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Monday, January 19, 2009

Choosing a Power Inverter

For boondockers who shun the use of a generator but still have a need for "shore power," the way to get that juice is through a power inverter. The inverter takes DC current from the rig's deep cycle "house" batteries and converts it to a form that most AC devices can use.

How do you choose a power inverter? Let's start with the two major types of inverters, true sine wave inverters and modified sine wave inverters. The former produce electrical power that "walks, talks, and looks"--not like a duck--but like the power you'd get from an electric utility company. On the other hand, the output of a modified sine wave inverter is not a smooth, rounded form (photo, left) but a bit choppy--as you can see from the drawing on the right. Most AC devices can use power from either of the two types of inverter, but the cost is what most differentiates the two. As an example, we found a modified sine wave inverter rated to provide 2,500 watts for sale at a cost of $250, but a similarly "sized" true sine wave inverter cost $1215.

Why spend so much for a true sine wave inverter? Some devices will ONLY operate on true sine wave--laser printers, for example, and some medical equipment. If you need an oxygen concentrator in your rig, it might not operate on modified sine wave power. Some cordless equipment battery chargers won't work with modified sine wave either. If there's any doubt, you'll need to contact the manufacturer of the device you'll be connecting to inverter. This doesn't mean you'll be forced to run out and by a costly sine wave inverter to run everything--you could install the lower cost inverter to operate most of your rig; then purchase the more expensive inverter and size it only as large as required to operate equipment that won't operate on the modified sine wave power.

Once you know which type of inverter, then it's time to pick the size: The "size" of the inverter should be measured by the amount of output power (in watts) the inverter can produce on a continuous basis. Another figure, the "peak output power" tells you how much juice the inverter can kick out for a short period of time to start up devices. For example, most corded power tools draw a lot more peak power to start up then they use while running. Here's where a lot of folks can get into hot water. They look at their electric drill, and it draws say, 6 amps. So using the calculation of 6 amps times 120 volts (the operating voltage of the device) they come up with a need for 720 watts. Figuring they can use a 1,000 watt inverter, they're sorely troubled when the drill won't start. Some electric tools can need several times their rated operating power to start up. Here again, you'll probably need to contact a tool manufacturer to find out just what the start-up power requirements are.

Happily, for most equipment RVers use, from computers and televisions to microwave ovens, the start-up power us usually well within twice the operating power of the equipment. Hence, adding up the power consumption of all the devices you'd use at any one time and purchasing an inverter that can continuously produce that amount of power, will do the trick. Better too much than too little, however. You can find the power consumption figure on the manufacturer's ID plate, and if it's not rated in watts, then apply the amps times volts formula previously mentioned.

An accessory that can really help:  Small power inverters can be plugged into a "lighter" outlet, or into the TV antenna socket. But beware, those sockets can produce only a small amount of power--8 amps at 12 volts is typical. It doesn't take much to "overrun" that level, so you'll find it best to wire your inverter directly to the RV low voltage power  supply. We'll take that topic up in our next blog, but suffice to say, your inverter will likely be very close to your RV batteries. Since you won't want to leave it turned on when not using it, a remote switch on the inverter is a real essential, unless you like running off your calories getting from inside the coach to the inverter and back. Many inverters that are designed to be hard-wired to the electrical system have the remote switch option--but not all do.

We'll take up the discussion of how to install and use an inverter in our next entry.

picture credits: sine wave, Rochester Institute of Technology; modified sine wave, oynot solar. 

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