Light in a Storm

Consider Capacity, Condition of Your Standby Electric Power System

Dan Miller
By  Dan Miller , Progressive Farmer Senior Editor
Perform regular maintenance on your power system. Check the oil, change the fuel every six months, and check for insect and rodent infestations. (Progressive Farmer photo by Jim Patrico)

Spring has rolled into view with its windy thunderstorms and tornados. It's a good time to consider the capacity and condition of your standby electric power system. In good running order, it will help fend off a serious hit to your bottom line if the power fails.

There are three types of standby electrical power systems: permanent standby generators; portable, engine-driven generators; and manual generators driven by a PTO. You'll want to purchase a reliable backup system with the generating power to protect the most critical functions of your operation.

Generators are sized by the wattage and voltage they produce. There are portable generators that produce as few as 1,000 watts of power to generators 100 times larger—big enough to operate large livestock operations.

The power required to run a piece of machinery is calculated in watts (amps times volts). By adding up the watts to run each piece of equipment, you'll determine the minimum supply of power needed to keep critical circuits energized. Electric motors require special consideration—three to seven times the current needed by the motor when it's fully operating.


Here's how to calculate your backup power needs. Add the following four categories.

-- The starting wattage of largest motor. If there are two or more motors of the same size, list the starting wattage of only one.


-- The running wattage of all other motors.


-- The nameplate wattage of appliances and equipment.


-- The wattage of lights.

You also want to consider the electricity needed to power functions that would be useful to have in operation but not critical to the farm's ongoing function.

Overloading a generator because of a poorly executed backup plan causes poor power quality that damages both the generator and critical equipment systems.

On the other hand, it's not a good idea to purchase a generator with an electrical capacity well beyond your power needs.

If, during its operation, your generator is under only a partial load, the engine may not reach full operating temperature. Over time, that can damage the engine.

A backup generator, whether portable or permanently installed, requires a double throw, or isolation switch. The switch serves several functions.

It's installed for reasons of safety. Connecting a generator directly into any point of electrical wiring without a transfer switch is illegal and dangerous, because it may cause back-feeding.


Back-feeding is the result of sending electricity from your backup system out onto commercial power lines. It energizes the transformer serving your farm and could cause the death of anyone working to restore power to your home and farm.

That's because electrical transformers work the same in either direction. The transformer reduces the 14,000 volts of power on an electrical line to the 240-volt service delivered to your operation. Putting power onto the line from your backup generator through the transformer energizes the line to 14,000 volts.

The transfer switch isolates a handful of critical electrical circuits from the incoming electrical service. If the generator is powering those circuits, and regular power is restored, that electricity will not flow to those isolated circuits until the generator is shut down, and the transfer switch is set to receive commercial electrical service.

Finally, the switch prevents power from your generator and the power company from energizing your electrical system at the same time, which risks the catastrophic failure of your generator and equipment.

Installation of the switch and wiring of the critical circuits is a job for a qualified electrician. This is not a job for shortcuts.

Regular maintenance of all three types of generators is critical to their performance.

Permanent standby generators generally feature an automatic, self-checking function that runs once a week or so. These self-checks indicate maintenance that needs to be performed.

Portable, engine-driven generators and PTO-driven generators (those operated with a power-take-off) require periodic maintenance.

On a regular schedule:

-- Check both the oil and fuel levels.

-- Change the fuel stored in the generator every six months.

-- Check the units for animal and insect infestations.

Mice may build nests inside the protective casing, even around the electrical windings. They may also eat into the windings.

-- Check exhaust systems for nests and other obstructions.

-- Portable generators should be run every month. PTO-driven generators should be exercised every quarter. In both cases, run the generators for about an hour under load.

If you are looking for more information, visit for a good tutorial on standby electric power systems from the University of Georgia.

Dan Miller