To size a diesel generator, add the running load of everything it will power, allow for the startup surge of the largest motor, and add about 20 to 25 percent of spare capacity. Towable generators are rated in kVA: multiply kVA by 0.8 to get kilowatts. A 25 kVA unit delivers about 20 kW, and a 125 kVA unit about 100 kW. The table below shows the sizes we sell most and what used units cost.
Generator sizes, output and used prices
| Size | Approx. output | Typical use | Used price range |
|---|---|---|---|
| 25 kVA | 20 kW | Small jobsites, tools, lighting, mobile offices, events | $8,500 to $16,500 |
| 45 kVA | 36 kW | Mid-size jobsites, pumps, small commercial backup | $14,000 to $22,500 |
| 70 kVA | 56 kW | Larger crews, compressors, commercial buildings | $19,500 to $24,800 |
| 125 kVA | 100 kW | Large jobsites, cranes, big commercial standby | $20,500 to $36,500 |
| 150 kVA | 120 kW | Industrial loads, facilities, large events | $29,500 to $51,500 |
Price ranges are the asking prices of used units listed by IronPower Industries. Hours, year and trailer configuration explain most of the difference within each size.
How to calculate the size you need
- List every load. Write down the running watts or amps of each tool, motor, light and HVAC unit that will run at the same time.
- Account for motor starting. Electric motors draw several times their running current for a moment when they start. Size for the largest motor starting while everything else is running.
- Convert to kVA. Divide total kilowatts by 0.8. A 40 kW load needs about 50 kVA.
- Add headroom. Leave 20 to 25 percent spare so the generator is not running at its limit and you have room to add loads.
- Check voltage and phase. Confirm whether your equipment needs single-phase or three-phase power and at what voltage. Many towable units can be switched between configurations.
Why bigger is not always better
A diesel generator that runs for long periods at a very light load does not reach proper operating temperature, and unburned fuel builds up in the exhaust. This is called wet stacking, and it reduces performance and engine life. We explain it in wet stacking explained. The goal is a generator that works comfortably in the middle of its range.
Frequently asked questions
How many kW is a 25 kVA generator?
About 20 kW. Multiply kVA by 0.8, the standard power factor for these generators.
What size generator do I need for a construction site?
Small crews with hand tools and lights usually work with 25 kVA. Sites with pumps, welders or several trades often need 45 to 70 kVA, and large sites with cranes or heavy motors move to 125 kVA or more. Always confirm with your actual load list.
Prime or standby rating: which one matters?
Use the prime rating if the generator is your main power source for long hours. Use the standby rating only for emergency backup during outages.
Is a used diesel generator reliable?
Yes when it has been load tested and serviced. Ask for the load bank test results, the hour meter reading and the service record. Our guide for new generator owners covers what to do after you buy.
See our current used diesel generators for sale, or send us your load list and we will help you choose the size.
