Generator sizing
Generator size is decided by the load you need to carry, not by the size of the building. This page explains how sizing actually works, and what to have ready before you buy.
Get help sizing itIn a hurry? Use the size calculator to get an estimate in about a minute, then come back for the reasoning.
The most common sizing mistake is starting with square footage. A 4,000 square foot warehouse with a few lights and an overhead door needs far less generator than a 1,500 square foot shop running a compressor, a welder, and refrigeration. The generator has to match what is actually drawing power.
Write down every piece of equipment that must run during an outage. Take the running watts from each nameplate. Add them up. That total is your running load, and it is only half the answer.
Anything with an electric motor — an air conditioner, a well pump, a compressor, a walk-in cooler, a hydraulic power unit — pulls a large inrush of current the moment it starts. Started across the line, that surge commonly runs three to six times the motor’s running current, and it lasts only a second or two.
That short surge is what stalls undersized generators. In practice the sizing question becomes: what happens when the largest motor on the system starts while everything else is already running? If the generator cannot absorb that, voltage sags, breakers trip, and equipment does not come up. Soft starters and variable frequency drives reduce the surge, and sometimes staggering start-up order solves it without buying a bigger unit.
These figures are approximate and vary by make, model, age, and efficiency. Use them to get oriented, then use your own nameplates for the real number.
| Load | Approx. running watts | Notes |
|---|---|---|
| Refrigerator or freezer | 150 – 800 W | Compressor surge on start |
| Well pump, 1 hp | ~1,000 W | Can surge to 3,000 W+ |
| Central air conditioner, 3 ton | 3,000 – 4,000 W | Large starting surge unless soft start is fitted |
| Electric water heater | 4,500 W | Resistive, no surge |
| Electric range | 8,000 – 12,000 W | Rarely all elements at once |
| Walk-in cooler | 1,500 – 3,000 W | Cycles, surges on start |
| Air compressor, 10 hp | ~7,500 W | Starting draw is the sizing driver |
| Small CNC or shop machine | Varies widely | Use the nameplate, not a guess |
Generator sets are rated in kilowatts (kW) and often also in kVA. kW is the real working power; kVA is apparent power. For typical three-phase equipment at a 0.8 power factor, kW equals kVA multiplied by 0.8 — so a 100 kVA set is usually an 80 kW set.
Voltage and phase have to match your service. Homes and small shops are generally single phase at 120/240 volts. Commercial and industrial sites are usually three phase at 208 or 480 volts. Many industrial gensets have a reconnectable alternator that can be configured for more than one voltage, but this is something to confirm on the specific unit before it is delivered, not after.
The same physical generator carries different ratings depending on how it is used. A standby rating assumes emergency use with a varying load and limited hours per year. A prime rating assumes it is the main power source with a varying load and unlimited hours. A continuous rating assumes a steady load around the clock. A unit advertised at 100 kW standby is not a 100 kW continuous unit. If the generator is going to be your regular power source rather than a backup, say so early — it changes which units are appropriate.
Buyers often reach for the largest unit in budget. That costs more up front, burns more fuel, and can shorten engine life. A diesel engine that runs for long periods at very light load tends to wet stack: unburned fuel and soot accumulate in the exhaust, and the engine eventually needs to be loaded properly or serviced to clear it. Diesel gensets generally prefer to work somewhere around half to three quarters of their rating.
The goal is sensible headroom over your real peak — enough to absorb motor starting and leave room to grow — not the biggest number available.
As a rough planning figure, a diesel genset burns in the neighborhood of 0.07 gallons per hour for each kW it is actually producing. A 60 kW unit at full load is therefore in the range of four to five gallons per hour, and considerably less at partial load. Work out how many hours you need to cover, and confirm the tank on the specific unit. A generator with a base tank sized for eight hours is a very different plan from one sized for three days.
You do not have to arrive with any of it worked out. Tell us what needs to stay running, what the site looks like, and where in Florida it is going. GTG Power has 30 years in generator repair and sales — we ask the practical questions, explain which sizes are realistic, confirm the specification and price of the actual unit, and arrange delivery.
It depends on whether you want to run the whole house or only essential circuits. Many Florida homes are covered by a 20 kW to 30 kW standby unit running the full house including central air conditioning, while an essentials-only setup covering refrigeration, well pump, lights, and a window unit can be much smaller. The correct answer comes from adding up the actual loads, not the square footage.
kW is real working power and kVA is apparent power. Generator sets are commonly rated in both, and for typical three-phase equipment kW equals kVA multiplied by a 0.8 power factor. A 100 kVA generator is usually an 80 kW generator.
An electric motor started across the line draws several times its running current for the first moments of startup, commonly in the range of three to six times. A 10 hp compressor that runs comfortably on a small generator can stall or trip that same generator at the instant it starts. Starting load, not running load, is what usually decides the size.
Oversizing costs more up front and can cause problems. A diesel engine that spends its life lightly loaded can suffer wet stacking, where unburned fuel and soot build up in the exhaust. Diesel gensets generally run best somewhere around half to three quarters of their rating. A generator sized with sensible headroom is better than one sized with enormous headroom.
Most homes and small shops are single phase at 120/240 volts. Commercial and industrial buildings are usually three phase at 208 or 480 volts. Check the equipment nameplates and your existing service. Putting a three-phase generator on a single-phase service, or the reverse, will not work.
Anything you have helps: the equipment nameplates, a photo of the main breaker panel and its main breaker rating, your utility bill, the voltage and phase of the service, whether the generator is for standby or daily use, and the delivery ZIP code. If you do not have these, we can work through it on the phone.
Generator sales and delivery in Florida