How to Calculate What Size Generator You Need (Step-by-Step)

One of the most common and costly mistakes in generator procurement is choosing the wrong size.

Buy too small, and your generator runs at over-capacity — overheating, failing early, and leaving your operation exposed. Buy too large, and you're paying for capacity you'll never use, burning excess fuel, and running the engine at inefficiently low loads.

The right size generator is one that matches your actual load — with a sensible margin for safety and future growth.

Here's how to calculate it correctly, step by step.

Step 1: List Every Piece of Equipment That Will Run Simultaneously

Start with a complete inventory. Don't think about what could run — think about what will run at the same time during normal peak operation.

For each piece of equipment, you need two numbers:

· Rated power (kW or kVA) — found on the equipment nameplate or in the technical manual

· Starting surge multiplier — motors and compressors draw 2-6x their rated power at startup

If you're sizing a generator for a construction site, include every power tool, site office load, lighting circuit, and water pump that could be running simultaneously during peak hours.

If you're sizing for a facility, include HVAC systems, production machinery, lighting, IT equipment, and any other continuous loads.

  Tip: If you are unsure about a piece of equipment's power draw, check the nameplate on the motor or control panel. It will show rated current (Amps) and voltage — multiply them together to get VA, then divide by 1000 for kVA.

Step 2: Understand Running Watts vs. Starting Watts

This is the step most buyers miss — and it's the one that causes the most problems.

Every piece of electrical equipment has two power values:

Running watts (continuous load):  The power the equipment draws during normal, steady-state operation. This is the number you see on most spec sheets.

Starting watts (surge load):  The peak power drawn for 1-3 seconds when the equipment first starts. Motors, compressors, and pumps are the main culprits — they can draw 3-6 times their running wattage at startup.

Your generator must be able to handle the surge — not just the continuous load.

Example: A 10kW air compressor motor has a running load of 10kW, but a starting surge of 25-35kW. A 15kW generator would fail to start it, even though it appears large enough on paper.

Typical Starting Surge Multipliers by Equipment Type

Step 3: Apply the Sizing Formula

Once you have your equipment list with running and starting loads, apply this formula:

  Required Generator Size (kW) = Total Running Load (kW) x 1.25

The 1.25 multiplier (25% safety margin) accounts for:

· Starting surge from the largest single motor on your list

· Simultaneous startup of multiple loads

· Derating due to altitude, ambient temperature, or fuel quality

· Future load additions as your operation grows

For facilities with large motors (above 30kW rated), you may need to apply a higher surge factor. In these cases, we recommend calculating the starting kVA of your largest motor separately and confirming the generator can handle it.

  Important: Always size based on kW output, not kVA. Generator spec sheets often show kVA as the headline figure. To convert: kW = kVA x Power Factor (typically 0.8 for industrial loads).

Step 4: Worked Example — Industrial Workshop

Let's walk through a real calculation for a mid-sized manufacturing workshop.

Equipment List

Calculation

Total running load:  55 kW

Apply 1.25 safety margin:  55 kW x 1.25 = 68.75 kW

Largest starting surge check:  Air compressor 15 kW x 4 (surge factor) = 60 kW peak. Generator must handle this without shutdown.

Recommended generator size:  80 kW (next standard size above 68.75 kW)

  Result: Specify an 80kW diesel generator. A 60kW unit would be under-sized; a 100kW unit would work but runs inefficiently at ~55% load during normal operation.

Quick Reference: Typical Generator Sizes by Application

Common Sizing Mistakes — and How to Avoid Them

Mistake 1: Sizing based on the largest single load, not the total load.  Your generator needs to power everything running simultaneously — not just the biggest machine. Add up all concurrent loads before applying the safety margin.

Mistake 2: Ignoring starting surge.  A generator that handles your running load perfectly may fail to start your largest motor. Always check the starting kVA requirement of motors above 5kW.

Mistake 3: Not accounting for altitude and temperature.  Diesel generators lose approximately 1% of rated output for every 100m above sea level, and derate in high ambient temperatures. If your site is above 1,000m or in a region with temperatures above 40°C, derate your calculation accordingly.

Mistake 4: Leaving no headroom for growth.  If you expect your operation to grow in the next 3 years, factor in projected additional loads now. Upgrading a generator is expensive and disruptive — it's cheaper to size correctly from the start.

Mistake 5: Confusing kW and kVA.  Always confirm whether your equipment specs are in kW or kVA. For motor loads, use a power factor of 0.8 to convert. Mixing up these units can result in a 20-25% sizing error.

We'll Do the Calculation for You

If you're not confident in your load calculation, don't guess — ask us.

Send us your equipment list and site conditions, and our technical team will provide a complete load analysis and generator recommendation at no charge. We've sized generators for operations in over 60 countries, from 10kW construction site units to 2,500kW industrial installations.

What to send us:

· List of equipment with rated power (kW or Amps + Voltage)

· Which equipment runs simultaneously

· Site altitude and typical ambient temperature

· Whether you need prime power or standby

· Any future expansion plans

We'll come back to you with a generator size recommendation, model options, and pricing within 24 hours.

Leading Power has supplied industrial diesel generators to buyers in over 60 countries since 2008. Our products are CE and ISO certified, with Cummins, Perkins, Volvo, and Baudouin engine options available from 5kW to 3000kW.