What Size Backup Generator Do I Need for My Business?


Choosing the right commercial backup generator starts with one question:

How much power does your facility actually need during an outage?

For commercial electricians, facility managers, and industrial buyers, generator sizing is not based on square footage alone. It depends on your critical electrical loads, starting amps, voltage, phase, fuel type, runtime needs, and whether the generator will be used for standby, prime, or continuous power.

At ARC Power Systems, we help businesses source used and surplus industrial diesel generators, natural gas generators, towable generators, and standby power systems for commercial and industrial applications.

If you are trying to size a generator for a warehouse, manufacturing plant, data center, quarry, cold storage facility, medical building, or commercial property, start with these three questions.


1. What Size Generator Do I Need for My Commercial Building?

The right generator size depends on what equipment must stay running during an outage.

Start by separating your loads into two groups:

Critical Loads

These may include:

  • Emergency lighting
  • Fire pumps
  • Elevators
  • Refrigeration
  • IT rooms and servers
  • Security systems
  • HVAC for critical areas
  • Production equipment
  • Process controls
  • Sump pumps
  • Medical or lab equipment

Non-Essential Loads

These may include:

  • General office lighting
  • Breakroom outlets
  • Non-critical HVAC
  • Comfort cooling
  • Extra warehouse lighting
  • Non-essential machinery

Most commercial sites do not need to power the entire building. In many cases, the generator only supports selected loads through an automatic transfer switch, emergency panel, or dedicated backup-power distribution system.

That is why two facilities with the same square footage can need completely different generators.

One site may need a 250kW generator. Another may need a 500kW generator, 1000kW generator, or multiple megawatts of backup power.

Need help narrowing it down? Use ARC Power Systems’ PowerMatch Tool to send us your voltage, fuel type, application, and estimated load.



2. How Do You Calculate Commercial Generator Size?

For three-phase commercial power, electricians often use these formulas:

kW = Volts × Amps × 1.732 × Power Factor ÷ 1000

kVA = Volts × Amps × 1.732 ÷ 1000

Most industrial generators are rated at a 0.8 power factor, which means:

  • 500kW = 625kVA
  • 800kW = 1000kVA
  • 1000kW = 1250kVA

Example

A 480V, three-phase facility with 600 amps of critical load at 0.8 power factor would require approximately:

480 × 600 × 1.732 × 0.8 ÷ 1000 = 399kW

But that does not automatically mean a 400kW generator is enough.

You still need to account for:

  • Motor starting
  • HVAC compressor startup
  • Fire pump starting
  • Elevator loads
  • Load sequencing
  • Voltage dip
  • Altitude and temperature derating
  • Future expansion
  • Standby, prime, or continuous rating

The biggest mistake buyers make is sizing a generator only by running load and ignoring startup demand. Large motors can require much more power to start than they need once running.


3. Is It Better to Oversize or Undersize a Generator?

Neither.

A backup generator should be properly sized with enough reserve capacity to handle startup loads and future growth.

If the Generator Is Too Small

An undersized generator can cause:

  • Failed motor starts
  • Voltage drop
  • Breaker trips
  • Overloaded windings
  • Frequency instability
  • Equipment shutdowns
  • Business interruption

This is the worst outcome: the business buys a generator, loses utility power, and the generator cannot carry the load.

If the Generator Is Too Large

An oversized generator can cause:

  • Higher purchase cost
  • Higher fuel consumption
  • Larger installation cost
  • Larger switchgear and cabling needs
  • Poor loading
  • Wet stacking on diesel engines under light load

A small amount of extra capacity is smart. Oversizing blindly is expensive.

The goal is simple:

Choose a generator that can carry the required load safely without running overloaded or severely underloaded.


What Information Should You Have Before Buying a Commercial Generator?

Before requesting pricing, gather as much of this information as possible:

  • Required voltage
  • Single-phase or three-phase
  • Estimated running load
  • Largest motor load
  • Starting method: across-the-line, soft start, or VFD
  • Fuel type: diesel, natural gas, or propane
  • Standby, prime, or continuous use
  • Indoor, outdoor, enclosed, skid-mounted, or towable
  • Required runtime
  • Emissions requirements
  • ATS or switchgear needs
  • Jobsite location
  • Installation timeline

The more information you provide, the faster a supplier can match you with the right generator.


Common Commercial Generator Sizes

Commercial and industrial buyers often search for:

  • 100kW diesel generators
  • 250kW commercial generators
  • 500kW standby generators
  • 800kW industrial generators
  • 1000kW diesel generators
  • 1250kVA generator sets
  • 1500kW generators
  • 2MW natural gas generators
  • Towable generators for quarry and aggregate operations
  • Backup generators for data centers
  • Standby generators for manufacturing plants
  • Emergency generators for commercial buildings

ARC Power Systems regularly sources and sells industrial generators across these categories, including diesel, natural gas, Tier 2, Tier 4 Final, enclosed, mobile, and high-voltage generator sets.

View available inventory here: Industrial Generators for Sale



Need Help Choosing the Right Generator?

If you are a commercial electrician, contractor, facility manager, or buyer trying to determine the right generator size, ARC Power Systems can help you narrow the search.

Send us:

  • Your voltage
  • Estimated load
  • Fuel preference
  • Application
  • Location
  • Required timeline

We will help match the project with available generator options.

Start here: Use Our PowerMatch Tool

Or browse current equipment here: ARC Power Systems Generator Inventory