You wake in the middle of the night to a loud clap of thunder. As you lean over to see if it's time for morning chores, you notice the light that normally shines in from the barn is not lit. A quick glance toward the alarm clock confirms the power's out. How will the cows get milked? How long have the fans in the hog confinement building been off?

If you have a critical need for uninterrupted power, you may want to consider purchasing a generator before the lights go out. The first step is to decide what type of system is right for your operation.

"Before buying a generator, evaluate your greatest risk for economic loss. Then match the generator size, fuel type, and start-up controls needed for this load," says Richard Hiatt, National Food and Energy Council (www.nfec.org) and author of Sizing and Selecting Your Standby Generator.

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There are two basic types of standby generators: engine-driven and tractor-driven.

Engine-driven generators can be permanently installed, or they can be portable. Permanent models are used to offset the potentially harmful effects of power outages or as an emergency power source for a home or farm. While portable generators can be used to run essential equipment during an outage, they are designed for use over short periods of time.

Unlike portable generators, permanently installed units are wired directly into an electrical system, which has two main benefits. When electricity is lost, it starts automatically with a short interruption (10 to 20 seconds) of power.

Essential devices are already connected to the generator. This means there's no need to run extension cords or to hook up any devices in an emergency.

You can have the generator wired to specific circuits for critical equipment or you can purchase a unit large enough to support your entire operation.

A tractor-driven generator uses a tractor's PTO drive to run the generator, which means there is no engine to maintain. The horsepower rating of the tractor engine should be at least twice the kilowatt capacity of the generator.

Generators wired directly into an electrical system have an installed source of fuel. There are generally three choices of fuel: diesel, natural gas, and liquid propane with a wide range of outputs. Portable generators typically run on gasoline, which cannot be stored for long periods of time. Portable units are available with outputs up to 17.5 kilowatts.

A qualified electrician should install a permanent unit. Portable generators should not be connected directly to your operation's electrical system during an outage. Electricity could back-feed into the power lines and put utility workers in danger and damage your equipment.

Once you've decided on the type of generator, size up your operation. Generators must be sized correctly to prevent overloading. A professional contractor or electrician is a good information source to help you with a load analysis. Look at the sum of the watts from all electrical devices that must operate at the same time. Remember that the biggest demand for power is when a motor-driven device first starts (peak load).

Because outages can happen in any season, calculate the total load during the time of year when you will be using the most electricity. The most common generator rating is in terms of kilowatts (kW). One kW is equal to 1,000 watts (W). A unit should run at approximately 80% of its peak load rating. For example, if you need 15 kW, purchase a 20 kW generator.

Any type of generator that serves loads through the permanent wiring system of the home or farm must use a suitable automatic transfer switch (ATS) and should be installed by an electrician. The rating of the switch (100, 200, or 400 amps) should match existing utility service. The ATS constantly senses the presence or absence of utility power. The switch also starts the generator periodically and runs it for 10 to 15 minutes as a self-test and exercise routine.

"Be sure to exercise your generator at least every few months so it will start when needed," Hiatt says.

Generally speaking, permanent generators can cost anywhere from $300 to $500 per kW, installed. A commercial-quality, portable generator (5 kW, gas powered) averages between $600 and $800.

A generator can last many years if it's well maintained. Changing the oil and spark plugs, and checking coolant will go a long way in preserving a unit's life.

Can your operation risk losing power for several hours or possibly several days? Invest in a system before you need it.