Generator Sizing Guide: How to Choose the Right Generator for Your Project

When hiring or purchasing a generator, choosing the correct size is critical. An undersized generator can lead to overloads, equipment damage, and costly downtime, while an oversized unit can result in unnecessary fuel consumption and increased hire costs.

This guide will help you understand how to properly size a generator and select the right accessories to ensure safe, efficient, and reliable power for your project.

Why Generator Sizing Matters

Generators are designed to supply a specific amount of power. Selecting the correct generator ensures:

• Reliable operation without overloading
• Reduced fuel consumption
• Improved equipment protection
• Lower operating costs
• Better overall site efficiency

Whether you need temporary power for a construction site, event, emergency backup, or industrial operation, proper sizing is the first step towards dependable performance.

Understanding Generator Power Ratings

Generator output is commonly measured in:

kVA (Kilovolt-Amperes)

The apparent power a generator can produce.

kW (Kilowatts)

The actual usable power consumed by equipment.

A common calculation is:

kW = kVA × Power Factor (PF)

Most generators operate at a power factor of 0.8.

Example:

• 100 kVA Generator
• 100 × 0.8 = 80 kW usable power

Step 1: Calculate Your Total Load

Begin by listing all equipment that will be running simultaneously.

For example:

EquipmentRunning PowerSite Cabins10 kWLighting5 kWPower Tools15 kWPumps20 kWWelfare Unit5 kW

Total Load = 55 kW

Step 2: Consider Starting Currents

Many electrical devices require significantly more power when starting than when running.

Equipment that typically has high startup currents includes:

• Electric motors
• Pumps
• Compressors
• Refrigeration units
• Air conditioning systems

A motor may require 2–6 times its running current for a short period at startup.

Failure to account for these surges can cause:

• Voltage drops
• Generator trips
• Equipment malfunction

This is one of the most common reasons generators are undersized.

Step 3: Add Spare Capacity

It is generally recommended to allow additional capacity beyond your calculated load.

Recommended Spare Capacity

• Minimum: 15%
• Preferred: 20–25%

Example:

55 kW Load

55 kW + 20%

= 66 kW Required Capacity

A generator rated around 80–100 kVA would typically be suitable depending on startup requirements.

Step 4: Determine Single-Phase or Three-Phase Power

Single-Phase

Commonly used for:

• Homes
• Small offices
• Small events
• Basic site equipment

Three-Phase

Required for:

• Industrial machinery
• Larger pumps
• Construction sites
• Manufacturing operations

Selecting the correct phase configuration is just as important as generator sizing.

Common Generator Applications

Construction Sites

Typical loads include:

• Site cabins
• Welfare units
• Lighting towers
• Power tools
• Dewatering pumps

Popular sizes:

• 20–60 kVA
• 100–200 kVA
• 250 kVA+

Events and Festivals

Power requirements often include:

• Stage power
• Caterers
• Lighting
• Sound systems
• Temporary facilities

Popular sizes:

• 20–100 kVA for smaller events
• 100–500 kVA for larger events

Emergency Backup Power

Businesses often require power for:

• Servers
• Refrigeration
• Security systems
• Manufacturing processes
• Critical operations

A professional load assessment is recommended to identify essential circuits and determine the correct generator size.

Generator Accessories: What Else Will You Need?

The generator itself is only part of the power solution. The correct accessories ensure efficient operation, site safety, and uninterrupted power supply.

Generator Cables

High-quality generator cables are essential for safely distributing power.

Options typically include:

• Single core power cables
• Distribution cables
• Extension leads
• Armoured cable solutions

When sizing cables, factors include:

• Generator output
• Cable length
• Voltage drop
• Load requirements

Using incorrectly sized cables can lead to overheating and power loss.

Fuel Tanks and Bunded Fuel Solutions

For long-term projects, external fuel tanks can significantly reduce refuelling requirements.

Benefits include:

• Extended run times
• Reduced site visits
• Improved fuel management
• Increased productivity

Available options often include:

• Bunded fuel tanks
• Long-run fuel tanks
• Fuel bowsers
• Automated fuel monitoring systems

These solutions are particularly useful for remote construction sites and critical standby applications.

Distribution Boards

Distribution boards allow power to be safely distributed across multiple circuits.

Common options:

• 63A distribution boards
• 125A distribution boards
• 400A distribution boards
• Event power distribution systems

These units help provide safe and compliant power distribution on site.

Automatic Transfer Switches (ATS)

An ATS automatically starts the generator and transfers power in the event of a mains failure.

Ideal for:

• Hospitals
• Data centres
• Retail facilities
• Manufacturing plants
• Commercial buildings

Benefits include minimal downtime and seamless power restoration.

Load Banks

Load banks are used to:

• Test generator performance
• Commission standby generators
• Prevent wet stacking
• Verify generator output

They are particularly important for backup generators that operate infrequently.

Lighting Towers

For construction sites and outdoor events, pairing generators with lighting towers provides:

• Improved site safety
• Increased productivity
• Enhanced visibility
• Reduced downtime

LED lighting towers offer excellent fuel efficiency and illumination.

Remote Monitoring Systems

Modern generators can be equipped with monitoring systems that provide:

• Live performance data
• Fuel level tracking
• Fault alerts
• GPS tracking
• Usage reports

This helps site managers maximise uptime and reduce operating costs.

Let Our Experts Help Size Your Generator

Every project is different. Factors such as starting loads, fuel requirements, cable runs, site conditions, and future expansion all influence generator selection.

Our team can help you:

✅ Calculate your power requirements

✅ Select the correct generator size

✅ Specify suitable cables and distribution equipment

✅ Choose the right fuel tank solution

✅ Ensure compliance with site safety requirements

✅ Provide a complete temporary power package

Whether you need a small portable generator, a large construction site power solution, or emergency backup power, we can provide expert advice and reliable equipment to keep your project running smoothly.

Contact us today to discuss your generator hire or power generation requirements and find the right solution for your site.