Every construction site runs on two things: a solid plan and reliable power. From the moment the site trailer is dropped off to the final interior finishes, consistent electricity is the invisible force that drives progress. It powers the welders, runs the water pumps, lights the site, and keeps the tower crane lifting.

Choosing the right generator isn’t just a preliminary task—it’s a critical decision that impacts your timeline and budget. In our decades of experience outfitting jobsites, we’ve seen firsthand how the wrong generator leads to costly delays, damaged equipment, and wasted fuel. An undersized unit will constantly trip, while an oversized one burns through diesel without providing any extra value.

This guide provides a straightforward, step-by-step process for selecting the right mobile generator to keep your project powered, productive, and profitable.

We’ll cover:

• How to accurately calculate your site’s total power needs (including motor startup).
• Understanding the difference between prime and standby power.
• Key features to look for in a durable, site-ready generator.

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Chapter 1: Sizing Your Generator, The Four-Step Method

This is the most critical step. Getting this right will save you headaches for the entire duration of the project.

Step 1: List All Your Electrical Equipment
Walk your site plan and list every single piece of equipment that will need power simultaneously. Be thorough.

• Site Office: Computers, A/C unit, microwave, lights.
• Tools: Air compressors, welders, concrete mixers, grinders, saws.
• Heavy Equipment: Tower cranes, man lifts, water pumps, submersible pumps.

Step 2: Find the Starting & Running Watts for Each Item
Every piece of equipment has two power ratings:

• Running (Rated) Watts: The continuous power it needs to operate.
• Starting (Surge) Watts: The extra burst of power an electric motor needs just to get started. This can be 2 to 3 times the running watts.

You can find these numbers on the equipment’s compliance plate or in its manual. If you can’t find them, this chart provides typical values for common tools:

Equipment	Estimated Running Watts	Estimated Starting Watts
Air Compressor (1 HP)	1,600W	4,500W
Welder (225 Amp)	7,000W	7,000W (No surge)
Submersible Pump (1.5″)	1,800W	3,000W
Tower Crane (150-ton)	120,000W	250,000W
Site Trailer (AC/Heat)	4,000W	6,000W

Step 3: Calculate Your Total Power Requirement
This is where most mistakes are made. Use this simple formula:

1. Add up the Running Watts of all the equipment you listed.
2. Identify the one piece of equipment with the Highest Starting Watts.
3. Your total requirement is: (Total Running Watts) + (Highest Starting Watts).

Example:

• You need to run an air compressor (1,600 Running / 4,500 Starting) and a submersible pump (1,800 Running / 3,000 Starting).
• Total Running Watts: 1,600 + 1,800 = 3,400W.
• Highest Starting Watts: The air compressor at 4,500W. (The additional surge needed is 4,500 – 1,600 = 2,900W).
• Total Requirement: (3,400 Running Watts) + (2,900 Additional Starting Watts) = 6,300 Watts.

Step 4: Convert to Kilowatts (kW) and Add a Buffer
Divide your total watts by 1,000 to get kilowatts (6,300W=6.3kW).

Expert Tip: Never plan to run a generator at 100% of its capacity. For safety and efficiency, add a 20-25% buffer. For a 6.3kW need, you should look for a generator rated for at least 8kW.

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Chapter 2: Prime Power vs. Standby Power

Not all generators are built for the same job.

• Standby Generators are for emergency backup power. They are designed to run only when the utility grid fails.
• Prime Power Generators are designed to be the main source of power, running for long hours every day.

Nearly every construction site requires a prime power generator. They are built with more robust engines and components to handle the demands of a full work week. Using a standby-rated generator as a prime power source can lead to premature failure and will void the warranty.

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Chapter 3: Key Features of a Jobsite-Ready Generator

Beyond the power rating, look for these features that signal a generator is built for the harsh realities of a construction site:

• Durability: A heavy-gauge, weatherproof steel enclosure is non-negotiable to protect the engine from dust, rain, and impacts.
• Portability: For most sites, a trailer-mounted (towable) generator is the most practical choice, allowing you to easily position it as the site evolves.
• Fuel Efficiency: Diesel is the standard for a reason. It’s fuel-efficient under heavy load and diesel engines are known for their longevity. A large, integrated fuel tank (100+ gallons) is essential to minimize daily refueling.
• Voltage Selector Switch: This is an incredibly valuable feature, allowing you to switch between single-phase power (for site trailers and standard tools) and three-phase power (for tower cranes and large motors) from the same unit.

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Generator Source in Action: Powering a Multi-Story Build

The Challenge: A general contractor for a new office building needed a single power source for a 150-ton tower crane (three-phase), multiple welding stations, and a two-story site office complex (single-phase).

Our Solution: Instead of the logistical headache of multiple smaller generators, our team analyzed their load list. We recommended a single, towable 500kW generator equipped with a voltage selector switch and a portable distribution panel.

The Result: The generator provided clean three-phase power directly to the crane while the distribution panel supplied multiple 120V and 240V single-phase circuits for the welders and site office. This centralized, fuel-efficient solution simplified their operations and provided flawless power for the entire 18-month project.

Choosing the right generator comes down to a simple process: do the math on your power needs (especially starting watts), confirm you need a prime power unit, and select a durable, versatile machine.

Don’t guess on this critical piece of equipment. The experts at Generator Source can help you analyze your jobsite’s power requirements and recommend the perfect mobile generator to keep your project moving forward.