Calculator
Example Data Table
| Device | Qty | Running watts | Starting watts | Power factor |
|---|---|---|---|---|
| Refrigerator | 1 | 700 | 2200 | 0.85 |
| Well pump | 1 | 1000 | 3000 | 0.80 |
| LED lights | 12 | 10 | 10 | 0.95 |
| Microwave | 1 | 1200 | 1200 | 0.98 |
Formula Used
Single phase watts: W = V × I × PF
Three phase watts: W = √3 × V × I × PF
Apparent power: VA = W ÷ PF
kVA: kVA = VA ÷ 1000
Running current, single phase: I = VA ÷ V
Running current, three phase: I = VA ÷ (√3 × V)
Derated output: Rated W × altitude factor × fuel factor
Recommended size: max(running target rating, surge watts) × safety margin
How to Use This Calculator
Enter the generator phase, voltage, rating, and power factor.
Add each connected device with quantity and watt values.
Use starting watts for motors, compressors, and pumps.
Enter derating values when altitude or fuel reduces output.
Press calculate to view load, current, reserve, and sizing.
Use CSV or PDF buttons to save the result.
Power Draw Planning
A generator works best when the load is known before startup. Power draw shows how much electrical work connected devices demand. Running watts describe the normal load after equipment is operating. Starting watts describe the brief surge needed by motors, compressors, pumps, saws, and refrigerators. Both values matter because a generator must handle the highest instant demand without tripping or slowing down.
Why Phase and Power Factor Matter
Single phase generators are common for homes, shops, events, and small sites. Three phase generators are common for larger motors and balanced industrial loads. The same watt load creates different current levels under each phase type. Power factor also changes the apparent load. A low power factor means the generator must supply more apparent power for the same useful output. This is why kVA is useful beside watts.
Safe Capacity Selection
A generator should not run at full rating for long periods. Heat, altitude, fuel quality, extension cords, and poor maintenance can reduce available output. A reserve margin protects the engine and alternator. It also helps voltage stay steadier when a motor starts. Many users target about eighty percent of rated output for continuous use. Sensitive electronics may need even more headroom or a cleaner inverter style supply.
Using Results Wisely
This calculator combines appliance quantities, running watts, starting watts, power factor, efficiency, voltage, and phase. It reports total running demand, surge demand, apparent power, current, load percentage, and recommended capacity. The result is an estimate. Always compare it with nameplate labels and manufacturer instructions. For critical medical, business, or safety loads, ask a qualified electrician to verify wiring, grounding, transfer equipment, and breaker protection.
Good Load Habits
Start the generator first, then connect loads one by one. Start the largest motor load early. Avoid running unnecessary heaters, chargers, and tools together. Keep cords short and properly rated. Balance three phase loads where possible. Recheck fuel consumption during real use. A measured clamp meter reading is helpful after installation. Good planning keeps power stable, lowers wear, and reduces nuisance shutdowns. Record each test run. Note weather, fuel type, voltage, and connected loads. These notes make future sizing easier and support faster troubleshooting during outages. They also reveal hidden demand.
FAQs
1. What is generator power draw?
Generator power draw is the electrical demand placed on the generator by connected loads. It is usually measured in watts or kilowatts.
2. Why are starting watts higher than running watts?
Motors and compressors need extra current when starting. This brief surge can be much higher than the normal running load.
3. Should I size a generator by watts or kVA?
Use both when possible. Watts show real power. kVA includes power factor and helps estimate current and alternator loading.
4. What is a safe continuous load?
Many users keep continuous load near eighty percent of rated output. This leaves reserve for heat, surges, and voltage stability.
5. Does power factor affect generator sizing?
Yes. A lower power factor increases apparent power. That can raise current and stress the alternator even when watts look acceptable.
6. How does altitude affect output?
Higher altitude can reduce engine power because air is thinner. Use a derate percentage from the generator manual when available.
7. Can this calculator replace an electrician?
No. It gives a planning estimate. Transfer switches, grounding, breakers, and critical loads should be checked by a qualified electrician.
8. Why include a safety margin?
A safety margin helps cover unknown loads, starting surges, voltage drop, fuel variation, and future equipment additions.