What Size Generator Does Your Construction Site Actually Need? Running watts, starting watts, kW versus kVA, and why the biggest unit on the lot is usually the wrong answer
Disclosure. We rent generators, and the bigger ones bill more than the small ones. So take it for what it is worth when we tell you that most people asking "what size generator do I need" end up needing less than they think.
Short answer: Add up the running watts of everything that will run at the same time, then add the single biggest starting surge on top, and pick a generator rated comfortably above that total. For a small crew with saws, a compressor and lights, that often lands around 10 to 20 kW. Big three-phase loads, pumps or a site trailer are a different conversation, and an electrician should be part of it.
The generator does not care what you planned to plug in. It cares what you actually plugged in, all at once, at 2 pm in August.
Undersize it and the breaker trips every time the compressor kicks on. Oversize it and you pay for capacity you never use, burn extra fuel, and on a diesel you can actually hurt the engine by running it too lightly. Both are avoidable with a pencil.
Jump to what you came for
Running Watts vs Starting Watts
Every tool has two numbers. Most people only look at one.
Running watts
This is what the tool draws once it is up and spinning. It is the steady load, and it is the number on most nameplates.
Starting watts
Anything with an electric motor pulls a short spike when it starts. Compressors, pumps, table saws, mixers and air conditioners are the usual offenders. As a rough rule of thumb, motor loads can surge to two or three times their running draw for a second or two, and some compressors and pumps pull more than that. Resistive loads like heaters and halogen lights do not surge much.
Peak vs continuous on the generator side
Generators have two ratings too. The bigger number is a short burst. The smaller one is what it can hold all day. Size your running load against the continuous number, and let the peak number cover the surge.
Adding Up the Loads
You only add what runs at the same time. That is the whole trick.
The ranges below are typical, not gospel. Use your own nameplates when you have them.
| Load | Running watts, typical | Starts hard? |
|---|---|---|
| 7 1/4 inch circular saw | 1,200 to 1,500 | Some |
| 10 or 12 inch miter saw | 1,500 to 1,800 | Some |
| Portable air compressor, 1 to 2 HP | 1,500 to 2,500 | Yes, the big one |
| Small submersible pump | 800 to 1,500 | Yes |
| Battery chargers | 100 to 300 each | No |
| LED work lights | 50 to 300 each | No |
| Portable AC unit or site trailer | Varies a lot | Yes |
A worked example
Hypothetical framing crew on an ADU in the Valley. Two circular saws at 1,400 each, one miter saw at 1,800, a compressor at 1,600 running, a bank of chargers at 300, and LED lights at 400. Running total: 6,900 watts. The compressor might surge to around 4,500 when it starts, so add the extra 2,900. Peak: roughly 9,800 watts.
That puts a 10 kW unit right at the edge. It might work. It might also trip every afternoon when both saws and the compressor line up. The honest move is either to stagger the loads or step up a size. If the crew sticks around for weeks, our ADU equipment guide covers the rest of what that kind of build tends to rent.
Leave headroom
A common rule of thumb is to keep your continuous load at roughly 70 to 80 percent of the generator's continuous rating.
Only add the biggest surge once
Motors rarely all start in the same second. Add every running load, then add only the single largest starting surge on top. If two big motors could start together, like a pump on a float switch and a compressor, count both.
kW vs kVA, Without the Lecture
Small gas units are rated in watts or kilowatts. Big diesel units are often listed in both kW and kVA, and the two numbers do not match.
The short version of the difference
kW is the real, usable power. kVA is the apparent power, which includes some that motors and other inductive loads need but do not turn into work. The ratio between them is the power factor. Many larger generators are rated at a power factor of 0.8, so kVA reads higher than kW. A unit listed at 100 kW and 125 kVA is a common example of that.
Amps are where cords and breakers live
For a quick sanity check, watts divided by volts gives amps. A 20 kW load at 240 volts single phase is a little over 80 amps. That number decides what cords, boxes and breakers you need, which is why the next sections exist.
Diesel or Gas
In most rental fleets, including ours, the line falls roughly by size. Small units run on gasoline. Bigger units run on diesel. The choice usually makes itself once you know your load.
Gas units: small, light, simple
Good for a few tools, a pump, some lights. The tradeoff is fuel burn and shorter run times per tank, and gas cans on a jobsite are their own small headache.
Diesel units: the long day and the big load
Towable diesel units handle a site trailer, multiple crews, three-phase equipment or anything running around the clock. They tend to sip fuel better per kilowatt and run longer between fills. They also need somewhere to park.
Do not run a big diesel lightly
This is where oversizing actually costs you. Diesel engines that run for long stretches at very light load can build up unburned fuel and carbon in the exhaust, often called wet stacking. If your real load is 8 kW, a 100 kW diesel is not insurance. It is a problem you are paying extra for.
Quiet Units Near Homes
Backyard jobs in LA County live and die by the neighbors.
A standard open-frame gas generator on a residential lot is loud. On a tight backyard ADU or remodel, it sits 15 feet from somebody's bedroom window.
Super quiet inverter-style gas units
For light loads like chargers, lights, a laptop, a small saw, the enclosed super quiet units are the polite option. We rent them from 1 kW up to 7 kW. They will not run a big compressor and two saws at once.
Check the local rules
Construction hours and noise limits are set city by city across Southern California. Check your city or county rules before you plan on starting the generator at 6 am.
Sometimes you do not need one at all
If the house already has power and the owner agrees, a properly installed temporary power setup or the existing panel may cover the job. Cordless tools have also taken over a lot of what used to need a generator. A charger bank on house power can beat a rental. We will happily lose that rental.
Cords and Distribution
The generator is half the setup. The other half is getting power across the site without melting anything or tripping everything.
Long, thin cords waste power
Voltage drops over long, undersized cords. Motors run hot and saws bog. Use cords rated for the load and the distance. For bigger units, we rent 50 and 100 foot 6-gauge power cords, and you should ask the counter what fits the unit you are renting.
Distribution boxes for multiple crews
When several trades need power in different spots, a power distribution box breaks one big feed into protected circuits. We rent 200 amp boxes. Ask which receptacles and breakers it has and whether that matches your generator and your tools.
Lights are often the forgotten load. In winter, the sun goes down before quitting time. If you need a tower instead of a string of work lights, our light tower guide covers when a self-contained tower beats hanging lights off the generator.
Renting One Without Surprises
Our rentals catalog lists diesel generators at 20, 40, 60, 100, 150, 180 and 240 kW, gasoline units at 3, 6.5 and 10 kW, and super quiet gas units from 1 to 7 kW.
Watch the hours on long runs
Generators that run a dewatering pump or a site trailer often run far longer than a work shift. PDQ's posted rental policies say hour-metered items are based on 8 hours a day, 40 a week and 160 a month, with use over 8 hours a day charged at 1.5 times the regular rate up to 16 hours and 2 times up to 24 hours. Ask whether the unit you want is billed on the meter. The signed rental contract is what governs. For the daily versus weekly math, see our rental rates guide.
Fuel, delivery and pickup
Ask how the unit goes out fueled and how refueling is handled at return. Ask about delivery for towable units. Delivery rates vary by equipment and location, per the posted policies. Our delivery checklist covers where a towable generator should land, and the return guide covers the back end. Anything we did not cover is on the rental policies page, or just ask.
The Short Version
Add running, then one surge
Total what runs at once, add the largest starting spike, and keep the continuous load around 70 to 80 percent of the rating.
Bigger is not safer
An oversized diesel costs more, burns more and can wet stack at light load. Rent for the load you have.
Quiet units for backyards
Near homes, a super quiet gas unit for light loads keeps the neighbors and the inspector off your back.
Electricians for tie-ins
Anything that connects to a panel or a building needs a licensed electrician and a proper transfer switch.
Seven Questions Before You Rent a Generator
- What will run at the same time, and what are the running watts on each nameplate?
- Which load has the biggest starting surge, and could two big motors start together?
- What voltage and phase does my equipment need?
- Is the unit's rating I am looking at continuous or peak, and is it in kW or kVA?
- How long will it run each day, and is it billed on an hour meter?
- What cords, distribution box and ground-fault protection does the setup need?
- Is it going near homes, and what are the local construction hours and noise rules?
Straight Answers
What size generator do I need for a construction site?
Add the running watts of everything that runs at the same time, add the single largest starting surge, and choose a generator with a continuous rating comfortably above that. Small crews with saws, a compressor and lights often land around 10 to 20 kW.
How do I calculate starting watts vs running watts?
Running watts are roughly volts times amps from the tool nameplate. Starting watts for motor loads can be two to three times running watts or more, so check the manual or manufacturer data for compressors, pumps and saws.
What is the difference between kW and kVA on a generator?
kW is usable power and kVA is apparent power. Many larger generators are rated at a 0.8 power factor, so the kVA number is higher. Size your load against the kW rating.
Is a diesel or gas generator better for a jobsite?
Gas units are lighter and simpler for small loads. Diesel units suit bigger loads, longer run times and three-phase equipment. Avoid running a large diesel at very light load for long periods.
Can I plug a generator into a house to power a remodel?
Not without a proper transfer switch installed by a licensed electrician. Backfeeding a panel can energize utility lines and is dangerous. Use cords and a distribution box from the generator instead.
Will a 10 kW generator run a compressor and two saws?
Sometimes, but it is tight. Two saws and a small compressor can approach 10 kW at peak when the compressor starts. Staggering loads or stepping up to a larger unit avoids tripped breakers.
We Rent Generators. You Noticed.
A rental company just spent a couple thousand words teaching you how to rent a smaller generator. It is still a sales page. It just takes the long way around.
Bring us your tool list, your voltages and how long it needs to run. We will help you size it, including telling you when the little quiet unit or no generator at all is the right call.
Seventy-some years of renting things has taught us one thing about generators. Nobody calls back to thank you for the 100 kW unit that idled all week.
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