More Power to You
Getting the most out of an inverter
By Paul Abelson
About 10 years ago, power inverters were just starting to appear on the market. They convert 12-volt direct current (DC) from your batteries to 120-volt alternating current (AC) similar to household current. Previously, all powered accessories had to operate on 12-v or just be left behind. Household current is more efficient for powering a host of appliances we take for granted at home, appliances that make extended stays in your sleeper cab much more tolerable. When an inverter is properly wired into your truck, you can plug in a microwave, a hair dryer, a space heater, a computer, DVD player or television without the extra cost or limited performance of special 12-v models.
Using the inverter’s wall plugs eliminates the misuse of multiple 12-v power point sockets. Many truck fires have been caused by overloading 12-v wiring. Multi-socket plugs plugged into other multi-socket plugs draw far more amps than the 10 or 20 amps the circuit was designed for. Too much amperage generates excessive heat, melting insulation and causing short circuits.
Inverters offer advantages, but not without problems. Improper selection, poor installation or inadequate wiring can create serious problems, and even inverters can cause fires if not properly installed.
When they first started to appear at travel centers and truck dealers, inverters could cost as much as $0.70 for each watt of capacity. A 3,000-watt capacity model used to cost more than $2,000, but even then, many drivers found inverters were worthwhile investments. You can run a TV, VCR, hot plate, refrigerator/freezer and other plug-in appliances, all at the same time. You can bring frozen meals from home, filling in with fresh dairy, fruits and vegetables, saving more than $100 a week compared to buying all your meals on the road.
Most electronic items’ prices come down over time while performance improves. Inverters are like that. A 3,000-watt model that once cost $2,000 is now less than $400. For powering just your office equipment, a computer and printer, a 400-watt inverter can be had for less than $100.
Inverters range from 50-watt capacity, useful for recharging cell phone and computer batteries, up to 3,000 -watt models capable of powering a full range of household products. While price is always a consideration, capacity and quality are more important selection criteria. I remember getting a letter from a husband and wife team. They purchased an inexpensive inverter and plugged it in to a lighter outlet, then plugged in a Crock-Pot slow cooker. After a few minutes of cooking, they smelled burning insulation. The current running to the power plug that the new inverter was plugged into overheated those wires.
When selecting an inverter, according to TMC Recommended Practice RP 163, “Power Inverter Selection Recommendations,” make sure it is rated for the peak wattage of all the devices you plan to run at any one time. If that includes a 900-watt microwave, a 300-watt 20-inch TV, an 800-watt toaster and a 900-watt coffee maker, you’d better have a 3,000-watt inverter. If you’re only planning on running a 13-inch TV (80 w) and VCR (100 w), a 200-watt model will still leave a safety margin.
Inverters are rated for both continuous and surge output. Surge during start-up is the initial rush of current needed to kick-start the device. Ratings vary from 1-1/4 times to more than double continuous current. More is better, but 1-1/3 to 1-1/2 times is most common, and should be adequate.
The quality of current coming from an inverter can be as important as capacity. Computer chips depend on smooth delivery of alternating current in the form of a pure sine wave. Lower priced inverters may be identified as delivering modified sine wave current. That’s a polite way of saying that the current is not regulated well enough for sensitive electronics. Computers, newer TVs, VCRs, DVDs and especially CPAP machines can be damaged by improper current. Modified sine wave inverters are fine for heaters and hair dryers, electric frying pans, power drills and electric blankets.
Read labels carefully, and avoid unknown brands or unknown vendors. That bargain inverter at the discount tool shop won’t be much of a bargain if it burns out your electronics.
Better inverters work with shore power and can charge your batteries.
When installing an inverter, wire size is critical. For a given size, the more current a wire carries, the hotter it will get. The larger the wire, the more current it can safely carry. Calculate current flow from the power source all the way to the point of use. For safety, install fuses/circuit breakers on the positive cable as close to the batteries as possible.
Wattage is the amount of current (amps) times the force behind it (volts). 1,200 watts is only 10 amps in a 120-volt circuit, but in a 12-volt circuit, it’s 100 amps. The wiring bringing current from the battery to the inverter is on the 12 volt side, so the amp load is roughly 10 times that in the appliance.
Inverters are not perfectly efficient devices. Current inside the inverter generates heat, so even the best inverters are only about 20 percent efficient. Lesser quality units may lose 30 percent or more. That means that for each 100 watts you draw from the inverter, you need to put 120 to 135 watts in.
If you have a 3,000-watt inverter, you’ll need to put 300 to 350 amps into it at 12-v. All inverters should be wired on a fused circuit directly to the batteries. For even the smallest units 2/0 battery cable is recommended. Large size units need 4/0 cable or larger with a properly sized fuse. The ground wire needs to be the same size as the hot wire.
If you’ll use an inverter for hotel loads with the engine off, make sure you have enough reserve capacity in your batteries. Otherwise, you may run them down and be unable to start. You might consider extra batteries and dedicated AGM (absorbed glass mat) for the inverter.
When properly selected and installed, inverters require as much maintenance as an outlet in your house — virtually none. If not working, check the fuse first. If that’s OK, you may need a new voltage regulator, part of the alternator-charging system. Better inverters have protection circuits to protect appliances from over voltage and to prevent batteries from excessive drain.