Choosing the correct generator size isn’t just about keeping the lights on — it’s about protecting your equipment, saving fuel, and ensuring reliability. Proper generator sizing requires understanding power demand, power factor, and starting surges.
This detailed guide walks you through the entire process with formulas, calculations, and real-world examples — perfect for homeowners, engineers, or facility managers.
🔹 Step 1: Understand the Key Terms
⚙️ Real Power (kW) vs Apparent Power (kVA)
Generators are rated in kVA (kilovolt-amperes), while most electrical loads are rated in kW (kilowatts).
The relationship between them depends on the power factor (pf):
$$\text{kVA} = \frac{\text{kW}}{\text{Power Factor (pf)}}$$
Typical values:
- US industrial & commercial loads: pf ≈ 0.8
- Residential & purely resistive loads: pf ≈ 1.0
So, a 100 kW load at pf = 0.8 requires:
$$\frac{100}{0.8} = 125 \text{ kVA generator}$$
⚙️ Running Load vs Starting (Surge) Load
Many appliances and motors draw 2 – 6× their normal current for a few seconds during startup.
- Air conditioners: 2×
- Compressors: 3×
- Submersible pumps: 4 – 6×
Your generator must handle these short surges without tripping or voltage drops.
🔹 Step 2: List All Electrical Loads
Create a load inventory including each device’s power rating, voltage, and type.
| Equipment | Voltage | Phase | Running Power | Starting Multiplier |
|---|---|---|---|---|
| Air Compressor | 400 V (EU) / 240 V (US) | 3φ | 5 kW | ×3 |
| Lighting | 230 V / 120 V | 1φ | 2 kW | — |
| Water Pump | 230 V / 240 V | 1φ | 1 kW | ×4 |
| Computers & Office | 230 V / 120 V | 1φ | 1.5 kW | — |
If only current (A) is known, convert to power:
- Single-phase: $$P = V \times I \times pf$$
- Three-phase: $$P = \sqrt{3} \times V \times I \times pf$$
🔹 Step 3: Calculate the Total Running Load
$$P_{\text{run, total}} = \sum P_i$$
Example:
$$P_{\text{run, total}} = 5 + 2 + 1 + 1.5 = 9.5 \text{ kW}$$
🔹 Step 4: Add the Starting (Surge) Load
For the 5 kW air compressor (3× surge):
$$\text{Starting Surge} = (3 - 1) \times 5 = 10 \text{ kW extra}$$
Total peak demand:
$$P_{\text{total}} = 9.5 + 10 = 19.5 \text{ kW}$$
🔹 Step 5: Convert to kVA (Generator Rating)
Assume pf = 0.8:
$$\text{kVA} = \frac{19.5}{0.8} = 24.38 \text{ kVA}$$
🔹 Step 6: Add a Safety Margin
Generators should not run at 100 % continuously. Add 10–25 % margin:
$$\text{kVA}_{\text{final}} = 24.38 \times 1.15 = 28.04 \text{ kVA}$$
Round up → 30 kVA generator
🔹 Step 7: Adjust for Local Standards
| Parameter | United States | Europe |
|---|---|---|
| Frequency | 60 Hz | 50 Hz |
| Common Voltage (1φ / 3φ) | 120/240 V or 120/208 V / 277/480 V | 230 V / 400 V |
| Plug / Connector Standards | NEMA | IEC |
| Fuel Type | Gasoline, Diesel, Natural Gas | Diesel, LPG, Natural Gas |
| Derating Factors | >1,000 m altitude | >1,000 m |
🔹 Step 8: Example – US Home Backup System
A US homeowner wants a standby generator for these loads:
| Appliance | Voltage | Running (W) | Surge (W) |
|---|---|---|---|
| Refrigerator | 120 V | 700 | 2,100 |
| Lighting & TV | 120 V | 1,000 | — |
| Central A/C | 240 V | 3,000 | 9,000 |
| Water Pump | 240 V | 1,500 | 4,500 |
| Misc. outlets | 120 V | 1,000 | — |
Total running load: 700 + 1,000 + 3,000 + 1,500 + 1,000 = 7,200 W
Largest surge load: A/C = 9,000 W (already includes running)
Total peak: $$7,200 + (9,000 - 3,000) = 13,200 \text{ W} = 13.2 \text{ kW}$$
Convert to kVA (pf = 0.8): $$\frac{13.2}{0.8} = 16.5 \text{ kVA}$$
Add 20 % margin → ≈ 20 kVA generator (≈16 kW)
🔹 Step 9: Common Mistakes to Avoid
- Ignoring motor starting loads — the #1 sizing error.
- Selecting generator by energy (kWh) instead of power (kW) — wrong basis.
- No margin — continuous overload leads to early failure.
- Ignoring site derating — high temperature or altitude reduces power output.
- Voltage mismatch — always match your system voltage & frequency.
🔹 Step 10: Derating for Altitude and Temperature
| Condition | Typical Derating |
|---|---|
| +10 °C above 25 °C | −1 % |
| +1000 m above sea level | −3 % |
| +2000 m | −6 % |
Example: 30 kVA at 2,000 m → $$30 \times (1 - 0.06) = 28.2 \text{ kVA}$$
🔹 Step 11: Single-Phase vs Three-Phase Selection
- Single-phase (230 V EU / 120–240 V US): Homes, offices, small shops.
- Three-phase (400 V EU / 208–480 V US): Industrial machinery, HVAC systems, motor loads.
🔹 Step 12: Prime vs Standby Rating
| Type | Description | Use Case |
|---|---|---|
| Standby | Backup during outages (not continuous). Rated for 70–80 % average load. | Residential, emergency |
| Prime | Continuous operation with varying load. | Industrial, off-grid |
| Continuous | 100 % constant load, 24/7 use. | Process plants, remote power |
🔹 Step 13: Practical Generator Selection Tips
- Load Management: Use ATS for safe switching.
- Soft Starters / VFDs: Reduce motor starting current.
- Fuel Efficiency: Diesel for heavy loads, gas for residential.
- Future Expansion: Add 15–20 % extra capacity.
- Service & Maintenance: Choose brands with local support.
🧮 Quick Reference Formulas
- Single-phase: $$P = V \times I \times pf$$
- Three-phase: $$P = \sqrt{3} \times V \times I \times pf$$
- Convert to kVA: $$\text{kVA} = \frac{P_{\text{kW}}}{pf}$$
- Add margin: $$\text{kVA}_{\text{final}} = \text{kVA} \times (1 + \text{margin})$$
❓ Frequently Asked Questions (FAQs)
Q1: What’s the best power factor to assume?
For mixed loads, use pf = 0.8. For purely resistive loads (heaters, lamps), use pf = 1.0.
Q2: Should I oversize the generator?
Slightly — yes. Oversize by 10–25 % to avoid overload and allow for future expansion.
Q3: What if I use both 120 V and 240 V loads (US)?
Choose a split-phase (120/240 V) generator with a proper transfer switch.
Q4: Can I connect a 50 Hz generator to a 60 Hz load (or vice versa)?
No. Frequency affects motor speed and voltage. Always match frequency to your region.
Q5: How often should I test my generator?
Run it monthly for 15–30 minutes under load to keep the engine and battery healthy.
✅ Final Thoughts
Correctly sizing your generator ensures stable voltage, long equipment life, and fuel efficiency.
➡️ List loads → Add surges → Convert to kVA → Add margin → Choose next standard size.
