What is the difference between kW and kVA?

kW (kilowatts) is real power — the actual work done. kVA (kilovolt-amperes) is apparent power, which includes reactive power from inductive loads like motors. Generator nameplates are usually rated in kVA. To convert: kW = kVA × power factor.

What power factor should I use?

Most residential and light-commercial generators are rated at 0.8 power factor. Industrial generators may be rated at 0.9 or 1.0. If unsure, use 0.8 — it gives a conservative (larger) kVA result.

Why do I need a startup surge factor?

Electric motors (air conditioners, refrigerators, well pumps, power tools) draw 1.5–3 times their running current for 1–3 seconds on startup. The generator must handle this peak without stalling. Only one motor typically starts at a time, so the calculator applies the highest single-load surge.

Why is a 10% safety margin added?

Running a generator at 100% capacity degrades its lifespan and leaves no headroom for measurement error or future load additions. A 10% de-rating buffer is standard industry practice.

What are typical running watts for common appliances?

Refrigerator: 150–400 W. Window AC (10 000 BTU): 900–1200 W. Microwave: 600–1200 W. Sump pump: 800–1050 W. LED lighting (per bulb): 8–15 W. Electric range burner: 1500–2000 W. Laptop: 45–65 W.

What generator sizes are standard?

Common portable sizes: 1 kW, 2 kW, 3.5 kW, 5 kW, 6.5 kW, 8 kW, 10 kW, 12 kW. Standby home generators: 10 kW, 14 kW, 18 kW, 20 kW, 22 kW, 24 kW. Commercial/industrial: 30 kW, 40 kW, 60 kW, 80 kW, 100 kW+.

Generator Size Calculator

Enter your electrical loads to instantly calculate the minimum generator size (kVA and kW) you need, including startup surge.

Electrical Loads

Add each appliance or device. Running watts are required; set a surge multiplier for motors.

Appliance / Device Name

Running Watts

Qty

Surge Multiplier

Power Factor (0.5 – 1.0)

Use 0.8 for most home/commercial generators.

Safety Margin (%)

Recommended: 10% de-rating buffer.

Quick Add Common Loads

Add at least one load and click Calculate.

Recommended Generator Size

— kVA

— kW

Nearest standard size above your calculated requirement (incl. safety margin).

Calculation Breakdown

Total Running Load — W

Peak Startup Load (largest surge) — W

Design Load (running + surge delta) — W

With Safety Margin — W

Required kW — kW

Required kVA (÷ power factor) — kVA

Load Summary

Appliance	Qty	Running W	Surge W
Total	—	—	—

Summary

This generator size calculator helps homeowners, contractors, and engineers determine the minimum generator capacity needed to power a given set of electrical loads. Enter each load's running watts and startup surge factor, then the tool applies your chosen power factor to compute the required kVA and kW rating. The results guide you to a correctly sized standby or portable generator, preventing under-sizing (which causes overloads) and over-sizing (which wastes money).

How it works

Add each electrical load (appliance or device) with its running watts.
Set a startup surge multiplier for motors and compressors (typically 1.5–3x).
Optionally adjust the power factor (default 0.8 for most residential/commercial generators).
The tool sums all running watts and adds the largest single startup surge delta to establish the peak design load.
It divides by 1000 to get kW, then divides by power factor to get kVA.
A 10% safety margin is added and the recommended next-standard generator size is displayed.

Use cases

Sizing a home standby generator for a panel transfer switch.
Selecting a portable generator for a job site.
Calculating generator requirements for an off-grid cabin.
Verifying that an existing generator can handle a new appliance.
Estimating generator needs for an outdoor event or temporary power.
Pre-purchase checks before buying a backup generator.

Frequently Asked Questions

Last updated: 2026-06-04 · Reviewed by Nham Vu