How to Size a Generator for Your Jobsite: A Contractor's Guide

Why Generator Sizing Matters More Than You Think

A generator that's too small trips breakers, damages sensitive equipment, and sends your crew home early. One that's too large burns excess fuel and costs you money every hour it runs. Getting the size right is one of the most important decisions you'll make before equipment hits your site.

The good news: sizing a generator isn't complicated once you understand the basics. Here's the method we walk every customer through before they take delivery.

Step 1: List Every Load You'll Run

Start with a complete inventory of every piece of electrical equipment on your site. Don't guess — pull the nameplates or spec sheets. For each item, you need two numbers:

• Running watts (kW): The continuous power draw during normal operation
• Starting watts (kW): The surge demand when a motor or compressor kicks on — typically 2–3× the running load

Common jobsite loads and their approximate running draws:

• Portable air compressor (5 HP): 3.7 kW running / 9–11 kW starting
• Concrete mixer (1/2 HP): 0.4 kW running / 1.2 kW starting
• Submersible pump (1 HP): 0.75 kW running / 2.2 kW starting
• Portable light tower: 2–4 kW running
• Welding machine (200A): 8–12 kW running
• Temporary office trailer (HVAC + lighting): 5–15 kW running

Step 2: Calculate Your Peak Demand

Add up all running loads that will operate simultaneously. Then identify your single largest motor load — that's your starting surge. Your generator needs to handle the total running load plus the starting surge of the biggest motor without dropping voltage.

Example: If your total running load is 45 kW and your largest motor (a 10 HP pump) has a 22 kW starting surge, you need a generator rated for at least 67 kW. In practice, you'd spec a 75 kW or 80 kW unit to give yourself a 15–20% headroom buffer.

Step 3: Account for Power Factor

Most generator ratings are in kVA (kilovolt-amperes), not kW. For typical construction loads with a 0.8 power factor, multiply your kW requirement by 1.25 to get the kVA rating you need. A 75 kW load requires a generator rated at approximately 94 kVA.

Step 4: Consider Your Fuel Runtime

A generator running at 75% load burns significantly more fuel than one at 50% load. If you're on a remote site or need 24/7 operation, factor in tank capacity and refueling logistics. Our larger units carry 100–500 gallon sub-base tanks for extended runtime without interruption.

Common Sizing Mistakes to Avoid

• Forgetting motor starting loads: This is the #1 cause of generator trips on jobsites. Always account for starting surge.
• Not planning for growth: If you're adding equipment mid-project, size for the peak load you'll eventually reach.
• Ignoring altitude and temperature: Generators derate at high altitude and in extreme heat. At 5,000 feet elevation, expect 10–15% less output.
• Skipping the load bank test: For critical applications, ask us about load bank testing before delivery to confirm the unit performs to spec.

When to Call Us Instead of Calculating

For complex sites — multiple buildings, phased construction, or mixed 120V/240V/480V loads — let our team run the numbers. We've sized power solutions for data center builds, large-scale construction projects, and emergency response operations across the country. A 15-minute call can save you a costly swap mid-project.

Call us at 888-702-0440 or request a quote online. We'll spec the right unit and have it on your site when you need it.