An industrial generator is a diesel or natural gas engine coupled to an alternator on a common skid. The engine spins the alternator at a fixed speed, usually 1800 RPM for 60Hz power, and the alternator turns that rotation into AC power, three-phase on most commercial units. A controller, voltage regulator, and transfer switch handle the rest. On a standby system, the whole sequence from grid failure to building power takes about 10 to 30 seconds without anyone touching it.
A few numbers worth knowing up front:
- Standby units are rated for emergency use of 200 hours a year or less. Prime and continuous units are built to run indefinitely.
- Diesel runtime depends on tank size and load. A 24-hour fuel supply at full load is a common spec, and critical facilities often store much more.
- Diesels run best at 50% to 80% of rated load. Below about 30% for long stretches, they wet-stack.
- Weak or dead starting batteries are the most common reason a standby generator fails to start.
This post is for people buying, specifying, or maintaining commercial and industrial units from 20kW to 4MW+. It covers the startup sequence, all ten major systems on a genset, fuel types and ratings, and sizing. If you want the physics underneath it, how a spinning magnetic field makes current in the first place, our earlier article on how a generator creates electricity covers the basics of electromagnetic induction.
What Happens When the Power Goes Out
On a standby system, the whole sequence from blackout to building power usually takes 10 to 30 seconds, and nobody has to touch anything. Here’s what’s going on in that window.
- The transfer switch senses the loss. The automatic transfer switch (ATS) sits between the utility feed, the generator, and your building. It’s watching utility voltage and frequency constantly. When either drops out of range for more than a couple of seconds (the delay keeps it from reacting to a flicker), it signals the generator controller to start.
- The controller cranks the engine. The control panel closes the starter circuit. The starting batteries, kept topped off by a battery charger, spin the engine. On a healthy engine this usually takes a few seconds.
- The engine comes up to speed. For 60Hz power the engine has to hold a fixed RPM, typically 1800 for most industrial units. The governor manages fuel delivery to lock in that speed, because engine speed is what sets the output frequency.
- The alternator builds voltage. As the rotor spins, the voltage regulator ramps up the field current until output hits the target, say 480V. Once voltage and frequency are both stable, the controller tells the ATS the generator is ready.
- The ATS transfers the load. The switch opens the utility side and closes the generator side. Lights come back on. Depending on the controller settings, this may happen all at once or in steps so motors don’t all start together and bog the engine.
- Utility returns, and the system waits. When grid power comes back, the ATS doesn’t flip immediately. It watches for a few minutes to confirm the utility is stable, then transfers the load back. The generator keeps running unloaded for a cooldown period, usually five to ten minutes, before shutting itself down.
On a prime or rental unit at a job site there’s no utility to lose and often no ATS. Someone starts it from the panel, lets it warm up, and closes the breaker. Same machine, less automation.
The ATS deserves the same attention as the generator. Contacts wear and controls fail, and a switch that never gets exercised may not transfer cleanly when it’s finally needed, so it should be tested along with the generator during regular exercise runs.
The 10 Main Components of an Industrial Generator
A generator set, or genset, is really an engine and an alternator bolted to a common frame with the systems needed to keep both alive. Here’s what each piece does and what to pay attention to when you’re evaluating a unit.
1. Engine
The engine is the prime mover. It turns chemical energy in fuel into rotation, and its horsepower largely sets how much power the generator can deliver. Industrial gensets run on diesel or natural gas, with propane and bi-fuel setups in some applications. You’ll see engines from Caterpillar (CAT), Cummins, Volvo, John Deere, MTU, Perkins, and Kohler on units in our yard.
Two things matter more than the brand. First, the engine has to hold its rated speed, 1800 RPM on most 60Hz units, under changing load, because that’s what keeps you at 60Hz. The governor handles this, mechanically on older units and electronically on anything recent. Second, hours and maintenance history tell you more about remaining life than the model year does. A 15-year-old diesel with 400 hours and clean oil samples is often a better buy than a 5-year-old unit that ran hard as prime power.
2. Alternator
The alternator is where mechanical energy becomes electrical energy. In the trade it’s also called the genhead or generator end. It has two main parts: a rotor that spins with the engine and a stator that holds the output windings. The rotor carries a magnetic field. As it turns inside the stator, the moving field induces current in the stator windings, and that’s your output.
Nearly every industrial alternator is brushless. Instead of brushes feeding current to the rotor, a small exciter generator on the same shaft produces AC, a set of rotating rectifiers converts it to DC, and that DC energizes the main rotor field. No brushes means no wear parts in the field circuit and far less maintenance. Many units add a permanent magnet generator (PMG) to feed the regulator, which gives better performance on motor starting and non-linear loads like VFDs and UPS systems.
The alternator’s nameplate tells you kW, kVA, voltage, phase, and temperature rise. Marathon, Stamford, and Leroy-Somer are the common brands.
3. Voltage Regulator (AVR)
The automatic voltage regulator holds output voltage steady as load changes. When a big motor starts and voltage sags, the AVR increases current to the exciter, the rotor field strengthens, and voltage recovers. When load drops off, it does the reverse. Modern digital AVRs also handle under-frequency rolloff, which pulls voltage back if the engine starts to bog so it can recover without stalling.
When lights flicker or electronics trip on generator power, the AVR and the engine governor are the usual suspects.
4. Fuel System
On a diesel unit, the fuel system is the tank, a lift pump, primary and secondary filters, a water separator, and the injection system. The tank is usually a double-wall sub-base tank built into the skid. Runtime depends on tank size and load. Many standby units are specified for 24 hours at full load, and critical facilities often add bulk storage tanks feeding a day tank for longer runs.
Diesel degrades in storage. Water condenses in the tank, microbes grow at the fuel-water line, and the result is clogged filters at the worst possible moment. Fuel polishing and regular sampling are part of any serious maintenance program. Our fuel consumption chart shows how much diesel a unit burns at 25%, 50%, 75%, and 100% load, which is how you figure out how long your tank actually lasts.
Natural gas units skip the tank. They connect to the utility gas line through a regulator and fuel train, and runtime is limited only by the gas supply. The tradeoff is that gas pressure and supply can be affected by the same event that took the grid down.
5. Cooling System
An engine turning 1800 RPM under load makes a lot of heat. Nearly all industrial gensets are liquid-cooled: a water pump circulates coolant through the block, an engine-driven or electric fan pulls air through a radiator, and a thermostat regulates temperature. Larger installations sometimes use remote radiators or heat exchangers fed by a cooling tower.
Cooling problems are a common cause of automatic shutdowns. Low coolant, a failed belt, a plugged radiator core, or a fan clutch that won’t engage will all trip a high-temperature alarm. Block heaters keep the coolant warm so a cold engine can start and accept load quickly, which matters a lot in Colorado in January.
6. Exhaust System
The exhaust system carries combustion gases away from the engine and the building. It includes the manifold, flex connector, a muffler or silencer, and piping to the outdoors. Silencers are rated by how much noise they cut, from industrial grade up to critical and hospital grade.
On newer diesels, exhaust also runs through emissions aftertreatment: diesel oxidation catalysts, diesel particulate filters, and selective catalytic reduction (SCR) that injects diesel exhaust fluid. EPA Tier 4 Final units use some combination of these, depending on the engine. Stationary emergency units aren’t required to meet Tier 4 under federal EPA rules, so most are Tier 2 or Tier 3, though some state and local air districts set stricter limits. Which tier you need depends on how the unit will be used and where it’s installed, and it affects both price and maintenance.
7. Lubrication System
The oil pump, filter, cooler, and sump keep every moving part in the engine from grinding itself apart. Oil also carries away heat and holds contaminants in suspension until the next change.
For standby units the interval is usually annual or every 250 hours, whichever comes first. Low oil pressure triggers an automatic shutdown on industrial controllers, so a leak or a weak pump can take the unit offline in the middle of an outage. Oil analysis is cheap and tells you what’s happening inside the engine without pulling it apart.
8. Battery and Battery Charger
Industrial gensets start on 12V or 24V lead-acid batteries, the same basic type that’s in a truck. A battery charger wired to utility power keeps them at float voltage so they’re ready to crank the moment the controller calls for it.
Dead batteries are the single most common reason a standby generator fails to start. Not fuel, not the engine, batteries. They age out in three to five years regardless of use. They should be load tested at every preventative maintenance visit.
9. Control Panel
The controller is the brain. It starts and stops the engine, monitors oil pressure, coolant temperature, battery voltage, RPM, output voltage, frequency, and current, and shuts the unit down if any of them go out of range. It also talks to the ATS and logs alarms and run hours.
Modern controllers from Deep Sea, ComAp, and the OEMs (Cummins PowerCommand, CAT EMCP, Kohler Decision-Maker) support remote monitoring over Modbus or Ethernet, so a facilities team can see generator status from a phone. On a used unit, the controller’s event log is one of the first things we read.
10. Frame, Enclosure, and Base
Everything mounts to a steel skid with vibration isolators between the engine-alternator assembly and the frame. Outdoor units sit inside a weather-protective or sound-attenuated enclosure. Sound attenuation matters in cities and near hospitals or residential areas, and local code often sets the dB limit at the property line.
The frame also provides the grounding point for the whole system, and the sub-base fuel tank on most diesels is built into it. If you’re buying used, look at the skid and enclosure for corrosion, especially on units that have lived in coastal Florida or Texas.
Diesel vs. Natural Gas, and Standby vs. Prime vs. Continuous
Two decisions shape almost everything else about a genset: what it burns and how often it’s expected to run.
Fuel
Diesel is the default for larger units and for anything where fuel independence matters. Diesel engines are more fuel-efficient, respond faster to load changes, and the fuel is on site under your control. The downsides are emissions compliance, fuel storage and upkeep, and noise. Most units over about 500kW are diesel.
Natural gas wins where a gas line is already there and runtime is a priority. No tank to fill, cleaner exhaust, quieter operation, and lower fuel cost over long runs. Gas engines are less tolerant of sudden load steps, take a bit longer to start, and depend on the utility gas supply staying up. They’re common in the 20kW to 500kW range for commercial buildings, and increasingly common at larger sizes.
Rating
Every genset carries a rating that says how it’s meant to be used. The same engine and alternator will carry a different kW number depending on the rating.
| Rating | What it means | Typical use |
|---|---|---|
| Emergency standby (ESP) | Runs only when utility fails. Rated for 200 hours a year or less, variable load, no overload capacity. Highest kW number. | Hospitals, data centers, commercial buildings |
| Prime (PRP) | Runs as the primary source, unlimited hours, variable load, 10% overload for 1 hour in 12. Rated about 10% below standby. | Job sites, remote facilities, rentals |
| Continuous (COP) | Runs as the primary source at a constant load, unlimited hours. Rated lowest, often 20% to 30% below standby. | Mining, oil and gas, utility peaking |
Buying a standby-rated unit and running it as prime power shortens its life and can void the warranty. It happens more than you’d think, usually when a rental or temporary installation turns permanent. If you’re not sure how a used unit was run, the controller’s hour meter and run log will tell you.
How Big a Generator Do You Need?
Understanding how a generator works is the easy part. Picking the right size is where most people get into trouble, in both directions.
Too small and the engine bogs when a chiller or elevator motor starts, voltage sags, and sensitive equipment trips offline. Too big and you’ve overpaid, and a diesel that runs at 20% load for years will wet-stack: unburned fuel builds up in the exhaust, the engine runs rough, and you’re looking at an expensive cleanup. Diesels like to run at 50% to 80% of rated load.
The short version of sizing:
- Add up the running load of everything the generator needs to carry, in kW. Your electric bill’s peak demand is a decent starting point for a whole-building load.
- Find the largest motor that will start on generator power. A motor started across the line can pull six times its running current or more for a few seconds, and the generator has to absorb that surge without an unacceptable voltage dip. Where there’s sensitive equipment, that dip is often capped at 15% to 20%.
- Add 20% to 25% headroom for future growth and to keep the engine out of its worst operating range.
- Check the voltage and phase match your service. Most commercial buildings are 480V or 208V three-phase.
Our Generator Size Calculator walks through this with your actual numbers and converts between kW, kVA, amps, and horsepower. For a quick lookup there’s also the kVA to kW to Amps chart.
For anything over a few hundred kW, or any facility where downtime is expensive, it’s worth a conversation with someone who sizes these for a living. Our team does it every day. Talk to a rep or browse current inventory to see what’s available in your range right now. it. Talk to a rep or browse current inventory to see what’s available in your range right now.
Frequently Asked Questions
How fast does a standby generator turn on after an outage? Most commercial standby systems pick up load 10 to 30 seconds after utility power fails. Life-safety circuits in hospitals and similar facilities fall under NFPA 110 Level 1, which requires power restored within 10 seconds. A short delay before startup is intentional: it keeps the system from reacting to a brief flicker.
What’s the difference between a generator and an alternator? In the generator business, the alternator is the part of the genset that makes electricity. The engine supplies the rotation, the alternator converts it to AC power. People also call it the genhead or generator end. The whole package of engine plus alternator plus controls is the generator set.
How long can an industrial generator run continuously? It depends on fuel and rating. A diesel standby unit with a sub-base tank typically runs 24 to 72 hours at full load before refueling. A natural gas unit runs as long as the gas supply holds. Prime and continuous-rated units are built for unlimited hours; standby-rated units are meant for under 200 hours a year.
Why does a generator run at 1800 RPM? Output frequency is set by engine speed and the number of magnetic poles in the alternator. A four-pole alternator at 1800 RPM produces 60Hz, which is the US standard. Two-pole units run at 3600 RPM, which is common on small portable units but louder and harder on the engine. Fifty-Hz countries run four-pole alternators at 1500 RPM.
What is an automatic transfer switch? The ATS is the device that moves your building’s load between utility power and generator power. It senses when utility fails, tells the generator to start, transfers the load once the generator is stable, and transfers back when utility returns. Without one, somebody has to switch over manually.
What is wet stacking? Wet stacking happens when a diesel runs at light load for long periods. The engine doesn’t get hot enough to burn all the fuel, and unburned fuel and carbon build up in the exhaust system. Symptoms are black exhaust, oil weeping from the exhaust joints, and poor performance. The fix is running the unit under heavy load, usually with a load bank, and the prevention is sizing the generator correctly in the first place.
What is load bank testing? A load bank is a resistive load connected to the generator to simulate a real building load. Testing with one proves the unit can actually carry its rated kW, burns off wet stacking, and catches cooling or fuel problems before an outage does. Most standards call for it at least annually. More on load bank testing here.
Why won’t my generator start? Batteries, nine times out of ten. Dead or weak starting batteries are the most common cause of standby failure. After that: bad fuel or clogged filters, a tripped controller alarm from a previous run, low coolant, or a failed block heater in cold weather. If the engine cranks but won’t catch, think fuel. If it doesn’t crank at all, think battery or controller.
Can I run a 50Hz generator in the US? Not without changes. A 50Hz unit’s alternator and controls are set up for 1500 RPM. Some can be converted to 60Hz with a re-rate and controller reprogramming, but not all, and the kW rating changes. We only buy and sell 60Hz equipment. Our voltage and frequency by country chart shows which standard applies where.
How often does an industrial generator need maintenance? For a standby unit: weekly or monthly exercise runs, a full service visit at least annually or every 250 hours, oil and filter changes at the same interval, battery load testing every visit, and annual load bank testing. Prime units on more aggressive intervals set by run hours. Our preventative maintenance program covers all of this with a 31-point inspection.