Views: 0 Author: Site Editor Publish Time: 2026-09-24 Origin: Site
Choosing the right generator size is important for reliable backup power. A generator that is too small may struggle to handle the required load, while an oversized unit can increase equipment and operating costs.
To calculate what size generator you need, start by identifying the equipment you want to power, adding its running wattage, accounting for starting loads, and allowing enough reserve capacity. For commercial applications, you can also consider the building's square footage as an initial sizing reference.
If you are planning a permanent installation, you also need to consider the physical dimensions and installation space of the genset, not just its electrical output. This is where factors such as genset size in feet, maintenance clearance, ventilation, fuel storage, and access become important.
Table of Contents
The right generator size allows the generator to supply the required electrical load without unnecessary oversizing or overload.
A generator must provide enough power for the equipment connected to it. However, the electrical demand is not always the same as the rated running wattage of each device.
Motors, compressors, pumps, air conditioners, and other inductive loads can require significantly more power when they start. GenPower notes that starting loads for some motors and compressors can reach several times their normal running load.
A generator that is too small may experience:
Voltage instability
Overloading
Excessive heating
Equipment shutdowns
Difficulty starting motors
Reduced generator life
An oversized generator can also create problems. It may operate inefficiently at low loads and result in unnecessary equipment and operating costs.
For this reason, generator sizing should consider both electrical load and operating conditions.
You need to know the equipment load, starting requirements, voltage, phase, operating schedule, and installation conditions before selecting a generator.
Start by making a list of all equipment that may need generator power.
For each load, collect:
Information | Why It Matters |
|---|---|
Running watts | Determines continuous power demand |
Starting watts | Determines startup capacity |
Voltage | Used for electrical calculations |
Amperage | Helps calculate watts or kW |
Phase | Determines generator configuration |
Power factor | Important for motor and industrial loads |
Operating schedule | Helps determine actual load profile |
Motor size | Helps estimate starting demand |
Location | Affects cooling and generator derating |
Installation space | Determines physical generator fit |
For larger commercial or industrial systems, utility bills and historical maximum demand can also help estimate the building's actual power requirement.
Calculate generator size by adding the required running load, accounting for the largest starting load, and allowing appropriate reserve capacity.
A basic calculation can follow this process:
Add the running watts of all equipment that may operate simultaneously.
Total Running Watts = Load 1 + Load 2 + Load 3 + ...
For example:
Equipment | Running Load |
|---|---|
Lighting | 10 kW |
HVAC | 25 kW |
Pumps | 15 kW |
Computers | 5 kW |
Other equipment | 10 kW |
Total | 65 kW |
The total running load in this example is 65 kW.
Next, identify equipment that requires additional power during startup.
Motors and compressors are particularly important because their starting demand can be several times their normal operating load.
A generator should not normally be selected exactly at the calculated load.
One commercial sizing approach uses a reserve capacity of approximately 25%:
Generator Size = Full Load kW + 25% Reserve
For example:
65 kW × 1.25 = 81.25 kW
The final generator would then need to be selected from an available rating that can safely support the calculated requirement.
GenPower uses this 25% reserve approach in its basic commercial sizing example.
Actual generator selection should also consider motor starting performance, power factor, voltage drop, altitude, ambient temperature, and the manufacturer's application data.
Running wattage is the amount of electrical power an appliance or machine needs after it has started and reached normal operation.
If the equipment label provides watts, use the rated wattage directly.
If only voltage and amperage are available, a basic single-phase calculation is:
Watts = Volts × Amps
For three-phase equipment, the calculation also needs to account for the phase relationship and power factor.
For example, if a single-phase appliance operates at 240 V and draws 10 A:
240 V × 10 A = 2,400 W
Therefore, its approximate running load is 2.4 kW.
For commercial and industrial generator sizing, however, a qualified electrical professional should verify the actual load calculation rather than relying only on simple nameplate calculations.
Starting wattage represents the temporary electrical demand required when equipment starts.
This is one of the most important parts of generator sizing.
Equipment such as:
Electric motors
Air compressors
Water pumps
HVAC systems
Refrigeration equipment
Fans
Blowers
may require substantially more power during startup than during continuous operation.
For example, if a motor requires 10 kW during normal operation but needs 30 kW during startup, the generator must be capable of handling the additional starting demand.
For multiple motor loads, you should determine whether they start simultaneously or sequentially. A staggered-start strategy can reduce the instantaneous demand. GenPower specifically recommends considering the largest starting load when estimating generator requirements.
Commercial generator sizing should consider the building's electrical load, critical systems, motor starting requirements, and building size.
Commercial buildings may have much more complex loads than residential properties.
Typical loads include:
Emergency lighting
Fire alarm systems
Security systems
Elevators
HVAC systems
Pumps
Refrigeration
IT equipment
Production equipment
Lighting systems
For larger installations, generator sizing may require an engineering study and compliance with applicable electrical and fire codes. GenPower notes that large commercial installations can require engineering oversight and consideration of NEC and NFPA requirements in the United States.
Square footage can provide a preliminary estimate when detailed equipment loads are not yet available.
One commercial sizing guideline cited by GenPower uses:
Commercial Application | Base Requirement | Additional Load |
|---|---|---|
Retail | 50 kW | +10 W/sq ft |
Other commercial | 50 kW | +5 W/sq ft |
For example, a 10,000-square-foot retail facility could be estimated using:
50 kW + (10,000 × 10 W ÷ 1,000) = 150 kW
This is only a preliminary sizing method. The actual generator requirement should be confirmed from the building's electrical load and equipment characteristics.
Building square footage can help estimate generator demand, but it should not replace a detailed electrical load calculation.
Two buildings with the same floor area can have very different power requirements.
For example, a:
20,000 sq ft warehouse
20,000 sq ft restaurant
20,000 sq ft manufacturing facility
may require very different generator capacities.
A warehouse may have relatively low electrical demand, while a manufacturing facility may operate large motors, compressors, pumps, and production machinery.
Therefore, square-footage calculations work best as an initial planning tool.
For an accurate generator specification, calculate the actual connected and operating loads.
The physical size of a generator depends on its kW/kVA rating, engine, alternator, fuel system, enclosure, and accessories, so generator capacity alone does not determine its installation footprint.
This is also where the keyword genset size in feet can naturally fit into the discussion.
When planning a generator installation, measure:
Genset length
Genset width
Genset height
Service clearance
Air intake space
Exhaust clearance
Fuel system space
Control panel access
Cable connection space
Maintenance access
Do not select a generator based only on whether the enclosure physically fits into the available room.
The installation needs enough space for technicians to inspect, service, repair, and safely operate the equipment. Generator installation guidance commonly requires additional clearance around the engine and generator components for maintenance access.
It is important to distinguish between generator capacity and physical dimensions.
A generator's:
kW / kVA rating = electrical output
while:
Length × Width × Height = physical size
For example, two generators with similar power ratings may have different dimensions because they use different engines, alternators, cooling systems, fuel tanks, or acoustic enclosures.
Therefore, if you are researching genset size in feet, always check the manufacturer's dimensional drawing or datasheet for the exact model.
Choose a generator that can handle the required running load, the largest starting demand, and the expected reserve capacity under actual site conditions.
A practical selection process is:
Determine which equipment must continue operating during a power outage.
Add the power requirements of equipment operating at the same time.
Pay special attention to motors, pumps, compressors, HVAC equipment, and other equipment with high startup demand.
Add an appropriate margin rather than operating continuously at the generator's maximum rating.
Compare your calculated requirement with available generator kW or kVA ratings.
If your calculated requirement falls between two standard generator ratings, selecting the next appropriate capacity may provide additional operating margin. GenPower recommends moving to the next higher capacity when requirements fall between common ratings.
Confirm the actual:
Length × Width × Height
of the selected genset.
Also confirm that the installation area provides adequate maintenance and ventilation space.
Altitude, ambient temperature, ventilation, fuel system design, and installation conditions can affect generator performance.
The most common generator sizing mistakes are ignoring starting loads, relying only on square footage, and failing to consider installation conditions.
A generator may have enough continuous capacity but still struggle to start a large motor.
Square footage provides only a rough estimate. Actual equipment loads should be used whenever possible.
Operating close to maximum capacity leaves little room for additional loads or unexpected demand.
Electrical capacity is only one part of generator selection. The genset must physically fit while maintaining required service and ventilation clearances.
High temperatures and high elevations can affect generator output and should be considered during equipment selection.
If the facility may add HVAC equipment, production machinery, refrigeration, or other electrical loads, future demand should be considered during planning.
The required generator size depends on the building's actual electrical load, critical equipment, starting loads, and reserve capacity. Square footage can provide an initial estimate, but a detailed load calculation is more reliable.
For a basic single-phase calculation, multiply volts by amps to estimate watts. For three-phase systems, use the appropriate three-phase formula and account for power factor.
A 25% reserve is used in some basic commercial generator sizing approaches, but the appropriate margin depends on the load profile, motor starting requirements, operating conditions, and generator manufacturer's recommendations.
Usually, "generator size" refers to electrical output such as kW or kVA. Physical dimensions are normally specified separately as length, width, and height. If you need the genset size in feet, check the manufacturer's dimensional specifications for the exact model.
Yes, square footage can be used as a preliminary sizing method for some commercial applications. However, actual electrical loads should be used for final generator selection.
A generator that is undersized may overload, overheat, experience voltage instability, fail to start large motors, or provide unreliable power.
An oversized generator can operate inefficiently at low loads and increase equipment and operating costs.
Want to learn more about generator sizing, electrical load calculations, and backup power systems? These industry discussions and technical articles provide additional perspectives on generator selection and power requirements:
Diesel Generator Sizing Calculation — An engineering discussion covering running loads, motor starting loads, power factor, spare capacity, and diesel generator sizing.
Why Is There No Existing Standard for Backup Generator Sizing? — An engineering forum discussion about generator sizing formulas, demand factors, motor starting loads, and different approaches to calculating generator capacity.
Sizing a Diesel Generator Based on Metering Data — Discusses using actual power measurements together with load calculations when determining the appropriate diesel genset size.
Redundancy Reassurance — An industry article covering the importance of properly sizing, locating, and installing backup generation systems for critical facilities.
Generator Sizing Discussions — Community discussions covering generator load calculations, starting loads, HVAC requirements, load management, and real-world generator sizing considerations.
Energy Storage System 
