How to Choose the Right Electric Generator

Choosing an electric generator can be a confusing process for many people. The options and models available from a wide variety of manufacturers and retail outlets daunt many consumers and deter them from actually choosing and purchasing a generator. This "analysis paralysis" occurs despite a very real desire to obtain the convenience, security, and peace of mind that an electric generator can provide.

We present the following information in an effort to reduce the complexity of the generator selection process by identifying a few key questions and providing answers that are as simple as the subject matter allows. In no way is the information presented here intended to be an exhaustive treatment of the topic.

What’s the single most important thing I must consider?

Without a doubt, the most crucial piece of information that you need to know is how much electrical power you will need in your specific situation. Nothing will be more disappointing to you than to buy a generator that does not produce adequate power for your needs. 

In order to determine your power requirements, you will need to understand a little about how electricity is measured. It may help to understand electricity flowing through a wire by comparing it to water flowing through a hose. To keep the following explanation from being quite so dry (pun intended,) we will use the water hose analogy.

The first unit of electrical measurement is one you already know — volts. Your home has appliances that run on  240 volts. In our water hose analogy, volts may be thought of as the pressure of the water in the hose.

The next electrical term you need to know is amperes, often shortened to amps. Amps are used to measure the flow of electric current. Think of electricity flowing in a wire just as water flows in a hose.

The final unit of electrical measurement you will need to consider is watts, which should be somewhat familiar to you. Watts ratings are printed on light bulbs; other devices like hair dryers and electric heaters are also defined by their wattage requirements. Watts are a measure of electrical power and are calculated by multiplying volts times amps. In our water hose example, the water coming from the end of the hose, taking into account both pressure (volts) and flow (amps), provides the power (watts) to do the work.

How do I determine how many I need?

That’s probably the hardest question of all to answer because there are so many variables to consider. But in the simplest terms possible, follow these steps.

First, determine what electrical devices you will want to run from the generator and look at the application guide to determine the watts required for each electrical device. (Remember that the guide is just a guide; actual wattage requirements vary significantly among brands and types. The most accurate way to determine the actual power requirements for any given device is to read the label on the device itself or the manual that came with it.)

Second, look to see if the electrical device you want to run has a starting wattage requirement that is larger than the running wattage requirement. If it does, use the starting wattage requirement.Some appliances with electric motors (refrigeration, air conditioners), require up to 2 to 3 times the listed wattage. The generator should be 20% larger than calculated requirements. Also, additional capacity allows more circuits to be added for future use.

Finally, add together the wattage requirements for all the electrical devices that you want to run or start at the same time. This is the minimum wattage you will need from a generator. You should also consider the current ratings of the electrical outlets (receptacles) on the generator as well as the outlets’ mechanical configuration. Be sure that the outlets are the right match for the electrical devices you want to power. And remember that watts are like money— it’s always better to have more than you think you will need.

What are the major parts of a generator?

There are four major components of an electric generator: the engine which provides the mechanical power, the alternator (often called a "genhead") which is attached to the engine and turns the mechanical energy into electricity, an output/control panel which provides a place for the electrical outlets (receptacles), circuit breakers, and controls (switches), and a metal frame to hold the whole thing together. In addition to these four major components, there may also be a fuel tank separate from the engine and often mounted on top of the frame.

What do I need to know about engines?

Your first concern should be the types of fuel they can use. Generators are available with engines that run on petrol, diesel fuel, liquefied propane (LP) gas, or natural gas. Unless you have expertise with some other fueling system, petrol or diesel power is your best choice

With engines, the second thing you need to think about is the brand name. Choose one that you know and trust, and for which you can obtain service in your area.

There are two technical issues that you should also consider. First and most important is whether the engine is an OHV (Overhead Valve) engine. OHV engines start easier, run quieter, last longer, and produce lower emissions than non-OHV engines. They also cost more, but if your budget allows it, an OHV engine is the best choice. The second issue is whether or not the engine has a cast iron sleeve (often abbreviated CIS) in the cylinder. This CIS is actually a liner in the cylinder of the engine which reduces wear and makes the engine last longer. Most OHV engines also have cast iron cylinder sleeves, but you should check to be sure; the added cost of a CIS is small and is well worth looking for. Both features are very important if you expect to use your generator often or for long periods of time. 

What do I need to know about alternators?

The alternator is the "business end" of the generator and actually produces the electricity, so it’s important to look for an alternator that is built to last. Look for an all-metal design because plastic housings can warp over time and cause the moving parts of the alternator to come out of alignment and break or wear excessively. Get an alternator that uses "ball bearings" instead of "needle bearings;" it will last longer. "Brushless" designs require less maintenance and produce cleaner power that is more suitable for electronic equipment.

What other generator features should I look for?

There are a few other features that fall into the "nice to have" category. We will provide a short description of some of them and you can decide which ones are important to you.

• Low oil shutdown is a feature that helps protect the generator engine from the user’s negligence. It shuts the engine down if the oil level drops below a safe operating level. We especially recommend it on generators with large fuel tanks.
• Electric start generators use a battery to provide the starting power so the user doesn’t have to pull a starter rope. Keep in mind that the battery must be charged regularly (either by running the generator or with an external charger) for the feature to be worthwhile.
• Idle control allows a generator engine to automatically throttle down when no power is being drawn from the alternator. It reduces noise and engine wear but can be annoying if you are using the generator for emergency power.
• An hour meter keeps a record of how long the engine has run (in hours) and can help with oil change scheduling and other maintenance timing.
• A portability kit consists of wheels and handles for moving the generator around and can be a real back saver when dealing with units that weigh over 45 kilograms.
• Lift kits or "skyhooks" are valuable for loading and unloading generators from trucks but usually aren’t needed by homeowners.
• Transfer switches aren’t part of the generator itself but are required when generators are connected into home wiring. If you’re using extension cords, a transfer switch isn’t needed.

Some appliances need a "surge" of energy when starting. This means that the amount of electrical power needed to start the appliance may exceed the amount needed to maintain its use. Electrical appliances and tools normally come with a label indicating voltage, cycles/Hz, amperage (amps) and electrical power needed to run the appliance or tool. Check with your nearest dealer or service center with questions regarding power surge of certain appliances or power tools.

• Electrical loads such as incandescent lamps and hot plates require the same wattage to start as is needed to maintain use.
• Loads such as fluorescent lamps require 2 to 2 times the indicated wattage during start-up. Loads for mercury lamps require 2 to 3 times the indicated wattage during start-up.
• Electrical motors require a large starting current. Power requirements depend on the type of motor and its use. Once enough "surge" is attained to start the motor, the appliance will require only 50% to 30% of the wattage to continue running.
• Most electrical tools require 2 to 3 times their wattage for running under load during use. (For example, a 9,000 watt generator can power a 3,200 to 7,000 watt electrical tool.)
• Loads such as submersible pumps, air conditioners and air compressors require a very large force to start. They need 3 to 5 times the normal running wattage in order to start. (For example, a 5,000 watt generator would only be able to drive a 1,800 to 3,100 watt pump.)

If the power consumption of electrical appliances exceeds the operating range or if there is short circuit or other problems in the appliances, the AC breaker could trip "OFF" or the rotation of the generator could be abnormally reduced. In this case, stop the generator to see if the power consumption of the appliances is too large and if there is a problem in the appliances.

What type of fuel is best?

This depends on many factors - shelf life, cost, storage location, availability, etc.

LP (PROPANE):
Advantages

• Long shelf life
• Clean burning
• Easily stored in both large tanks or in smaller 25-50 litre cylinders
• Obtainable during power outages - petrol stations may be unable to pump other fuels during an area wide outage but, LP tanks are usually stocked full
• Home delivery available for larger tanks is commonplace

Disadvantages

• Pressurized cylinder of flammable gas
• Fuel system is more complicated (increased possibility of failure)
• Larger tanks are not aesthetically pleasing (unsightly)
• Fuel systems plumbing results in higher installation cost
• Some local ordinances prohibit the use of high pressure LP in residential applications

NATURAL GAS:
Advantages

• Usually unlimited fuel source - refueling not necessary
• Clean burning
• Almost always available during power outages
• No unsightly tank required

Disadvantages

• May be unavailable during natural disasters (earthquakes, etc.)
• Lower power output (30% less BTU's per unit than gasoline)
• Larger tanks are not aesthetically pleasing (unsightly)
• Fuel system plumbing results in higher installation cost
• Not available in many areas

DIESEL:
Advantages

• Least flammable fuel source
• Clean burning
• Easily obtained
• On site fuel delivery available
• High BTU output fuel

Disadvantages

• 18-24 month shelf life
• Lower power output (30% less BTU per unit than petrol)
• Installing large storage tanks raises cost of system
• May not be available during power outages
• Unpleasant smell

by DeVILBISS AIR Power Company, Cummins Onan