Generator Selection TT119
Published on May 8, 2014

Generator Selection - Tech Talk 119

If your electric power goes off for a few hours, will you lose your animals? Most modern aquaculture is heavily dependent on public electricity (the "grid") because it is an extremely good value. Our 1981 Aeration Handbook and Catalog states, "[O]ne kilowatt-hour is equivalent to about two days of hard work by one man." Hmm, let's day's work lifting water by hand or 5¢ for electricity?

To size a generator for your single-phase electrical items you must add up all the loads it will be asked to handle. Some loads, such as light bulbs, can be determined simply by their watt rating (volts x amps = watts). Other loads, such as electric motors, will have both a watt rating (for normal running operation) and a starting watt rating. The starting rating can be more than 4 times the running watts!

Electricity supplied by the grid has a surge capacity limited only by the circuit protection provided, such as a 200-amp circuit breaker. When power has been off and comes back on, all motors will start within a few seconds. A generator, on the other hand, is limited by its engine torque and the inertia of the rotating parts. To get a motor turning (inertia) by your generator you will have to have the electricity needed for that starting inertia, or "starting rating," in watts. If you have numerous motors, you will want to start them one at a time.

To size a generator make a chart like the one below with all the items you wish to operate. We suggest that you select a generator by its rated "continuous duty watts;" that is, 25% larger than what you calculate you will need (as it gets old or if operated at a higher altitude than sea level it will not produce full rated power). Remember, a few seconds after a motor is started it is only using its running watts, so you can reduce the size of the generator needed by turning on your motors one at a time, starting with the largest first.

Note: This chart is provided to suggest some typical values. Equipment age, manufacturer, condition and size can make a significant difference.

Generators are purchased for use in an emergency, so don't let anything prevent them from working. When you get a new one, test it by operating everything that you expect it to run. Then keep them dry and either run them every month for a half hour or so (don't let the fuel get too old), or "pickle" them so that they will not corrode and be ready to operate with just the addition of fuel. Have power cords tested and ready, and do a full test at least once a year. Portable generators have one or more outlet receptacles, and you must be careful to not overload them by dividing the load between them. Generators produce carbon monoxide, a poisonous gas that must be kept well away from people, animals, compressors and building inlets. You will want to operate them only outdoors. Remember that electricity and water do not mix. Follow all the warnings supplied with your new generator, especially grounding instructions. Keep the instruction manual in a Ziploc® bag near the generator and in it record the hours of operation, oil changes, etc.

Generator Selection  

Generator Selection