Commercial Backup Generators: Types and How to Choose the Right One

A generator does not keep a building running by itself. A commercial backup generator is one part of a system that also includes a transfer switch, a fuel supply, and a plan for which loads get power when the grid goes down. Buyers who shop on generator size alone often end up with equipment that starts fine but cannot carry the loads that matter.

That system view is the point of this guide. Power outages in California are no longer rare events; utilities such as PG&E now shut off power on purpose when high winds and dry vegetation raise wildfire risk, and PG&E’s own guidance tells customers to plan for an extended outage. Below, you will find the main generator types, the fuel and sizing decisions that follow, and what installation involves in the Central Valley. Standby generator work is part of Sebastian’s commercial electrical services, so the electrical side of each decision is covered here as well.

Why a Backup Generator for Business Is a Continuity Decision

A backup generator for business pays off when the cost of downtime is higher than the cost of owning the equipment. Refrigerated product, point-of-sale systems, security and access control, servers, and life safety systems all stop when utility power stops.

Planned shutoffs add a second reason. PG&E describes a Public Safety Power Shutoff as a temporary outage used when conditions such as low humidity, forecast high winds, and dry vegetation create fire risk, and it says forecasts can change the timing and the number of affected customers. A business cannot assume the lights will be back on within a shift.

The practical question is which functions must keep running, and for how long. Answering that first makes every later choice in this guide easier.

Types of Commercial Backup Generators

Commercial backup generators fall into four working categories: standby, portable, inverter, and industrial. Standby units are permanently installed and start automatically; the other three trade automation or capacity for flexibility.

Type	How it is installed	Typical commercial use	Main limitation
Standby	Permanently mounted on a pad or in an enclosure, wired through a transfer switch	Whole-building or critical-load backup that starts without anyone on site	Needs permitting, a fuel supply, and professional installation
Portable	Moved into place and connected manually	Short-term or temporary loads, job sites, small shops	Someone must be present to set it up and refuel it
Inverter	Compact unit with electronics that clean up the power it produces	Small, sensitive loads such as registers or networking gear	Limited capacity for larger commercial loads
Industrial	Large, permanently installed set built for heavy or long-running loads	Processing plants, packing houses, cold storage, large campuses	Highest planning, space, and permitting demands

Standby generators

A standby generator is the default choice for most commercial buildings because it detects an outage and starts on its own. That automatic response matters in places where nobody is on site overnight or on weekends. The related search term “commercial standby generator” usually describes this category.

Industrial backup generators

An industrial backup generator serves facilities where the load is large, continuous, or tied to production or product safety, such as processing and cold storage. These sites often need a custom design that coordinates the generator with switchgear and motor loads. Sebastian’s industrial electrical page covers that kind of work.

Portable and inverter generators

Portable and inverter generators suit temporary or small loads. For a business that cannot tolerate a gap in service, they work better as a supplement than as the main plan, because they depend on a person to place, connect, and refuel them.

Fuel Options: Diesel, Natural Gas, and Propane

Each commercial generator fuel trades storage convenience against supply risk. The right choice depends on how long the generator must run and what the site can store.

• Diesel: Fuel is stored on site, so runtime depends on tank size and load. Diesel engines in the San Joaquin Valley also carry air permit conditions, including a requirement to burn CARB diesel with very low sulfur.
• Natural gas: Fuel arrives by pipeline, so there is no refueling during an outage. Runtime then depends on the gas utility staying in service, which is worth testing as a risk for the site.
• Propane: Fuel is stored in tanks on site, which suits locations without gas service. Tank size sets the runtime, and tanks need space and clearances.

A dual-fuel arrangement or a generous on-site fuel plan can reduce the risk in any of these cases. The point is to match the fuel to the longest outage the business must ride through.

How to Size a Commercial Backup Generator

Sizing a commercial backup generator starts with a load list, not the building’s square footage. Sizing for the loads that must keep running costs less and fits more reliably than sizing for everything.

Most projects choose between two approaches:

1. Critical-load backup: The generator feeds only selected panels, such as refrigeration, life safety, data, security, and a share of lighting and HVAC.
2. Whole-building backup: The generator carries the entire facility, which usually means a larger unit and a larger transfer switch.

Motors deserve special attention. Pumps, compressors, and large HVAC equipment draw far more current when they start than when they run, so a generator that looks adequate on paper can struggle when several motors start together. A load calculation that accounts for starting current and for future growth keeps the system from being outgrown in a few years. Sebastian’s posts on three-phase vs. single-phase power and surge protection cover related power-quality questions.

Transfer Switches: Automatic vs. Manual

A transfer switch connects the generator to the building and keeps generator power from flowing back onto the utility lines. An automatic transfer switch senses an outage, starts the generator, and moves the load without anyone present. A manual transfer switch needs a person to make the change.

For unattended sites, an automatic switch is usually the practical choice. The switch also sets how the load is divided, so it belongs in the same conversation as sizing. Plan for the switch as a core part of the system, not an accessory.

Commercial Generator Installation in California

Commercial generator installation in California involves building and electrical permits from the local jurisdiction and, for many engine-driven units, an air permit. Both should be sorted out before equipment is ordered.

In the San Joaquin Valley, the Valley Air District’s policy for diesel emergency standby engines covers new units above 50 horsepower that power generators. Permit conditions in the district’s engineering evaluation include a non-resettable hour meter, operating and fuel records, and limits on testing hours. If the site is near a school, additional restrictions on non-emergency operation apply.

Federal rules set their own limits for emergency engines. EPA allows up to 100 hours per year for maintenance, testing, and certain non-emergency uses, while California’s diesel rules, applied through the district permit, generally limit maintenance and testing to 50 hours.

Physical planning matters as much as paperwork. A pad or enclosure needs clearances, a fuel path, ventilation, exhaust routing, and a conduit route to the transfer switch. Deciding these early avoids rework once equipment is on order.

Maintenance and Testing for a Commercial Backup Generator

A commercial backup generator that is never exercised is an unproven generator. Standby power is only valuable if it starts and carries load on the day it is needed.

NFPA 110 is the national standard for emergency and standby power systems, and it is a common reference for inspection and testing programs. The required test frequency, duration, and load for a specific system depend on its classification, the code edition the local fire authority enforces, the manufacturer’s instructions, and the air permit. In Fresno, the City of Fresno Fire Department’s testing table lists emergency and standby power systems as an annual item under NFPA 110. Ask your service provider and your local fire authority for the written schedule that applies to your system, and keep it on file. A sound program typically includes:

• Scheduled exercise under load, so the engine reaches normal operating temperature
• Transfer switch operation on a regular schedule
• Fuel checks, including quality and quantity
• Battery, coolant, and filter service on the manufacturer’s schedule
• Written records of every run and service visit, which the air permit may also require

Because California permit conditions limit testing hours, the exercise schedule should be planned against the permit, not improvised. Sebastian’s commercial electrical services page lists scheduled exercise runs, load testing, and inspections as part of a standby generator project, so the testing plan can be set up alongside the installation. Sebastian’s commercial electrical maintenance program guide explains how maintenance contracts can include this kind of recurring work.

Frequently Asked Questions

What is the difference between a standby generator and a portable generator for a business?

A standby generator for a business is permanently installed and starts automatically through a transfer switch, while a portable generator must be moved into place, connected, and refueled by a person. Standby units suit sites that must keep running unattended; portable units suit temporary or small loads.

How long can a commercial backup generator run during an outage?

A commercial backup generator can run for as long as it has fuel and stays within the manufacturer’s service limits, so runtime depends on tank size or gas supply and on the connected load. The Valley Air District’s diesel standby policy does not cap hours for true emergency operation, though it limits hours for maintenance and testing.

Does a commercial backup generator need a permit in California?

A commercial backup generator in California generally needs local building and electrical permits, and many engine-driven units also need an air permit. In the San Joaquin Valley, new diesel emergency standby engines above 50 horsepower go through the Valley Air District, so confirm requirements with the district and your local building department before ordering.

How often should a commercial backup generator be tested?

A commercial backup generator should be tested on a written schedule that matches the system’s classification, the manufacturer’s instructions, the code edition your local fire authority enforces, and the air permit’s hour limits. Your service provider and your local fire authority can confirm the frequency, duration, and load that apply to your system.

What size backup generator does a commercial building need?

The size of a backup generator for a commercial building depends on the loads that must keep running, including the starting current of motors, not on square footage alone. A load list and a decision between critical-load and whole-building backup come first; capacity follows from there.

Planning Backup Power That Holds Up When the Grid Does Not

The most reliable backup power plans start with the loads that cannot go down, then work outward to the generator, fuel, and transfer switch that serve them. That order keeps equipment right-sized and keeps surprises out of the permit process.

Sebastian’s commercial electrical team handles standby generator work as part of its commercial electrical services, including coordination with the rest of the building’s electrical system. For a Fresno or Central Valley facility, that means one team can look at the generator, the switchgear, and the loads together.

If you are weighing options for a facility, you can request a proposal or call (877) 253-7835 to talk through your site and which loads need backup.