The right generator for construction is sized to the site’s real running load at roughly 70 to 90 percent of its rated capacity, runs on diesel unless gas service is already connected, and can ship now instead of waiting on a manufacturer build queue. Start with your voltage, amps, and largest motor loads. Then decide whether to rent, buy used or surplus, or order new.
The rest of this guide takes those decisions in the order most contractors and project managers run into them. If you already know what you need, browse construction generators in stock or request a quote.
What size generator do I need for a construction site?
Sizing starts with the load, not the site. Have your electrician confirm the voltage your site runs at and the amps within that voltage, then list everything that will run at the same time. Once you have voltage and amps, a generator can be sized. Our generator size calculator converts between kW, kVA, and amps.
Smaller sites often land around 100kW. On projects of $20 million and above the requirement climbs fast, because every subcontractor needs to work at once, and big sites often need more than one voltage.
- Target about 70 to 90 percent of the unit’s rated capacity, a common rule of thumb also used by Cat Rental. A 200kW running load points to a unit somewhere around 225 to 285kW.
- Account for startup loads. Cranes, compressors, pumps, and welders draw far more when they start than when they run, and a unit sized only to the running total can bog down or trip. Give your supplier the equipment list, not just a total.
- Leave 10 to 20 percent for growth. Trailers, tools, and temporary lighting tend to show up as a project ramps.
- Don’t go far oversized. A badly underloaded diesel can wet stack, which means unburned fuel and carbon building up in the exhaust. Load bank testing is the fix once a unit is in service.
- Expect altitude and heat to cut output. Ask for the derated figure at your site’s elevation and temperature, which matters in Colorado, Florida, and Texas alike.
Diesel or natural gas for a construction generator?
Diesel is the default on most jobsites. The fuel sits in the unit’s tank, so it doesn’t depend on a gas connection, and a new site often doesn’t have one yet. Natural gas fits when reliable gas service is already at the site and the project would rather skip fuel deliveries. Our full diesel vs natural gas buyer’s guide goes deeper.
| Diesel | Natural gas | |
|---|---|---|
| Fuel supply | Stored on site, delivered by truck | Piped in, needs adequate supply and pressure at the site |
| Best fit | New or remote sites, towable units, heavy or sudden loads | Sites with gas already connected, long runs, urban or noise-sensitive areas |
| Load response | Handles big load swings well | Can need a larger unit for the same output |
| Upfront cost | Generally lower | Often higher for comparable output |
| Emissions and noise | Higher, so check local rules | Generally cleaner and quieter |
| Watch for | Fuel storage and fuel quality on long projects | Gas supply interruptions and underloading |
Does a generator for construction need to be Tier 4 Final?
It depends on how the unit is classified. A trailer or skid-mounted generator that moves between sites is generally treated as a nonroad engine, and new nonroad diesel engines have to meet EPA Tier 4 Final standards. Stationary emergency standby units are treated differently and can be Tier 2 or Tier 3 under federal rules, while stationary units used for non-emergency power face stricter requirements. State and local rules can be tighter still, California most of all. Caterpillar’s Tier 4 white paper lays out the federal framework.
Tell your supplier where the unit will run, for how many hours, and whether it will move. Compliance follows the engine’s certification at the time it was built, so on a used towable unit, read the EPA label on the engine and check with your local air quality agency before you commit.
Should I rent or buy a generator for construction?
Rent when the project is short or the load is still uncertain. Buy when you run projects back to back or the unit will keep working after this job. A generator is an asset that holds value, so a used or surplus unit bought well can come back out at resale when you’re done.
| Rent | Buy used or surplus | Buy new | |
|---|---|---|---|
| Best for | Short projects, uncertain loads, a single phase of work | Back-to-back projects, long timelines, permanent standby | Specific spec, warranty, or emissions requirements |
| Upfront cost | Lowest | Lower than new | Highest |
| Availability | Fast when the size is on hand | Often in stock and ready to ship | Manufacturer build queue |
| After the project | Return it | Keep it or resell it | Keep it or resell it |
| Watch for | Who services and fuels it | Hours, condition, and inspection | Delivery dates that slip |
Generator Source rents generators as well as selling them, and a rental only helps if someone keeps it running, so ask who handles service and fuel delivery before you sign. If you’re buying, ask your tax advisor whether Section 179 applies to your purchase.
Why do towable construction generators cost more?
People often assume a towable jobsite unit is just a generator on wheels. It’s built differently. The engine and alternator are made to run continuously under prime power, with better insulation classes on the windings. A voltage selector switch lets one unit run several voltages, and each voltage needs its own breakers. Most also carry 120V and 240V outlets that stay live while the unit runs, and those need their own transformer. If your project doesn’t need that flexibility, a stationary unit on a skid can cost far less, so it’s worth asking whether mobile power is a necessity or a convenience.
How long does it take to get a generator for construction?
A water site in the Denver area ordered a new generator from the manufacturer almost two years ago. The build finished. The generator still hadn’t arrived, and the site kept running on temporary power.
Manufacturer lead times move with tariffs, parts, and production schedules, and the date promised at order doesn’t always hold. For a general contractor, that delay lands on the whole project.
Buying from stock avoids most of it. Generator Source buys and owns its inventory outright and keeps roughly 14 acres of it in Brighton, so large units are often ready to load on a truck. Every unit goes through a 31-point inspection before it’s sold, and units on the lot can be modified before they ship, with distribution panels, lighting, onboard load banks, battery heaters, or control panel changes. Surplus is common too, since customers order equipment for projects like telecom sites or surgical centers and then the site moves or the requirements change. Long relationships with manufacturers mean we’re often one of the first calls when an order is canceled.
The best time to buy is before you need it. Ask about lead time during design and procurement, not at closeout. You can browse new surplus, used, and portable units now.
What to have ready before you request a quote
- Voltage and amps from your electrician
- Your equipment list, including anything with a big startup load
- Diesel or natural gas, and whether gas service is connected yet
- Where the unit will run, for how long, and whether it will move
- Rent, buy used, or buy new
- Whether the unit is in stock, with the delivery timeline in writing
- Who will service and fuel it
Frequently asked questions
What size generator do I need for a construction site? Get the voltage and amps from your electrician, list everything that will run at the same time, and size the unit so that load lands at about 70 to 90 percent of its rated capacity. Add 10 to 20 percent for growth, and account for the startup draw of big motors like cranes, compressors, and pumps.
Is diesel or natural gas better for a construction generator? Diesel is the usual choice because the fuel is stored in the unit and doesn’t depend on a gas connection. Natural gas makes sense when reliable gas service is already at the site and you want cleaner, quieter operation with no fuel deliveries.
Can I use a used generator on a construction site? Yes, and used and surplus units are often available sooner and for less than new. For a towable unit, check the engine’s EPA emissions label and confirm it meets the rules where you’re working, since emissions compliance is set by the engine’s certification at manufacture.
How long does it take to get a generator for construction? Manufacturer lead times vary with parts availability and production schedules, and delivery dates promised at order don’t always hold. Units in stock can be sized, configured, and shipped much sooner. Generator Source keeps roughly 14 acres of inventory in Brighton, Colorado.
What voltages do construction generators run? Many towable construction generators have a voltage selector switch that lets one unit run 120/240 single phase, 120/208 three phase, or 277/480 three phase. Confirm which voltages your site needs before you request a quote.
Generator Source sells, services, rents, and buys commercial and industrial generators from 20kW to 4MW+, with headquarters in Brighton, Colorado and branches in Jacksonville and Pensacola, Florida, and Austin, Texas. Send us your voltage, amps, and project timeline, and our team will help you find a unit that fits.
Request a quote | Size your generator | Browse construction generators
Sources
- U.S. EPA nonroad diesel engine standards (40 CFR Part 1039) and stationary engine rule (40 CFR Part 60, Subpart IIII)
- Caterpillar, EPA Tier 4 and the Electric Power Industry
- Cat Rental, Generators for Construction