Electrical Generator

This will likely be the single largest expense in your off-grid power system, so choose wisely. The generator converts the energy in a fuel such as gasoline or diesel oil into electricity using an internal combustion engine. The primary characteristics of a generator will be type of fuel and power output.

Power Output

Bigger is better, but it is possible to get too much of a good thing. Larger motors have a higher idle fuel consumption so when little power is needed they are less efficient than smaller generators. But the flip-side is that bigger generators are more efficient at higher power output than are small generators.

A good balance is achieved with a generator that is about 30 percent bigger than you normally require. For example, if we normally need 10 KW of power then we should have a generator of about 13 KW. This will put the generator running at about 70 or 80 percent of capacity, which is the most efficient operating point for most motors.

Choice of Fuel

Diesel is by far the best choice. Gasoline is more popular because of the wide availability of cheap generators, but gasoline has numerous disadvantages compared to diesel.

• Cost. Diesel, purchased as heating oil--it is the same stuff--is far cheaper than gasoline. Not only is the price per gallon less, but the diesel fuel actually contains more energy, typically 20% more.

• Safety. Gasoline vapors are explosive. Diesel is barely burnable. Here's a stupid trick: take a lighted match and stick it in a cup of diesel fuel. It'll go out! Do NOT try this with gasoline!

• Storage. All fuel must be treated for long term storage, but diesel fuel lasts longer than gasoline under all circumstances and it is half the cost to treat.

Fuel Efficiency

Fuel will be the major expense of a generator-based power system, so the machine should be as efficient as can be found. Diesel generators will produce the highest power output per gallon of fuel consumed, largely because of the higher energy content of diesel fuel. There will be quite a bit of variation between different makes and models, but a good average is 10 KWH per gallon. This assumes a quality unit run at about 70% of full load.

Other Issues

Finding and buying a unit is only about half the fun. We need to have a place to put it and it has to be wired into the rest of our power plant. It would be a good idea to think of these things before the truck arrives and unloads a thousand pound crate in your driveway.

You'll also need a fuel storage tank and piping to the generator and some quantity of fuel must be on hand to startup and test the new generator. Most of this preparation can be done before the generator arrives.