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To calculate what size generator you need, add the running power of all equipment operating simultaneously, account for motor starting loads, and include an appropriate capacity margin. For commercial and industrial applications, convert the required power from kilowatts (kW) to kilovolt-amperes (kVA) using the relevant power factor.
However, generator sizing involves more than electrical output. If you are planning a generator installation, understanding the genset size in feet, including its length, width, height, and required installation space, is equally important.
This guide explains how to calculate generator capacity, compare typical power ratings, estimate physical dimensions, and plan installation space for residential, commercial, and industrial power systems.
Table of Contents
The right generator size depends on the electrical equipment you need to operate, the maximum simultaneous power demand, and the generator's intended operating conditions.
Generator size is generally described in two ways:
Sizing Type | Measurement | What It Determines |
|---|---|---|
Electrical Capacity | Watts, kW, kVA | How much electrical power the generator can supply |
Physical Dimensions | Feet, inches, meters | How much physical space the genset occupies |
Installation Space | Square feet or square meters | Space needed for equipment, ventilation, access, and maintenance |
For example, a 100 kVA generator rated at a power factor of 0.8 typically corresponds to 80 kW of rated real power. Its actual physical dimensions, however, depend on the engine model, alternator, enclosure, radiator, and fuel tank arrangement.
Basic Generator Sizing Formula:
Required Generator kW = Simultaneous Running Load × (1 + Reserve Margin)
Required Generator kVA = Required Generator kW ÷ Rated Power Factor
These equations provide an initial estimate. Motor starting performance, load characteristics, and site conditions must also be evaluated before confirming the final rating.
If you are new to generator terminology, our article on what a genset means explains how engines, alternators, and supporting systems work together to produce electricity.
Calculating generator size requires more than adding the wattage of every appliance or machine. You need to identify which loads operate simultaneously, how much power they consume, and whether their starting demands exceed normal operating requirements.
Running watts represent the electrical power required to keep equipment operating under normal conditions.
Start by listing the equipment that must receive generator power, including lighting, motors, pumps, air-conditioning systems, computers, and industrial machinery.
For a basic single-phase AC load:
Watts = Voltage (V) × Current (A) × Power Factor
For balanced three-phase equipment:
kW = √3 × Voltage (V) × Current (A) × Power Factor ÷ 1,000
If the equipment nameplate already specifies input power in kW or watts, use that value rather than estimating it from the current alone.
Example: Commercial Facility Running Load
Equipment | Running Load |
|---|---|
Lighting | 10 kW |
HVAC System | 25 kW |
Water Pumps | 15 kW |
Computers and IT Equipment | 5 kW |
Other Electrical Equipment | 10 kW |
Total Running Load | 65 kW |
In this example, the facility requires 65 kW when all listed equipment operates simultaneously.
For facilities with varying operating schedules, calculate the realistic maximum simultaneous demand rather than assuming every installed device runs at full capacity continuously.
Some equipment requires considerably more current when starting than during normal operation.
Electric motors, air compressors, refrigeration systems, and large water pumps are common examples.
Starting demand depends on motor type, starting method, load torque, and the electrical system.
For example:
Total simultaneous running load: 65 kW
Additional active-power demand during the largest motor start: 20 kW
Estimated active-power demand during that starting event: 85 kW
This simplified example illustrates how an individual starting event can temporarily increase demand.
However, starting kW alone is not sufficient for final generator sizing. Engineers must also evaluate the motor's starting kVA, inrush current, generator voltage dip, and transient response.
As explained in Schneider Electric's motor starter configurations guide, different starting methods can influence motor starting behavior and electrical system requirements.
Where practical, staggering motor starts or using suitable reduced-current starting equipment can help manage peak starting demand.
Industrial generators are commonly rated in kVA because the alternator must supply both real and reactive power.
The relationship is:
kVA = kW ÷ Power Factor
For example, a load requiring 80 kW at a power factor of 0.8 corresponds to:
80 ÷ 0.8 = 100 kVA
The following table shows common conversions at a power factor of 0.8.
Real Power (kW) | Apparent Power (kVA) |
|---|---|
40 kW | 50 kVA |
64 kW | 80 kVA |
80 kW | 100 kVA |
120 kW | 150 kVA |
160 kW | 200 kVA |
200 kW | 250 kVA |
320 kW | 400 kVA |
400 kW | 500 kVA |
800 kW | 1,000 kVA |
These are mathematical conversions at 0.8 power factor, not a list of recommended generator models.
Power factor affects the relationship between real and apparent power. Schneider Electric's generator monitoring and power factor guide provides further technical information about generator operating conditions and the effects of special electrical loads.
For additional detail, see our guide on why generator sets are rated in kVA instead of kW.
A generator should not be sized solely by matching its nameplate rating to the calculated steady-state load.
A planning margin may help accommodate reasonable load variations, future expansion, and operating uncertainty.
For example, assume:
Total running load: 65 kW
Preliminary reserve margin: 25%
Generator-rated power factor: 0.8
Calculation:
Required kW = 65 × 1.25 = 81.25 kW
Required kVA = 81.25 ÷ 0.8 = 101.56 kVA
The preliminary steady-state requirement is approximately 102 kVA.
The next step is checking available generator ratings and confirming that the selected equipment can handle the largest starting load without unacceptable voltage or frequency deviations.
A 25% margin is an illustrative planning assumption rather than a universal engineering rule. Actual requirements depend on the load profile, generator duty rating, ambient conditions, and project specifications.
Different applications have different electrical load characteristics. A generator suitable for a commercial office may not be suitable for a manufacturing facility with heavy motor loads, even when the calculated running power is similar.
Application | Typical Electrical Loads | Main Sizing Consideration |
|---|---|---|
Home Backup Power | Refrigeration, lighting, pumps, air conditioning | Essential loads and starting watts |
Commercial Buildings | HVAC, lighting, elevators, IT equipment | Peak demand and critical circuits |
Construction Sites | Power tools, compressors, pumps | Variable loads and motor starting |
Manufacturing Plants | Motors, conveyors, compressors, production equipment | Starting current and load changes |
Data Centers | IT equipment, cooling, UPS systems | Load acceptance and power quality |
Agricultural Facilities | Irrigation pumps, fans, processing equipment | Motor starting and operating schedules |
Consider a commercial building with a measured maximum simultaneous backup load of 90 kW.
Assuming an illustrative reserve margin of 20%:
90 kW × 1.20 = 108 kW
At a generator-rated power factor of 0.8:
108 ÷ 0.8 = 135 kVA
This produces a preliminary generator requirement of 135 kVA.
The final generator must be checked against the manufacturer's available ratings and performance data, particularly if the building includes elevators, large HVAC motors, or other equipment with high starting demand.
For commercial and industrial power projects, explore DIYPOWER's low voltage diesel generator units to review power-generation configurations for different applications.
Building floor area can provide useful context for early planning, but it is not a reliable substitute for an electrical load calculation.
Two buildings with the same area may require very different generator capacities.
For example, a 20,000-square-foot warehouse primarily using LED lighting can have a much lower electrical load than a 20,000-square-foot manufacturing facility operating compressors, conveyors, and production equipment.
For commercial generator sizing, prioritize:
Historical peak electrical demand
Essential loads during an outage
Motor starting requirements
Required voltage and phase configuration
Expected operating schedule
Building square footage should be treated as supporting information, not the primary method for determining generator capacity.
Genset size in feet refers to the physical length, width, and height of a generator set. It is different from electrical generator sizing, which is measured in watts, kW, or kVA.
Industrial diesel generators vary considerably in size depending on the engine, alternator, enclosure, fuel system, cooling components, and power rating.
A generator's physical dimensions are generally expressed as:
Length × Width × Height (L × W × H)
Understanding these measurements helps engineers and buyers evaluate equipment-room layouts, transportation requirements, foundation dimensions, and maintenance access.
The table below provides a dimensional reference based on selected industrial generator configurations. It should not be treated as a standardized size chart for every generator within a particular power rating.
Generator Rating | Length (ft) | Width (ft) | Height (ft) | Configuration |
|---|---|---|---|---|
150 kVA | Approx. 8.2 | Approx. 4.3 | Approx. 5.3 | Reference open-type configuration |
200 kVA | Approx. 10.9 | Approx. 3.7 | Approx. 5.5 | Reference enclosed configuration |
1,250 kVA | Approx. 17.1 | Approx. 7.1 | Approx. 9.8 | Reference open-type configuration |
1,500 kVA | Approx. 17.7 | Approx. 8.2 | Approx. 10.6 | Reference open-type configuration |
Important: These are approximate dimensions from selected reference configurations, not standardized sizes or verified DIYPOWER product specifications. Actual dimensions must be obtained from the relevant model's technical datasheet. The table includes different configurations and is intended to illustrate physical-size variation.
A higher-capacity generator is not necessarily wider or taller than every lower-capacity model. Equipment packaging and enclosure design can significantly affect the overall dimensions.
A 150 kVA industrial diesel generator can measure approximately 8.2 feet long, 4.3 feet wide, and 5.3 feet high in one reference open-type configuration.
At a rated power factor of 0.8:
150 kVA × 0.8 = 120 kW
This means a 150 kVA generator corresponds to 120 kW of rated real power at that power factor.
However, two 150 kVA generators may have different physical dimensions.
An open generator set generally has different space and enclosure requirements from a sound-attenuated generator package.
Other factors that affect dimensions include:
Engine and alternator arrangement
Acoustic enclosure design
Integrated fuel tank capacity
Radiator and cooling system configuration
Exhaust and emissions-control equipment
Always confirm the exact length, width, height, weight, and lifting arrangement before planning transportation or installation.
Industrial generator datasheets frequently specify dimensions in millimeters or meters.
To convert measurements:
Feet = Millimeters ÷ 304.8
Feet = Meters × 3.28084
For example, consider a generator with the following physical dimensions:
Length: 3,000 mm
Width: 1,200 mm
Height: 1,600 mm
After conversion:
Dimension | Millimeters | Feet |
|---|---|---|
Length | 3,000 mm | 9.84 ft |
Width | 1,200 mm | 3.94 ft |
Height | 1,600 mm | 5.25 ft |
Therefore, the generator measures approximately:
9.84 ft × 3.94 ft × 5.25 ft
These measurements describe the physical equipment envelope, not the complete installation area.
Generator enclosure design can have a significant impact on overall size and site preparation.
Feature | Open-Type Generator | Silent Generator |
|---|---|---|
Physical Configuration | Engine and alternator mounted on a base frame | Generator enclosed within an acoustic housing |
Equipment Dimensions | Depends on engine and cooling arrangement | May require a larger housing envelope |
Noise Control | Usually requires external acoustic measures where needed | Enclosure provides integrated noise reduction |
Installation | Often installed in a dedicated generator room | Common in suitable outdoor or purpose-designed locations |
Maintenance Access | Depends on room layout and component positions | Depends on enclosure door and panel arrangement |
When comparing generator dimensions in feet, confirm whether the published measurements include the enclosure, base fuel tank, and other accessories.
The physical footprint of a generator is only one part of the installation requirement.
Commercial and industrial generator installations must also provide adequate space for ventilation, maintenance, exhaust systems, electrical connections, and fuel supply arrangements.
The basic footprint formula is:
Generator Footprint (sq ft) = Length (ft) × Width (ft)
For example, a generator measuring:
Length: 10 ft
Width: 4 ft
Height: 5 ft
has an equipment footprint of:
10 × 4 = 40 sq ft
However, a 40-square-foot installation area would generally not provide sufficient access for maintenance or support systems.
Suppose the preliminary equipment layout includes an additional 3 ft on each side for illustrative planning purposes.
The resulting space dimensions would be:
Required Length = 10 + 3 + 3 = 16 ft
Required Width = 4 + 3 + 3 = 10 ft
Estimated rectangular planning area:
16 × 10 = 160 sq ft
This example demonstrates the difference between generator footprint and overall planning space.
The 3 ft allowance is not a universal safety or maintenance clearance. Actual access requirements depend on the manufacturer's installation drawing, cooling-air path, exhaust arrangement, applicable codes, and service procedures.
Installation Factor | Planning Consideration |
|---|---|
Cooling Airflow | Adequate air intake and hot-air discharge |
Maintenance Access | Access to filters, batteries, controls, and service panels |
Exhaust Routing | Safe exhaust discharge and thermal protection |
Fuel System | Tank placement, fuel connections, and spill protection |
Electrical Connections | Suitable access for cables, panels, and switchgear |
Foundation | Appropriate structural support and vibration control |
Noise Requirements | Acoustic treatment where necessary |
Equipment Delivery | Suitable access for transportation, lifting, and positioning |
For general safety considerations, OSHA publishes guidance on using portable generators safely. This guidance is primarily intended for portable generator use; permanently installed industrial diesel generators require model-specific installation requirements and applicable local electrical, mechanical, and fire-safety codes.
A complete industrial generator installation may also include an automatic transfer switch (ATS). Learn more about what an ATS is for a diesel generator and how it supports automatic backup power operation.
Generator sizing problems often result from overlooking electrical demand characteristics or physical installation requirements.
Kilowatts measure power, while kilowatt-hours measure energy consumption over time.
For example, dividing monthly electricity consumption by total hours gives average power, not the maximum electrical demand.
Peak load measurements are more useful when determining generator capacity.
A generator may support the normal running load but experience excessive voltage drop when a large motor starts.
Starting current, load sequence, and generator transient performance must be considered.
Floor area does not directly represent electrical demand.
A commercial office, factory, and warehouse may have similar floor areas but significantly different power requirements.
Standby, prime, and continuous ratings describe different operating conditions.
The correct rating must match the intended duty, expected running hours, and permitted loading conditions.
For more information about equipment configuration and operating requirements, see our guide on how to choose the right generator set.
Electrical capacity alone does not guarantee that a generator will fit the available site.
Before purchasing, confirm the genset dimensions in feet, equipment weight, foundation layout, ventilation requirements, and maintenance access.
The first step is identifying all electrical equipment that must operate simultaneously. Record each load's normal power consumption, starting requirements, voltage, phase, and operating schedule. Use this information to establish the maximum expected demand.
Add the simultaneous running loads, consider the largest motor starting event, include an appropriate planning margin, and convert the required kW to kVA where necessary. The final rating must be validated against generator performance and operating requirements.
Genset dimensions vary by power rating, engine model, and enclosure. One reference 150 kVA configuration measures approximately 8.2 ft long, 4.3 ft wide, and 5.3 ft high, while some large industrial configurations exceed 17 ft in length. Always consult the selected model's dimensional drawing.
One reference enclosed 200 kVA generator measures approximately 10.9 ft long, 3.7 ft wide, and 5.5 ft high. Actual dimensions may differ significantly between models and enclosure types.
Generator size in kVA describes apparent electrical power capacity. Generator size in feet describes physical length, width, and height. Both are important when planning industrial generator installations.
Calculate the maximum simultaneous demand of essential equipment, including lighting, HVAC, elevators, pumps, and IT systems. Account for motor starting requirements and convert the required real power into the appropriate generator rating.
Required clearance depends on the generator model, radiator arrangement, access panels, exhaust system, and installation regulations. Use the manufacturer's drawings and applicable local requirements to determine the final space allocation.
Yes. A generator that is significantly oversized for its normal operating load may operate inefficiently and can experience low-load-related issues, depending on engine design and operating conditions.
Generator loading also affects operating costs and fuel consumption. Our article on diesel generator efficiency explains the relationship between load levels and generator performance.
Selecting the right generator requires a clear understanding of electrical demand, motor starting conditions, operating duty, and physical installation requirements.
DIYPOWER provides diesel generator sets and power-generation solutions for commercial, industrial, and other demanding applications.
Our team can assist with reviewing:
Required generator power in kW and kVA
Voltage, frequency, and phase configuration
Standby, prime, or continuous operating requirements
Generator dimensions and installation space
Engine, alternator, and enclosure configurations
Project-specific technical documentation
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