How to Size a Generator for a Small Commercial Building

Sizing a commercial generator is not a square footage problem. It is a load problem. The right size comes from adding up what actually has to run during an outage, accounting for the extra current motors pull at startup, and then deciding how much of the building you want covered. Two buildings the same size can land three sizes apart once you look at what is plugged in. A dental office in Viera and a small grocery in Cocoa with the same floor plan will not need the same machine.

Here is how the calculation works, what information makes it accurate, and what to look for in a proposal before you sign anything.

What goes into the load calculation

A proper calculation starts with your electrical service, not a guess. We pull the numbers from three places:

• Your utility bills. Twelve months of kilowatt demand data shows the real peak your building hits, including August afternoons when every rooftop unit is running at once.
• The panel schedule and service size. Whether you are on 200 amps single-phase or 400 amps three-phase changes the whole conversation, and three-phase service narrows the equipment options.
• A walkthrough of the equipment. Nameplate data on compressors, air handlers, walk-in coolers, elevators, pumps, and anything with a motor.

Demand data tells us what the building averages and peaks at. Nameplates tell us what happens in the first few seconds after the transfer switch closes, which is a different and usually bigger number.

Why motors drive the decision

Anything with a motor draws far more current starting up than it does running. A compressor that hums along at a modest load can pull several times that for a moment when it kicks on. On utility power you never notice, because the grid absorbs it. A generator is a much smaller power source, and if too many motors try to start at once, voltage sags, equipment faults out, and you get nuisance shutdowns during the exact storm you bought the thing for.

This is the single most common sizing mistake we see on quotes from other outfits. The generator matches the average load fine and then struggles the first time the AC and the walk-in call for cooling in the same second.

There are a few ways to handle it:

• Size up so the unit swallows the inrush without complaint.
• Use load shedding, where the controller sheds non-critical circuits (a second AC unit, exterior lighting) when the generator is near capacity and restores them when the load drops.
• Stagger the restart so equipment comes back on in sequence instead of all at once.
• Soft starters on the biggest motors, which can meaningfully lower the generator size you need.

Load shedding is often the smarter buy. Instead of paying for a larger machine to cover a peak that lasts seconds, you spend less on controls and get the same protection.

Optional standby versus legally required

Not every commercial generator carries the same obligation. Most small businesses in Florida are installing what code calls optional standby: you choose what gets backed up, and you choose how much. A restaurant might cover refrigeration, hood exhaust, POS, and a few lighting circuits and let the dining room ambient lighting go dark.

Some occupancies do not get that choice. Assisted living facilities, certain medical uses, and buildings with emergency egress or fire systems fall under required standby or emergency system rules, which come with tighter requirements on wiring separation, transfer times, and testing. If you are in one of those categories, tell your contractor before design starts. It changes the equipment, the wiring method, and the inspection.

Either way, the generator has to be sized and installed to the Florida Building Code and the NEC as adopted here, and it has to survive our wind loads and salt air. Coastal sites in Delray Beach or along the Indian River need corrosion-resistant enclosures and hardware. Our team handles commercial standby generator installation across both service areas, and the specification changes noticeably between an inland Wellington site and one two blocks off the water.

What a real proposal should show you

Before you sign, ask for these in writing:

• The load calculation itself, with the connected load, the calculated demand, and the starting load broken out.
• Exactly which circuits transfer and which do not, panel and breaker by breaker.
• Transfer switch type, size, and whether it is service-rated or downstream of your main.
• Fuel type, and for propane or diesel, tank size and expected runtime at roughly half load.
• Who pulls the permit, and whether gas piping and any structural pad work are included or excluded.
• Utility and gas coordination, especially if the meter or service needs upsizing.
• How much downtime the tie-in requires, and whether the crew can do it after hours.

That last one matters more than most owners expect. The actual power interruption for the transfer switch tie-in is usually short, but it is not zero, and we schedule it around your operating hours whenever the utility allows. A Boynton Beach retailer might take a Tuesday morning. A restaurant in West Palm Beach almost always wants it between lunch and dinner service or overnight.

Cost follows the same variables: service size, single-phase versus three-phase, how far the generator sits from the panel, fuel and gas line work, permitting, and whether we are working outside normal hours. Anyone quoting a price before doing the load calculation is guessing.

If you want a real number for your building, we will do the calculation first. Call us at (844) 432-2262 or request a free, no-obligation quote through our commercial electrical services page, and we will get back to you within one business day to schedule a walkthrough.

Frequently Asked Questions

Do I need to back up the whole building?

No. Most small commercial buildings use optional standby, where you pick the circuits that matter and leave the rest off the generator. Covering only critical loads is usually the better value, though some occupancies like assisted living and certain medical uses have required backup that is not optional.

What happens if the generator is undersized?

You get voltage sag and nuisance shutdowns when large motors start, which tends to show up at the worst time. Correct sizing accounts for starting current, not just running load, and load shedding controls can solve the problem without buying a larger unit.

Does a commercial generator need a permit in Florida?

Yes. Commercial generator installations require an electrical permit and, for gas units, a gas permit, with inspection by your county or city building department. We pull the permits and handle the inspections as part of the job.