“If you choose a diesel generator set with a smaller capacity, the machine won’t be able to carry the load and will break down as soon as you start it up. If you choose a larger capacity, you’ll end up wasting money and wasting fuel!”
How do you choose? There’s a scientific formula for this, but many people simply estimate it by multiplying the total power by 1.5, which can lead to significant errors. Today, we’ll use three core formulas and real-world examples to help you choose the right model!
First understand two concepts
Continuous Power (COP):
The generator’s ability to output stable power over a long period of time makes it suitable for 24/7 operation in environments such as hospitals and data centers.
Reserve Power (PRP):
This refers to the power at which a generator can operate at a short-term overload (generally allowing one hour of overload within 12 hours). It is suitable for occasional emergency use.
1. 3 formulas to choose capacity
Formula 1: Calculation Based on Stable Load
Applicable scenario: Stable load, no frequent starting of large motors (such as lighting or IT equipment).
Formula: Generator capacity (kVA) = Total load power (kW) ÷ (Load efficiency × Power factor)
For example: An office building has a total load of 800kW, a load efficiency of 0.85, and a power factor of 0.8: 800 ÷ (0.85 × 0.8) ≈ 1176kVA → Select a 1200kVA generator.
Formula 2: Calculation Based on Maximum Motor Starting
Applicable Scenario: Loads include high-power motors (such as pumps and fans).
Formula: Generator Capacity (kVA) = (Other Load Power + Motor Starting Power × Starting Factor) ÷ Power Factor
Key Points:
1. Starting Factor
For direct start, use a value of 5-7; for star-delta start, use a value of 4; for soft start, use a value of 2.5.
2. Motor Starting Power
Motor Rated Power × Starting Factor
For example: A factory has a 100kW motor (direct start) and 500kW other loads, a starting factor of 7, and a power factor of 0.8: Motor Starting Power = 100 × 7 = 700kW
Total Demand = (500 + 700) ÷ 0.8 = 1500kVA → Select a 1600kVA unit.
Formula 3: Verify by Voltage Drop
Applicable scenario: When the motor starts, you’re concerned about excessive voltage fluctuations (such as with precision equipment).
Formula: Generator Capacity (kVA) = Motor Starting Capacity × Transient Reactance ÷ (Allowable Voltage Drop Ratio)
For example: A laboratory requires a voltage drop of ≤15% during motor starting. The motor’s starting capacity is 900 kVA, and the transient reactance is 0.25: 900 × 0.25 ÷ 0.15 ≈ 1500 kVA → Select a 1500 kVA generator.
2. Pitfall Avoidance Guide: 4 Common Selection Mistakes
Mistake 1: Only considering total power, ignoring motor starting current.
Case study: A construction site purchased a 1000kVA unit with three 110kW water pumps. However, the unit tripped when they started simultaneously!
Cause: The starting current of a single pump is too high (110kW x 7 = 770kW), resulting in an overload.
Correct approach: Start the units in batches or use a soft starter to reduce the starting current.
Mistake 2: Ignoring the Impact of Altitude and Temperature
Case Study: A project in Tibet selected turbines based on plain standards, resulting in insufficient power.
Reason: Power decreases by approximately 10% for every 1,000-meter increase in altitude.
Correct Approach: Select turbocharged turbines for high-altitude locations, or increase capacity by 15%-20%.
Mistake 3: Using Standby Power as Continuous Power
Case study: A supermarket used a PRP unit for 24-hour power supply. The engine failed after three months.
Cause: Prolonged overload operation of the PRP unit can lead to overheating and damage.
Correct approach: For continuous operation, select a COP model!
Mistake 4: Mismatched Load Characteristics
Example: A data center UPS load was capacitive, causing the generator voltage to lose control.
Cause: Capacitive loads require specialized voltage regulation.
Correct: Choose a generator with AVR regulation or increase the capacity by 1.2x.
3. A table helps you quickly select
Load Type |
Recommended Formula |
Amplification Factor |
| Pure lighting/IT equipment | Formula 1 (Stable load) | 1.0 – 1.2 times |
| Motor + small load | Formula 2 (Maximum motor startup) | 1.3 – 1.5 times |
| Precision instruments | Formula 3 (Voltage drop verification) | 1.2 – 1.8 times |
| Plateau/high-temperature environment | Formula 1 + environmental correction | 1.15 – 1.2 times |
| ——END—— | ||
Final reminder:
1. Don’t be cheap! Using a small horse to pull a large cart saves initial costs, but it also increases maintenance costs.
2. Find a professional manufacturer! Submit a loadsheet and have them calculate it for you. This is more reliable than learning it yourself.
