by Rock Reed

Contractors on commercial jobsites depend on the power delivered by generators to keep projects on schedule and on budget. By extension, making sure these portable powerhouses start every time and run dependably—without costly, time-consuming breakdowns—is a challenge. The first step is to pick right generator for the job.

Understanding your power requirements is the most important factor in selecting the right generator for your job requirements. Once the generator is selected, establishing a routine maintenance schedule ensures the smartest starts and longest life from the generator.



Power Requirements

Simple power management allows a smaller generator to power a big job because very seldom do all tools or appliances operate simultaneously. When calculating power requirements, consider that tools generally require more wattage to start than they do to actually operate. Remember Ohm’s law: 

Watts = Volts x Amps

Amps = Watts/Volts



Several resources can help determine the rated power needed from your generator. See the chart on page 36 for commercial wattage specifications.  


Checks and Balances

Preventive maintenance begins with monitoring oil and air—two vital factors in efficient generator operation. 

On any generator—especially those used in commercial applications—the oil level should be checked daily and every time gasoline is added. Many generators have single-cylinder air-cooled engines that tend to consume more oil than those in other vehicles. These engines may see more dimensional changes as the load on the generator fluctuates and thermal expansion occurs.  


Also, the air filter’s condition should be monitored, especially for commercial generators that operate in dirty or dusty environments. A clogged air filter can cause overheating and a breakdown.  The filter must be clean and clear—it may need to be cleaned every day in dusty conditions.

For more detailed maintenance guidelines, your workers should refer to the generator’s shop manual. The manual is a handy reference guide that provides troubleshooting procedures for all common problems. 

For anything beyond routine maintenance, the generator should be taken to a dealer.  A person conducting a running test of a generator could be exposed to 120 or 240 volts of electricity, which could be fatal in certain conditions. Even if the generator is shut off, a generator with a condenser (capacitor) can cause a terrible shock.


Future Problems 


Along with routine maintenance checks, it is important to monitor a generator’s major wear items. A generator sometimes controls voltage very accurately at different loads, but the brushes can wear prematurely if dust gets on the rotor’s slip rings.  The result is usually an erratic output or no output. 

 Problems can be avoided by consulting the generator’s shop manual for the brushes’ wear-limit dimensions. There is usually no standard schedule for checking the brushes, but most rental shops will check them periodically. Replacement brushes are fairly inexpensive and can be replaced easily by removing the alternator’s end cover.

The generator’s valve clearance should also be checked routinely.  Left unmonitored, the clearance can become too large, and the recoil starter may become hard to pull. This is because the automatic compression release on the camshaft can no longer operate. Adjusting the valve clearance will fix this. 


Proper Operation 

Common sense goes a long way when using any type of generator. Be sure your crew follows these cautionary practices: 

•For proper operation, the model should be placed in an open, well-ventilated space. A generator should never operate in an enclosure, which could cause carbon monoxide poisoning. 

•A generator should operate within 3 feet of any wall since all generator alternator windings and engines need a steady flow of cooling air. If this is not followed, the engine might need a rebuild, or the rotor/stator might need to be replaced.  (In this case, it would be less expensive to buy a new generator).

•To minimize the risk of electric shock, the generator must stay dry and on a firm, level surface while running. The generator should run at least once a month. This lubricates the engine by circulating oil, runs fresh gas through the carburetor and recharges the battery. All of these actions ensure the generator will start.

•The fuel tank and carburetor float bowl should be drained if the generator will not be used in a month’s time.  Otherwise, the fuel may evaporate and cause the small passageways to gum, which will require a visit to a dealer for carburetor cleaning or replacement.

•If the generator will be connected to a building’s electrical system, it must be connected through a transfer switch. This switch breaks the connection to the electric utility company and makes the connection to the generator.  Without this switch, it will feed back into the generator and overheat the windings when the utility power comes back on—necessitating a costly repair. Or even worse, a worker repairing the electric lines would be exposed to high voltage from the generator as its output is increased and fed back through a transformer.

•A generator should never be refueled while it is running. Spilled fuel may ignite, so spills should be cleaned.

•The generator’s maximum output capacity should not be used more than 30 minutes at a time. Continuous operation should not exceed the rated load (generally 90 percent of the maximum load depending on the cooling system). Again, refer to your owner’s manual for additional specifics. 

•A generator should not be overloaded. Even though the breaker will trip, it has a time delay to allow starting electric motors. Repeated overloads may degrade the insulation on the windings over time, and it can be expensive to replace the rotor and stator.

•If the breaker keeps tripping, tools should be unplugged. (In this case, it might be wise to buy a generator with more power output).


Jobsite Tool/Application

Approximate Wattage

Required for Starting


Running Wattage

 Air Compressor    
1/2 hp  1,600  975 
1 hp  4,500  1,600 
Bench Grinder, 8.0 inches  2,500  1,400 
Circular Saw, Heavy Duty, 7.25 inches 2,300 1,400 
Concrete Vibrator    
1/2 hp  840 (avg.)  840 (avg.)
1.0 hp  1,080 (avg.)  1,080 (avg.) 
2.0 hp  1,560 (avg.)  1,560 (avg.) 
3.0 hp  2,400 (avg.)  2,400 (avg.) 
Demolition Hammer  1,260 (avg.)  1,260 (avg.) 
Drain Cleaner  250 (avg.)  250 (avg.) 
3/8 inch, 4.0 amps  600 440 
1/2 inch, 5.4 amps  900  600 
Electric Chain Saw, 14 inches, 2.0 hp  1,100  1,100 
Hand Drill, 1/2-inch  900  600 
High-pressure Washer, 1.0 hp  3,600  1,200 
Rotary Hammer    1,200 (avg.)     1,200 (avg.)   
Table Saw, 10 inches  4,500 1,800