Generator Sizing: How to Calculate the Correct KVA for Your Load
Choosing the right generator size is critical to avoid equipment damage, costly failures, or inefficient operation. This guide explains the difference between kW and kVA, categorizes load types (resistive vs. inductive), and provides a step‑by‑step calculation method—including the industry‑standard formula that accounts for motor starting surges. With a real‑world example and key considerations like altitude, temperature, and duty cycle, you’ll learn how to determine the exact kVA rating needed for reliable and cost‑effective backup or continuous power.
Generator Sizing Guide: How to Calculate the Right kVA for Your Load
Selecting the correct generator capacity is essential for reliable electrical power. An undersized generator may experience excessive voltage drops, overload trips, or unstable operation. An oversized generator can increase acquisition and operating costs and may contribute to wet stacking in certain diesel engines when operated continuously at very low loads.
The key to generator sizing is understanding kVA (kilovolt-amperes). While kW represents real power, kVA represents apparent power, which accounts for the electrical demands of both real and reactive power.
This step-by-step guide explains how to estimate generator capacity, calculate electrical loads, consider motor starting requirements, and select a suitable generator for projects in Saudi Arabia.
Step 1: Understand Different Electrical Load Types
Before calculating generator capacity, identify the types of electrical equipment that will be connected. Different loads have different power factors and starting characteristics.
1. Resistive Loads
Ideal resistive loads convert electrical energy primarily into heat. Their power factor is approximately 1.0.
- Examples: Electric heaters, incandescent lighting, and industrial heating elements.
- Impact: They generally do not require the large motor-starting currents associated with induction motors.
2. Inductive and Motor Loads
Electrical motors typically require higher current during startup than during normal operation. Motor starting demand is one of the most important considerations when selecting generator capacity.
- Examples: Air conditioning units, water pumps, air compressors, conveyors, and elevators.
- Impact: Direct-on-line starting currents can be several times greater than normal running current. The actual starting demand depends on motor design and the starting method.
- Important: Soft starters and variable frequency drives may reduce starting demand, but generator compatibility and power quality must be considered.
Step 2: Understand the Difference Between kW and kVA
One of the most common generator sizing mistakes is confusing real power, measured in kW, with apparent power, measured in kVA.
The relationship between these values depends on the load's power factor (PF).
kW = kVA × PF
kVA = kW ÷ PF
Many generators are rated at a nominal power factor of 0.8. However, the actual power factor of connected equipment may differ and should be obtained from equipment specifications whenever possible.
Example: A generator rated at 100 kVA with a rated power factor of 0.8 corresponds to 80 kW of nominal real power under its specified rating conditions.
Step 3: Calculate the Required Generator Capacity
Start by listing all connected loads, calculating their running requirements, and reviewing motor starting demand.
1. Create an Electrical Load List
Prepare a list of all equipment that may operate simultaneously. Record:
- Running power in kW or kVA.
- Operating voltage, frequency, and phase.
- Power factor where available.
- Motor starting current or starting kVA.
- Equipment quantity and starting method.
- Which loads will operate simultaneously.
2. Convert Voltage and Current into kVA
When equipment ratings are provided in amperes and volts, apparent power can be calculated using the following formulas for balanced electrical loads.
kVA = (V × A) ÷ 1000
kVA = (1.732 × V × A) ÷ 1000
For three-phase calculations, V represents line-to-line voltage and A represents line current.
When the load is provided in kW, divide its real power by its actual power factor to estimate apparent power. Do not automatically assume a power factor of 0.8 for every appliance.
3. Consider Motor Starting Requirements
Starting a large motor can temporarily increase apparent power demand and cause voltage and frequency disturbances.
For a simplified example in which the largest motor starts while all other equipment is already operating:
Estimated Starting Demand = Total Running kVA + (Motor Starting kVA - Motor Running kVA)
This formula estimates apparent power demand during the starting event. It is not sufficient by itself to determine the final generator rating.
Final selection must account for the generator set's transient load acceptance, allowable voltage dip, frequency response, motor starting method, and load sequencing.
Practical Generator Sizing Example for a Workshop
Consider a small workshop with the following electrical equipment. Assume all running loads remain connected when the air compressor starts. The values are illustrative assumptions.
Lighting and Outlets
Running power: 10 kW
Power factor: 1.0
Running load: 10 kVA
Air Conditioning
Running power: 5 kW
Power factor: 0.8
Running: 6.25 kVA
Starting: 25 kVA
Air Compressor
Running power: 7.5 kW
Power factor: 0.8
Running: 9.375 kVA
Starting: 37.5 kVA
A. Calculate Total Running kVA
10 + 6.25 + 9.375 = 25.625 kVA
B. Identify the Largest Starting Surge
The compressor has the highest starting kVA in this example.
37.5 - 9.375 = 28.125 kVA
C. Estimate Apparent Demand During Startup
25.625 + 28.125 = 53.75 kVA
This does not automatically mean that a 55 or 60 kVA generator will be suitable. Transient voltage limits, alternator response, engine performance, and motor starting characteristics may require a larger generator or a different starting arrangement.
The final generator selection should be verified using manufacturer motor-starting performance data and the actual load profile.
Important Generator Sizing Factors in Saudi Arabia
1. Site Elevation and Derating
Air density decreases at higher elevations, potentially reducing the available output of internal combustion engines. The required derating depends on engine design, turbocharging, and manufacturer specifications.
There is no universal altitude derating percentage suitable for every generator. Always consult the manufacturer's performance and derating tables.
2. High Ambient Temperature
High temperatures may reduce engine and alternator performance and affect cooling system capacity. This is especially relevant for generator installations in hot climates across Saudi Arabia.
Confirm the generator's ambient temperature rating, ventilation requirements, and required cooling clearances.
3. Generator Duty Rating: Standby, Prime and Continuous
- Emergency Standby Power (ESP): Typically intended for emergency backup during utility outages, subject to applicable operating-hour and load limits.
- Prime Rated Power (PRP): Suitable for extended operation with variable electrical loads under the manufacturer's specified conditions.
- Continuous Operating Power (COP): Intended for continuous operation at a constant load under its specified rating.
Do not assume that every generator provides a 10% overload allowance or identical annual operating-hour limits. Always check the exact model rating.
4. Future Load Expansion
If additional electrical equipment will be installed later, consider those future requirements during generator selection.
However, excessive oversizing should be avoided because prolonged operation at very low load can reduce efficiency and increase maintenance requirements.
Generator Sizing Checklist Before Selecting a Unit
Final Recommendations for Selecting Generator Capacity
Important: Simplified sizing calculations are useful for preliminary estimates but should not replace engineering verification for large motors, VFDs, UPS systems, or sensitive electrical equipment.
Learn more about our generator services or visit our contact page to discuss your project requirements.
Need Help Selecting the Right Generator Capacity?
Contact Ramz Rikaz Al Joudah Trading Est. (RJTE) to discuss your electrical load requirements, project duration, site location, and available equipment solutions. Final electrical design should be verified by a qualified professional where required.
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