All generators, from permanently installed standby units for backup power, to portable generators on job sites, have two very important specifications that all generator users should be aware of.
- Maximum Continuous Power is the power the generator is capable of supplying all the time.
- Maximum Surge Power is the total power the generator can supply for a few short moments.
Both specifications are given in watts or kilowatts.
Power and Electric Motors
When an electric motor starts, a momentary inrush of electric current gets the motor started. The inrush allows the motor to overcome inertial resistance of the motor and the load it is attached to.
For example, when an air-conditioner motor starts, it has to overcome the resistance of its own internal parts, and of the compressor and fan to which it is attached. When the motor is not moving, the inrush current establishes the magnetic fields the motor needs to run, and the motor begins to turn. The electric current at this moment is very high―many times the running current of the motor.
The high current flow only lasts a very short time. As the motor speed increases, the current spike continues to fall until the motor reaches its full speed, when it operates at its rated running watts.
Power is expressed in watts (P), which is voltage (V) multiplied by amperes (I). P = V x I.
Generator Surge Power
As current increases, so does the heat created by the flow of current. A generator has a maximum continuous power rating that prevents the generator from supplying so much current that it fails. Although the generating unit that creates electric current can supply higher amounts of current than the maximum continuous rating, it will fail if it does so for longer than a few seconds.
The main circuit breaker on the generator is the primary safety against overloading the generator and causing it to fail. If current rises higher than the main breaker rating, the breaker does not trip immediately. Instead, it allows the higher current for a short period of time before it trips. This allows the generator to supply a surge current to the load. If the high current continues, the generator’s main breaker will trip to protect the unit from overload.
Starting Watts, or Maximum Surge Power, are the number of watts a generator can supply for this very short period of time to start electric motors.
Example: The average 1/3 -horsepower motor uses 830 running watts. To start, it needs approximately 2500 watts, or about three times the running watts.
When choosing a generator for purchase or determining what loads a generator can power, take both specifications into consideration. Briggs & Stratton has a Starting Watts Worksheet for calculating the size of a generator required. The worksheet outlines a method for determining what size generator is required for any purpose. It also supplies a table of running watts and starting watts of common tools and appliances for reference. Remember that appliances and tools may vary; check each one for actual power requirements.
Your first step is to add up the running watts of all the appliances or tools the generator will run. You will need a generator that can supply the total running watts―Maximum Continuous Power― all the time. Next, find the appliance or tool with the highest starting watts and add the starting watts to the running watts. This is maximum surge power the generator will have to supply.
Purchase a generator that can supply the maximum watts required for a few seconds and the continuous running watts the rest of the time.