How do I calculate the generator size I need?

Add the running wattage of all appliances you plan to operate, include the largest additional starting surge, and then add a safety margin of approximately 20 to 25 percent.

What is the difference between running watts and starting watts?

Running watts are required during normal operation. Starting watts are the temporary higher power demand produced when a motor, compressor, pump or similar appliance starts.

Should I choose a generator larger than the calculated load?

Yes. A safety margin helps manage load changes, future appliances, voltage drops and continuous operation without running the generator at its maximum capacity.

What power factor should I use?

A power factor of 0.8 is commonly used for general generator estimates. Use the equipment manufacturer's stated power factor when accurate sizing is required.

Can this calculator size a three-phase generator?

Yes. Select three-phase power, enter the system voltage and power factor, and the calculator will estimate the corresponding current and generator capacity.

Electrical & Energy Calculator

Generator Size Calculator

Estimate the generator capacity required to power your appliances, tools or business equipment. Account for running watts, starting surge, power factor and reserve capacity in one calculation.

Running and starting loads

kW and kVA results

Single and three-phase

Surge-aware sizing

Safety margin included

Enter your electrical loads

Add each appliance and its rated power.

Power system

Voltage

Power factor

Safety margin

Appliance or equipment	Quantity	Running watts	Starting watts

Starting watts should be equal to or greater than running watts.

Your generator estimate

Updated from the loads entered.

Minimum calculated capacity

0.00 kW

Suggested standard generator: —

Running load 0 W

Peak load 0 W

Required kVA 0.00 kVA

Estimated current 0.00 A

Reserve capacity 0 W

Largest surge 0 W

Calculated generator utilisation 0%

Add your appliances and select Calculate Generator Size to see the recommended capacity.

What is a Generator Size Calculator?

A Generator Size Calculator estimates how much electrical capacity a generator should provide for a collection of appliances, machinery or essential circuits. Generator capacity is normally expressed in watts, kilowatts or kilovolt-amperes.

Selecting a generator based only on normal running power can result in undersizing. Refrigerators, air conditioners, pumps, compressors and power tools often require substantially more electricity for a short period when their motors start.

Running watts and starting watts

Running watts are the continuous watts needed after an appliance is operating normally. Starting watts, also called surge watts, represent its temporary demand at startup.

This calculator adds the normal operating load of all appliances and then includes the largest additional starting surge. This models a common scenario in which the highest-demand motor starts while the remaining equipment is already operating.

Professional sizing tip: Check the equipment nameplate or manufacturer documentation whenever possible. Nameplate values are more reliable than general appliance estimates.

Generator sizing formulas

Formulas applied by this calculator.

Total running load Running watts × quantity for every appliance

Peak load Total running load + largest additional surge

Required generator watts Peak load × (1 + safety margin ÷ 100)

Generator kVA Required kW ÷ power factor

Three-phase current Watts ÷ (√3 × voltage × power factor)

How it works

Calculate your generator requirement in four steps

Use appliance ratings and realistic operating conditions to produce a more practical generator estimate.

List your appliances

Add every appliance, tool or electrical circuit that may operate from the generator.

Enter wattage ratings

Enter normal running watts and the higher starting watts for motor-driven equipment.

Set electrical details

Select phase, voltage, power factor and an appropriate safety margin for your application.

Review the result

View the required kW, kVA, current, peak load and nearest common generator rating.

Capacity guide

Common generator size applications

These ranges are general examples. Actual capacity depends on appliance ratings, motor starting currents and how many loads operate simultaneously.

Generator range	Typical application	Possible loads
1–2 kW	Portable essential power	Lights, router, television, laptop and small appliances
3–5 kW	Basic home backup	Refrigerator, lights, electronics and selected kitchen equipment
6–10 kW	Expanded home or workshop backup	Pumps, refrigerator, lights, tools and a small air conditioner
12–20 kW	Whole-home or small commercial backup	Multiple circuits, larger air conditioning, pumps and office equipment
25–50 kW	Commercial or agricultural systems	Machinery, commercial refrigeration, motors, pumps and larger facilities
60 kW and above	Industrial or large commercial use	Production equipment, large motors, buildings and critical infrastructure

Frequently asked questions

Generator sizing FAQs

Understand the main electrical factors before selecting a portable, standby or commercial generator.

Add the running wattage of every appliance that may operate simultaneously. Then include the largest additional motor-starting surge and add a reserve margin, commonly around 20% to 25%.

Running watts are used continuously during normal operation. Starting watts are temporarily required when a motor, pump or compressor starts and may be several times higher than the running load.

A modest reserve is recommended. It prevents the generator from operating continuously at maximum output and allows for load fluctuations, future appliances and temporary voltage drops.

A value of 0.8 is commonly used for a general generator estimate. Resistive loads may be close to 1.0, while motors and other inductive equipment can have a lower power factor. Use the manufacturer’s value where available.

Kilowatts measure real usable power, while kilovolt-amperes measure apparent power. The relationship between them depends on power factor: kW equals kVA multiplied by power factor.

Yes. Select three-phase power and enter the correct line voltage and power factor. The calculator will use the three-phase current formula when displaying estimated amperage.

This calculator assumes the largest motor surge occurs while the remaining listed appliances are running. Where several large motors can start simultaneously, a qualified electrician should perform a detailed load and motor-starting study.

Important: This calculator provides an estimate for planning purposes. Generator installation, transfer switches, grounding, wiring, fuel systems and circuit protection should be designed or verified by a qualified electrician or generator supplier according to local regulations.