How to test the output of a portable generator

Written by ian kelly
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How to test the output of a portable generator
Use a multimeter to test the output of your portable generator. (multimeter image by dinostock from

To cope with the escalating severity of weather systems, owning a standby generator is a necessary precaution. However, if your power requirements are greater than your portable generator's output, lights will be dimmer, HVAC systems and water heaters will lose efficiency and electric motors will run slower. In addition, overloading a generator will cause premature generator failure and also damage household electrical systems. Test the output of your generator, and compare it to your emergency power requirements. If your generator produces too little power, either use fewer lights and appliances during a power outage, or upgrade to a more powerful generator.

Skill level:

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Things you need

  • Multimeter
  • Pencil
  • Paper
  • Calculator

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  1. 1

    Place your portable generator outdoors to guard against carbon monoxide poisoning. Set the generator on a stable base. Check fuel and oil levels and start the motor. Allow the motor to warm up and reach normal operating speed before carrying out tests.

  2. 2

    Adjust your multimeter to the highest amperage setting. Remove the red probe from the DC outlet and insert it into the red fused circuit designed to measure AC voltage.

  3. 3

    Insert the black probe into the neutral (or negative) orifice in the output socket on front of the generator, and the red probe into the live (or positive) orifice in the output socket on the front of the generator. Check the multimeter and make a note of the amperage your generator produces. Remove the probes.

  4. 4

    Test the voltage output of your generator. Set the knob on the multimeter to the "AC volts" setting and select the range to measure 120 volts.

  5. 5

    Insert the probes back into the generator output socket; red probe to the live (or positive) side and the black probe into the neutral (or negative) side. If your generator is functioning correctly, the output will be somewhere between 100 and 120 volts.

  6. 6

    Multiply the amperage by the voltage reading on a calculator to arrive at the wattage produced by your generator.

  7. 7

    Add up the maximum start-up wattage required from all the devices you wish to connect to the generator during a power outage. Since start-up power requirement of motors and compressors are up to three times greater than running power requirements, use the higher figure. Power specifications are stamped on the specification plate attached to motors and appliances. If any are missing, refer to the relevant owner's manual.

  8. 8

    Reduce the number of devices available if your generator produces too few watts. In the event of a power outage, switch the surplus devices off before attaching your generator to your household electrical system. Alternatively, buy a larger generator capable of meeting all your emergency requirements.

Tips and warnings

  • If you inadvertently reverse the multimeter probes during testing, the multimeter will produce the same reading without causing damage.

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