Probing the Mysteries of Power Inverters: Part II II

This second installment of a two-part article discusses how to connect power inverters to a home DC power generation system.

| May/June 1983

power inverters - inverter unit with switches, posts, and outlets

Power inverters like this one convert DC to AC.

Photo courtesy of Best Energy Systems for Tomorrow

After the lengthy discussion of inverter theory and characteristics in Probing the Mysteries of Power Inverters: Part I,  you should be well prepared to look into the actual process of fitting an inverter to your power system. So, in this article, we'll explore the factors that make for a proper hookup and discuss how to get the job done for the least possible expense.

There are six major categories of inverters to choose from: the square wave, modified square wave, stairstep wave, sine wave, motor/generator, and synchronous. The single most important factor determining your choice of a unit will be the amount of power you expect your system to produce. AC power is measured in watts, and the wattage requirements of the appliances you wish to operate must never exceed the output of the inverter.

Let's say, for example, that you plan to light three 100-watt bulbs with your inverter. Since the three fixtures will need 300 watts when burning simultaneously, the inverter must be able to handle at least that much power. In actual practice, however, it's a good idea to consider the worst case situation and even to make allowances for the unexpected. A good rule of thumb, then, is to estimate all your needs, and add 10%. Consequently, it'd be a good idea to look for an inverter rated at 330 watts if you plan to operate those three light bulbs. [EDITOR'S NOTE: Inverters with ratings of 350 watts are very common and would work nicely in this application.]

Lest you assume that "more is always better", though, remember that internal losses are a factor; the efficiency of an inverter is directly proportional to the ratio of the working output power to the maximum available AC power. For that reason, a grossly oversized inverter will be very inefficient.

The Rating Game

While we're on the subject of power rating, it should be pointed out that not all inverters are rated in the same way. Some manufacturers take the liberty of including the power factor in their computations, and express the inverter's rating in volt-amps (VA) instead of watts. As you may recall from the previous installment, volt-amps is the product of voltage times current, which can be different from watts if the two don't peak at the same time. We called this disparity the power factor, or the difference between apparent and true power.

Watts is the term used when the power factor is at unity, or equal to one. It therefore follows that one amp at 100 volts is equal to 100 watts. But if the power factor is less than one, the appropriate unit is volt-amps. Consequently, in our example of one amp at 100 volts, an inverter with a power factor of 0.7 gives 100 VA but only 70 watts! That is, only 70% of the available power is actually being used by the load.

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