Rent the wrong size generator and you’re either running an overloaded unit that shuts down mid-shift or burning fuel on capacity you never needed. Neither option is cheap. Getting the sizing right before your equipment arrives saves you the headache, and this guide will walk you through exactly how to do it.
Start With Your Total Power Load
Generator sizing comes down to one number: your total running wattage, with enough overhead for startup surges.
Every piece of electrical equipment on your site has two wattage figures. Running watts is what the equipment draws during normal operation. Starting watts (also called surge watts) is the spike it pulls when it kicks on, typically two to three times higher for motors and compressors. Your generator needs to handle the starting load of the largest motor on site while simultaneously powering everything else that’s already running.
To get your total:
- List every piece of equipment that will run on generator power
- Find the running watts for each (check the nameplate or equipment manual)
- Identify the single item with the highest startup surge
- Add running watts for everything, then add the startup surge for the biggest motor
That final number is your minimum generator requirement. Choosing a unit with 20 to 25 percent more capacity than that minimum gives you room to add equipment or handle variable loads without stressing the machine.
Common Power Loads on Colorado Construction Sites
To give this some grounding, here are typical wattage ranges for equipment commonly found on Colorado job sites:
Lighting: A string of work lights or a light tower draws 500 to 2,000 watts depending on configuration. LED towers are on the lower end.
Power tools: A circular saw runs around 1,400 to 1,800 watts. An angle grinder is similar. A large air compressor can pull 1,500 to 3,000 running watts with a startup surge three times that.
Portable heaters: Indirect-fired heaters used on cold-weather job sites here in Colorado typically run off their own fuel supply, so they don’t add much electrical load. Electric heaters are a different story and can draw 1,500 watts or more per unit.
Welding equipment: A MIG welder can require 3,000 to 8,000 watts depending on amperage. TIG and stick setups vary widely. If you’re running welding alongside other loads, this is typically your biggest draw.
Trailers and site offices: A temporary office trailer with lighting, a computer, and a small HVAC unit can run 3,000 to 5,000 watts sustained.
What Generator Size Covers What?
Here’s a practical range guide based on the fleet available for rental in Denver:
15KW (15,000 watts): Suited for smaller sites with a focused load. Light towers, basic power tools, and a trailer office can typically be managed at this size if you’re not running them all simultaneously at full draw. Good for a small crew doing finish or light construction work.
20KW: Adds enough headroom to handle a moderate combination of tools, lighting, and site infrastructure. This is a common choice for mid-size residential or commercial work where the load is predictable and manageable.
36KW: Where things start to get serious. A 36KW unit can power a well-equipped construction trailer, multiple power tool circuits, and a few larger pieces simultaneously. Good for general contracting sites with varied load profiles.
45KW to 75KW: For larger commercial sites, multi-trade operations, or any job requiring redundant or backup power alongside active site loads. These units give you the overhead to run heavy equipment, multiple welding stations, or a generator-fed temporary power distribution panel without worrying about overloading.
Does Altitude Affect Generator Output in Colorado?
Yes, and this catches some crews off guard. Internal combustion engines lose power as elevation increases because there’s less oxygen available for combustion. A generator rated at sea level loses roughly 3.5 percent of its output for every 1,000 feet of elevation gain.
When it comes to generator rentals in Denver, altitude matters. Denver sits at 5,280 feet, so a generator nominally rated at 70KW at sea level is producing closer to 52KW at Denver elevation if it hasn’t been derated for altitude. Mountain job sites in Summit County or along the I-70 corridor at 9,000 to 11,000 feet lose even more capacity.
When you’re sizing a rental generator in Colorado, it’s worth confirming with your rental provider whether the rated output reflects altitude-adjusted capacity or sea-level ratings. The team at One Call Rentals can advise on this for any unit in the fleet.
What If You’re Not Sure?
Call and ask. That’s the straightforward answer. A good rental provider will ask you the right questions: What’s running? How many simultaneous circuits? Do you have motor loads? What’s the site elevation?
If you try to size it yourself and round down to save on rental cost, you risk running the generator beyond its rated load. That causes voltage sag, overheating, and eventual failure. Renting up a size costs less than a day of downtime.
How Long Is Your Project? That Affects Generator Selection Too
Short-term jobs of a few days call for a different calculus than a multi-week project. For longer runs, you want a unit with proven fuel efficiency at your expected load level. Running a 75KW generator at 20 percent load burns fuel without delivering value. Matching your expected sustained load to the right unit size saves on fuel costs across the duration of a project.
On longer projects, on-site diesel delivery to keep the generator fueled is also worth factoring into your planning. Sending a driver off-site to fill fuel cans is a productivity drain that compounds fast.
FAQ: Generator Sizing for Job Sites
Will a 20KW generator run a construction site?
It depends on the site. A 20KW unit works well for smaller sites with moderate, predictable loads, such as basic tools, lighting, and a trailer office. Larger sites with welders, heavy air compressors, or multiple simultaneous circuits will typically need 36KW or more.
Does altitude reduce generator output in Colorado?
Yes. Generators lose approximately 3.5 percent of rated output per 1,000 feet of elevation. At Denver's 5,280 feet, that's a meaningful reduction from sea-level ratings. At higher elevations in the mountains, the derating is more significant. Always confirm whether the generator you're renting is rated at altitude or sea level.
What is the most common generator size rented for construction sites?
Mid-range units in the 36KW to 75KW range are the most common for commercial construction. Smaller residential or specialty jobs often run on 15KW to 20KW. The right size depends entirely on your specific load calculation.
Can I add a second generator if my load grows?
Yes. Rental generators can be deployed in parallel configurations to handle load growth on longer projects. If your scope increases, a rental provider can bring in additional units or swap to a larger unit depending on site logistics.
Final Thoughts
For job sites across the Front Range and beyond, the right generator size is the difference between a productive day and a scramble. If you’re planning a project and need help dialing in the right unit, reach out to the team at One Call Rentals. We have a high-quality fleet, the local knowledge, and the 24/7 support to keep your site powered and on schedule.