Generator Sizing

About the Generator Sizing Calculator

This generator sizing calculator helps you select the right backup or prime generator for motor loads, lighting, HVAC, and mixed industrial loads. The calculator accounts for two main factors: motor starting current (which can be 5–10 times running current and lasts a few seconds) and demand factor (the fraction of total connected load running simultaneously).

Outputs in kVA so you can match generator nameplate ratings directly. Choose between standby applications (intermittent backup power) and prime applications (continuous off-grid duty) since they have different sizing margins and duty cycles.

How to size a generator

The generator must handle two challenges simultaneously:

Generator kVA ≥ max(Largest motor starting kVA, Sum of running loads × Demand Factor)

(1) The largest single motor starting at any time — inrush current is 5–10× running current and lasts a few seconds. (2) The steady-state running load of all combined equipment, factored down by typical demand factor (rarely is 100% of equipment running simultaneously). Whichever is larger sets the generator size.

Worked example

A facility runs one 25 HP motor (~30 kVA running, ~120 kVA starting at NEMA code letter G) plus 40 kW of lighting and HVAC at 0.85 PF (~47 kVA total). Demand factor is 0.8 for the secondary loads.

Generator must be at least max(120, 30 + 47 × 0.8) = max(120, 67.6) = 120 kVA — dominated by motor starting requirement. Round up to a standard 125 or 150 kVA unit. The 150 kVA option provides healthy margin and supports future expansion.

Standby vs prime power generators

• Standby: emergency backup, intermittent use (~200 hr/year max). Sized for nameplate rating with brief overload tolerance. Most cost-effective for buildings with reliable utility power.
• Prime: continuous duty, variable load. Sized at 70–80% of nameplate for thermal margin. Required for off-grid applications, remote sites, and facilities where the generator runs daily.
• Continuous: 24/7 baseload, constant load. Sized at 60–70% of nameplate. Used for prime power in industrial process facilities.

Motor starting kVA by NEMA code letter

The NEMA code letter on a motor nameplate tells you locked-rotor kVA per HP at rated voltage. Common values:

• Code A: 0–3.14 kVA/HP — very rare, oldest motors
• Code B: 3.15–3.54 kVA/HP — older standard
• Code F: 5.00–5.59 kVA/HP — common for older general-purpose motors
• Code G: 5.60–6.29 kVA/HP — common for modern Premium-efficiency motors
• Code H: 6.30–7.09 kVA/HP — high-efficiency, high-inrush motors
• Code J: 7.10–7.99 kVA/HP — specialty applications

Multiply HP × code letter value to estimate locked-rotor kVA. Example: 25 HP × Code G (use 5.95 midpoint) = 149 kVA starting demand. This is what your generator must momentarily supply.

When to use this calculator

• Sizing a backup generator for an industrial facility, machine shop, or commercial building
• Specifying prime power for off-grid agricultural pumping, mining, or remote communications
• Right-sizing portable generators for construction sites with mixed motor loads
• Replacing aging gen-sets with modern, more efficient units
• Verifying that an existing generator can support a new motor or HVAC addition
• Preparing for utility outages by understanding actual peak demand vs nameplate load
• Specifying generator transfer switches and feeders to the building

Frequently Asked Questions

How do you size a generator for a motor?

Generator must handle motor starting kVA, not just running kVA. Multiply running kVA by 3–6 times for typical motor inrush. For a 5 HP motor (~3.7 kW running), the generator should be at least 22 kVA to start the motor without stalling.

What kVA generator do I need for a 5 HP motor?

5 HP equals about 3.7 kW running. With ~6× starting inrush, the generator should be at least 22 kVA minimum. For applications with multiple motors, size for the largest single motor’s starting demand plus the running load of others.

What is demand factor?

The fraction of total connected load that runs simultaneously. For mixed industrial loads, 0.7–0.85 of nameplate sum is typical. Lighting and HVAC rarely all run at once; demand factor accounts for this and prevents oversizing.

Should I size for kW or kVA?

Generators are rated in kVA (apparent power). Size in kVA, then verify the kW rating supports the load × power factor. Most generators have a kW rating equal to about 80% of kVA rating — matching the typical 0.8 PF design assumption.

Does motor starting affect generator size?

Yes — significantly. Motor inrush current is 5–10× running current and can stall a small generator. Use the motor’s NEMA code letter and locked-rotor kVA to estimate starting demand. For sites with many small motors, soft starters or VFDs reduce inrush and let you use a smaller generator.

What’s the difference between standby and prime power?

Standby is emergency backup (~200 hours/year max), sized at nameplate. Prime is continuous duty, sized at 70–80% of nameplate for thermal margin. Continuous is 24/7 baseload, sized at 60–70%. Pick the rating that matches how often the generator will run.

Can I use VFDs or soft starters to reduce generator size?

Yes. VFDs ramp motor speed gradually, eliminating inrush current. Soft starters reduce starting current to 200–300% of running (vs 600–800% for direct online start). Either can let you specify a smaller generator — payback is typically under 2 years on the gen-set savings alone.

Related calculators and Electracore products

Specifying motors for a generator-backed installation? Browse General Purpose motors, Farm Duty motors, or Pump motors from Baldor, WEG, and Marathon.

Related calculators: kW to kVA Calculator · kVA to Amps Calculator · Cable Sizing Calculator · 3-Phase Power Calculator.