In short: a commercial backup generator is sized to the loads your business can’t lose, not to the whole building. Supplier guides put many small businesses at 30 to 50 kW; one supplier’s 2026 estimates put a 50 kW system at $16,000 to $40,000 installed. Codes (NFPA 110, NEC 700 to 702), fuel, runtime and maintenance contracts drive the rest of the decision.
This guide is for owners and managers of offices, shops, restaurants, warehouses, farms, apartment buildings and hotels. It explains how to work out critical loads, which code category your system falls into, diesel versus natural gas, ownership options, permits and what to ask a commercial generator dealer. At the end, you can send us your project and we will reply ourselves.
Who needs a commercial backup generator
U.S. electricity customers averaged about 11 hours without power in 2024, nearly twice the yearly average of the decade before, mostly because of hurricanes (EIA). Customers in South Carolina averaged nearly 53 hours. For a business, each of those hours can mean lost sales, spoiled stock or unsafe conditions. Typical needs:
| Property | What usually has to keep running |
|---|---|
| Offices | Servers and network, phones, security, some lighting and HVAC |
| Retail and restaurants | Refrigeration and freezers, point-of-sale, lighting, security; cooking if you want to stay open |
| Warehouses and light industry | Cold storage, controls, dock doors, sump pumps, critical process equipment |
| Farms | Ventilation and heating in livestock barns, well pumps, milk cooling, grain drying |
| Multifamily buildings | Elevators, hallway and exit lighting, water pressure pumps, fire alarm and life-safety systems |
| Hotels | Life-safety systems, elevators, guest-room power, kitchens, front desk and key cards. See backup power for hotels. |
Sizing basics: start with critical loads
Sizing a commercial generator starts with a list, not a catalog. A licensed electrician or engineer does the final load calculation, but you can prepare the inputs:
- List the critical loads by what each one protects: life safety, legal requirements, money (refrigeration, servers) and comfort.
- Get the numbers from nameplates or your electrician: running power and starting (inrush) power for motors, compressors and elevators.
- Check your utility bills. Many commercial bills show peak demand in kW. Your peak is the most the building drew in a billing period; your backup need is usually well below it if you only back up critical circuits.
- Decide runtime. Hours or days? That decides fuel and tank size.
- Plan for growth, such as EV chargers or a new freezer, but don’t oversize too far: generators run better under a reasonable load.
Our generator sizing calculator gives a first range for smaller sites. For a mix of generator and batteries, which can cover the first minutes or short outages quietly, see commercial battery storage.
Codes: emergency, legally required or optional standby
This is an overview, not code advice; your engineer and the local authority having jurisdiction decide. The National Electrical Code (NFPA 70) splits backup systems into three articles, and NFPA 110 sets how the generator system must perform:
| Category | NEC article | NFPA 110 | Typical loads |
|---|---|---|---|
| Emergency systems | 700 | Level 1: failure could cause loss of life or serious injury | Exit and egress lighting in places of assembly, fire alarm and life-safety systems |
| Legally required standby | 701 | Level 2: failure is less critical to life safety | Heating, refrigeration, communications, ventilation and smoke removal, sewage, systems whose loss could hamper rescue or fire-fighting |
| Optional standby | 702 | Not required by code | Business continuity: servers, refrigeration, point-of-sale, comfort loads |
Most small businesses that just want to stay open are in the optional standby category. Hotels, apartment buildings, assembly spaces and health-care sites often have emergency or legally required systems too, with stricter wiring, transfer times and testing. Stationary engines also fall under EPA rules: emergency generators may run only a limited number of hours a year for maintenance and testing (100 hours combined with a few other uses) and need an hour meter and records. Your state air agency may require a permit, especially for diesel.
Fuel: diesel, natural gas or both
| Diesel | Natural gas | |
|---|---|---|
| Fuel supply | On-site tank; runtime set by tank size and refueling | Utility pipeline; no on-site storage |
| Equipment cost (supplier estimate, 2026) | About $180 to $400 per kW, plus fuel infrastructure | About $220 to $500 per kW, 10% to 20% higher |
| Trade-offs | Fuel ages, tank rules and spill rules; independent of the gas network | Depends on gas pressure staying up during the event; utility may need to upsize the meter |
Makers sell both. Generac’s industrial line, for example, runs from 15 kW to 3.25 MW single units, with natural gas models for small sites and diesel models for larger ones, plus bi-fuel units that start on diesel and switch to natural gas under load to stretch fuel. Propane is common for smaller rural sites and farms.
Transfer switches, placement and safety
The automatic transfer switch is the heart of the system. It senses the outage, starts the generator, moves the chosen loads over and moves them back when utility power returns. It also keeps generator power from flowing back onto the utility lines, where it can injure line workers. Buildings with both life-safety loads and business loads often end up with more than one transfer switch, because codes keep emergency circuits separate from everything else; your engineer lays that out.
Placement matters as much as size:
- Exhaust and air intakes. Generator exhaust contains carbon monoxide. Keep it away from doors, windows and rooftop or wall air intakes, and put CO alarms in occupied spaces nearby.
- Fire clearances. NFPA 37, the fire standard for stationary engines, sets distances from walls and openings; the local fire marshal applies it.
- Flooding. Put the unit and its switchgear above likely flood levels. A generator in a flooded basement or low parking lot won’t help after a hurricane.
- Noise and neighbors. Sound enclosures and placement help meet local noise ordinances, which matter for hotels and apartment buildings.
- Access. Room for service, fuel deliveries and, for large units, a crane on installation day.
Portable generators are no substitute for a business: running one near a doorway or loading dock is exactly how carbon monoxide gets inside. If you use a rental unit, have an electrician install a proper connection point and keep it outdoors, away from openings.
What commercial generators cost
We found no government data on commercial generator prices. The ranges below are one generator supplier’s published 2026 estimates (ZCC Power, June 2026), installed, including the unit, automatic transfer switch, electrical integration, foundation and exhaust. Fuel storage, permits and testing are often extra. Use them only to set a budget before quotes.
| Size | Installed estimate (supplier, 2026) |
|---|---|
| 50 kW | $16,000 to $40,000 |
| 100 kW | $28,000 to $73,000 |
| 150 kW | $46,000 to $106,000 |
| 200 kW | $63,000 to $143,000 |
| 500 kW | $135,000 to $305,000 |
What moves the price: size and fuel, the distance from the service entrance, switchgear and transfer switches (one or several), sound enclosures, a diesel tank, crane access, engineering and permitting, and whether the system must meet emergency (NEC 700) rules. Smaller sites can sometimes use the same air-cooled units sold for homes; see standby generator cost for those prices.
Buy, finance or rent: ownership options
- Purchase: you own the asset and its maintenance. Simplest when you have the budget.
- Equipment loan or lease: spreads the cost over a term; compare the total paid, end-of-term options and who maintains the unit.
- Rental: trailer-mounted generators for events, construction, or as a bridge while a permanent system is designed. You’ll need a safe hookup point; a pre-wired connection box (installed by an electrician) makes it faster when a storm is coming.
- Service agreements: almost everyone needs one, whichever way the unit is owned.
Tax: fuel-burning generators get no federal clean energy credit. Depreciation and other business tax treatment depend on your situation: ask a tax professional. Some utilities and states have resilience programs; see incentives and your state page.
Maintenance contracts and testing
A generator that isn’t tested may not start when it matters. Systems under NFPA 110 have set inspection, exercise and load-test routines, which industry guides summarize as weekly inspections and a monthly exercise under load. EPA rules require records of run hours. A good service contract covers scheduled visits, oil and filters, battery and coolant checks, load-bank testing when needed, and response times during storms. Read more in generator maintenance and repair.
Permits and the project timeline
Expect electrical and building permits, a fuel or fire permit (especially for diesel tanks), possibly an air permit, zoning review for noise and placement, and a utility review for natural gas. Larger systems need stamped engineering drawings. The work is done by licensed electrical contractors; check them on your state’s license lookup, linked from each state page. Plan months, not weeks, for larger systems, and order before storm season.
Questions to ask a commercial generator dealer
- Which loads did you include, and which code category (NEC 700, 701 or 702) applies?
- What runtime does the fuel plan give at our expected load?
- What’s included: transfer switches, pad, enclosure, tank, permits, engineering, start-up, load test?
- What are the electrical contractor’s license number and insurance?
- What does the service contract cover, and what is the storm-time response promise?
- What is the lead time for the unit?
Get help with your project
Tell us about your building, what must keep running and for how long. Tell us about your project and we will connect you with a licensed, independent installer who serves your area. It is free, there is no obligation, and only the installer you choose gets your details. See how it works.
Commercial generator questions
What size generator does a small business need?
It depends on the critical loads, not the building’s size. Supplier guides put many small businesses at 30 to 50 kW for refrigeration, lighting, point-of-sale and some HVAC, but a restaurant with walk-in coolers needs more than an office. List what must run, get running and starting power from nameplates, and have a licensed electrician do the load calculation before you buy.
How much does a commercial backup generator cost?
One supplier’s 2026 estimates put a 50 kW system at $16,000 to $40,000 installed and a 100 kW system at $28,000 to $73,000, with fuel storage and permits often extra. We found no government price data, so treat these as a budget guide only. Site work, transfer switches, enclosures and code category move the final price a lot.
Diesel or natural gas for a business generator?
Natural gas needs no on-site fuel storage and runs as long as the pipeline holds pressure; diesel stores fuel on site and doesn’t depend on the gas network, but tanks bring rules and fuel upkeep. Supplier estimates put natural gas equipment 10% to 20% higher per kW. Bi-fuel units start on diesel and switch to gas to stretch runtime.
Is a business backup generator required by code?
For most small businesses, no: a generator for business continuity is “optional standby” under NEC Article 702. Codes can require emergency (Article 700) or legally required standby (Article 701) power in places of assembly, high-rises, health-care sites and some multifamily buildings. Your engineer and local authority decide which applies, and NFPA 110 sets the performance rules.
Can a battery replace a commercial generator?
For short outages and for cutting demand charges, sometimes. For long outages, a battery runs out unless solar or a generator recharges it. Many sites pair them: the battery covers the first minutes or hours quietly, and the generator handles multi-day events. Compare both in our commercial battery storage guide before you decide.
Keep exploring
See energy for businesses, hotel backup power, solar and EV charging, commercial battery storage, commercial EV charging and our backup power guide. Back to the generators hub.
Sources
Outage hours: U.S. Energy Information Administration (EIA), December 2025, 2024 data. Code categories: National Fire Protection Association (NFPA), NFPA 110 overview and NFPA 70 article references, read October 2026. Emergency engine hours and records: U.S. EPA stationary engine fact sheets. Installed cost and $/kW by fuel: ZCC Power, commercial standby generator cost, June 2026 (supplier estimate, not verified independently). Product range: Generac Industrial Power, read October 2026. Federal credits: IRS; tax treatment: ask a tax professional. Estimates on this page are ours or the named supplier’s and are labelled.