Saturday, August 15, 2009

Choosing a Generator

For a preparedness-minded individual, it is easy to see the value of owning a generator, but actually choosing what type and size of generator to buy can be quite intimidating. Generators are application-specific tools, meaning that the right generator will vary depending on what it is intended to be used for. The most common generators that people think of are 10,000-watt portable gasoline generators, but there are many more that fit a variety of situations.

Generator Type


Portable generators are by far the most common generators out there. They range from 1,000 watts all the way up to 80,000-watt trailer mounted sets, but the most common are 3,000 - 10,000 watt gasoline-powered generators.
The smaller economy model generators are little more than a lawn mower engine attached to a dynamo. They have aluminum sleeves and use side valves, and are not manufactured for extended use. They often have minimal electronics and the power they generate can be very “dirty.” Larger commercial generators utilize overhead valves and steel sleeves. They are often computer-controlled so that the power produced is clean and consistent, and so the engine runs at maximum efficiency regardless of the load.


Commonly found at data centers, hospitals, prisons, and other locations that require uninterruptible power, fixed generators are large systems and are not designed to be moved once installed. They generally are hardwired into the building, and are setup to automatically start providing power once grid power has been down for a predetermined amount of time.


A less common type of generator is the Power Take Off, or PTO generator. The alternator in your car is actually a small PTO generator, but it won’t power much. Most PTO generators are mounted onto the main PTO of a tractor, or to some vehicle transmissions. Of course they are also available as just a plain PTO that could theoretically be mounted on any rotating drive shaft.

Industrial and Turbine

These generators are truly massive beasts. Often generating in excess of 250,000 watts, such generators are usually only found at large outdoor concerts, traveling carnivals, or permanently installed at large commercial installations. Turbines have the advantage of being relatively quiet compared to piston driven generators. Portable versions of these huge behemoths are either mounted on 18-wheeler trailers, or are have a trailer actually integrated into the generator itself. It’s very unlikely that a single individual would ever need such a large generator, but they are often trucked in for use by first responders when power is knocked out to entire cities for weeks at a time, such as in the aftermath of a hurricane or earthquake.

Fuel Type

One very important concern when choosing a generator is the type of fuel that it uses.


Most residential backup generators are able to use propane and natural gas interchangeably. Liquid propane is easy to store. In rural areas, large storage tanks are commonly found at every house and business, and propane delivery services are widely available to keep these tanks full. In urban settings large tanks are less common and more heavily regulated, though small versions are very common due to their prevalent use firing BBQ grills and other outdoor accessories.

Natural Gas

Most homes in cities in the US have access to natural gas through municipal gas lines. This makes it very convenient when installing a residential backup generator. Be aware that in certain situations, if the local power grid goes down, it is likely that municipal gas lines may lose pressure and go down as well, so an alternate fuel source such as propane may be necessary.


While diesel does not go bad as quickly as gasoline, it is still susceptible to hydration and fungal growth if not treated with an additive. Diesel has the advantage of being easier to store, and it is a more efficient fuel. Generators that are diesel-powered usually require less maintenance and have a longer operational lifespan. While not common yet, a few small portable diesel-powered generators are available.


Gasoline is the most common fuel used for small portable generators rated for less than 20,000 watts. Gasoline-powered generators require the most maintenance and have the shortest lifespan of any fuel type. Storing gasoline can be problematic, as gasoline decays within just a few weeks without being stabilized with additives. Even with the use of stabilizing agents, it is generally inadvisable to try to use gasoline stored longer than two years. Gasoline is the most dangerous fuel to store, due to its low flash point (the point at which fuel fumes form an ignitable mixture in the air) and its sensitivity to temperature fluctuations.


Understanding Ratings and Choosing the Right Size

All generators are rated for two wattages. There is a big difference between a generators’ peak capacity or surge wattage and its rated operating capacity. Most generators are designed to run at approximately 75% of their surge watt rating. If you run at more than 75% capacity for a long period of time, the generator will undergo excessive wear and tear and will eventually become damaged. Exceeding the surge watt rating for even a limited amount of time can damage or destroy a generator.

Knowing what your energy demands are will help you choose a properly sized generator. If you have a refrigerator (3,000 watt startup), a hot water heater (3,000 watts), and a well pump (1,500 watt startup) you need to run, you have a total energy demand of 6,500 watts. Given that demand, you will need a generator with a surge rating of at least 9,000 watts. This leaves you 250 watts to power some lights while keeping you within our 75% total load guideline. A recommended rule of thumb is to calculate your total need, add in the overhead so that your load is 75% of the rating, and then double that. That means that instead of a 9,000 watt we would buy an 18,000 watt generator, in order to allow for future energy needs.

Startup Load v. Running Load

Many appliances have a very high inductive startup load. The startup load is the power it takes to get an electric motor moving. All electric motors draw the most power and produce the most torque while starting up. These loads are often 2-3 times higher than the operating load. Naturally, this puts a higher demand on the generator. Startup loads should be considered when calculating the total load demanded of the generator.

Power QualityPower quality used to be a concern with older generators. Older generators attempted to create a specific frequency (usually 110v – 120v at 60 hertz) of AC current by governing engine speed. Obviously, this mechanical means of regulation was not perfect, and the result was fluctuations in the frequency and voltage, or “dirty” power. Dirty power isn’t an issue with light bulbs, heating elements, or most motor-driven appliances, but it can play hell with computers and computer-controlled appliances, as well as some transformer-powered appliances.

Small economy style generators still produce “dirty” power. Most modern generators however have computer-controlled power outputs and have no problem putting out perfect sine-wave 60 hertz AC power.

Wiring Concerns

Having a generator is great, but how do you get the electrical power to where you need it? Extension cords are a natural solution, but they are limited in their usefulness. A typical household outlet is capable of running 1,500 to 2,000 watts of continuous power. A single extension cord should never be used to supply more than 1,800 watts (approximately 15 amps) of power. If you need an extension cord run of 50 – 100 feet, make sure to use a 12 gauge cord and remain within the 1,800 watt guideline.
For large generators (3,500 watts or more) it makes more sense to install a transfer switch that will allow the generator to supply power directly to the house wiring. Transfer switches isolate the house wiring and generator from the grid. If the house were not isolated, the generator would be putting power down the power lines at potentially deadly voltages. This would obviously endanger any repair crews working on the lines, and would damage the generator which would be trying to power anything connected to that undamaged portion of the grid.

Choosing the Right Size Generator

Average Power Demands of Common Appliances
ApplianceStart-up WattsOperating Watts
Furnace fan1,400700
Portable Heater1,8001,500
Well pump (½ horsepower)1,500750
Sump Pump32001700
40-gallon water heater3,0003,000
Water Heater4,0004,000
Clothes Washer1,500750
Light Bulb6060
Home Security Alarm100100
Coffee maker850850
Electric Skillet1,2001,200
Crock pot250250
2-slice toaster1,1001,100
Plug-in heater1,5001,500
7¼-inch circular saw1,500750
Belt Sander1,9001,500
Bench Grinder1,800700
Jig Saw900600
3/8” Hand Drill750500

Safety Concerns

Fuel Storage

If you’ve got a generator you’re going to need fuel for it, and in a situation where the power is out most fuel stations are unable to pump fuel. This means that you’re going to have a fuel storage system. Fuel should be stored in a detached structure away from a house or living quarters. Always abide by local fire codes and safe storage laws. Even stored in a proper container, gasoline should never be stored in a building with any pilot lights or open flames. Stored gasoline and diesel should be rotated out on a yearly basis and restocked with fresh fuel.

Generators should always be run in a well ventilated area with the exhaust away from people. Never run a portable generator indoors or in an enclosed porch or garage. Install a carbon monoxide detector in your living area to detect if any dangerous fumes are infiltrating the house.

Grounding and Electrical Safety

When running the wiring for your generator, don’t neglect grounding requirements. If possible, the frame of your generator should be bonded to your house ground. If you are using an extension cord and not bonding to your house, then adequate grounding can be achieved by driving a 12” metal stake into the ground and wiring that to the generator frame.

Generator Maintenance

If you are using a gasoline powered generator, it is generally not advised to store it with fuel in the tank. After use, when preparing the generator for storage, you should drain the fuel tank and then run the generator until the fuel line and carburetor are dry. This prevents gummy deposits forming from fuel left in the system. Like most small engines, your generator should be run with a load at least twice yearly. Sparkplugs should be checked and cleaned once a year.

Oil Changes

Just like any other engine-driven device, generators require maintenance. The owner’s manual of your generator will have recommended service intervals. Be aware that some smaller economy model generators require oil changes as frequently as every 24 hours of use. This means that you should always have a ready supply of oil on hand for your generator, so that you can change the oil during a power outage without needing to go out searching for oil to buy.

Parts Compatibility

Some generators use common automobile based parts, like spin-on oil filters, and commonly found fuel and air filters. When purchasing a generator, find out if parts that wear out are easily replaced. A generator with a bad sparkplug becomes useless if you cannot easily find a replacement. Use particular caution when considering generators from smaller companies, as these often have parts that are difficult to find replacements for if they break.

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