to balance legs.
FAQs
Most homes start big motors at different times. The highest starting surge usually dominates momentary demand. If multiple motors may start together, add their surges accordingly.
Running a generator at roughly 70–80% of rated output improves efficiency, noise, voltage regulation, and lifespan. The calculator sizes your recommendation to hit your chosen load percent.
kW measures real power while kVA is apparent power. Their ratio is power factor. Many generators are rated in kVA assuming PF≈0.8; resistive loads may approach PF≈1.0.
Loads like deep well pumps or large AC units often need 230–240 V. If you only power 120 V circuits, choose 120 V. Always match voltage to your equipment and transfer switch.
They depend on engine size, carburetion, alternator type, ambient conditions, and load profile. Enter manufacturer consumption numbers for best results; defaults are typical.