Power Inverters: 7 Things Fleet Managers Want to Know

Editor’s note: This feature answers seven questions truck fleets should consider when choosing and installing power inverters. Another version, geared for owner-operators, is available upon request. Please contact Doug Siefkes and in the subject line, put “Inverters - Seven Questions for Owner-Operators.”

Power inverters—devices that convert standard battery (DC) power to AC household power—are becoming more common in the trucking industry. Depending upon whom you ask, that’s to the delight or disdain of fleet and maintenance managers.

Steve Carlson, OEM sales manager

Steve Carlson, OEM sales manager

Inverters have always been a magnet for controversy, says Steve Carlson, Xantrex’s OEM sales manager. Xantrex is a leading supplier of inverters, and Carlson says shipments have risen sharply since early 2012. The company expects this trend to continue.

“Fleets know that drivers love them, as they provide a measure of creature comfort that helps with driver retention,” Carlson says. “But some fleets have questions about inverters and their potential to damage the trucks electrical system.”

The distrust of inverters is really a thing of the past, he says, as long as the right inverter and installation practices are followed. “The key is for fleets to do their homework and know what’s best for their operation prior to making a purchase,” says Carlson. “One size does not fit all and inverter quality varies greatly.”

Carlson says several questions rise above all others as fleets try to determine which inverters to purchase:

What Size Should I Buy?

By far, Carlson says, “what size” is the first and most important question he hears from fleets.

“It’s easy to say get the biggest inverter on the market so you’ll be covered for every need,” Carlson says. “But that’s not the best advice. Inverter sizes range from 300-watt cigarette-lighter plug-in inverters to 5,000-watt units. Each fleet should do a survey on truck size and power usage and understand how their drivers will use an inverter, including what items they want powered and what items will be used at the same time. That will help right-size the inverter for your operation.”

As an example, Carlson says drivers will often run a microwave, TV, and laptop all at the same time. On each device you’ll see a wattage number: a microwave might be rated at 1,000 watts, a TV at 250 watts, and a laptop at 95. “Add them up to see how much continuous power you’ll need and then add 20%,” he says. “In this case, you’ll need just over 1,600 watts.

Next, round up to find an inverter that meets your power needs. “Xantrex, for example, offers an 1,800-watt unit,” says Carlson, “and that’s what we would recommend.”

While determining continuous power is an important consideration, so is surge power.

Whenever you power up a device, the initial load is more—sometimes double—what the continuous power requirement is. Carlson says the surge rating on quality inverters should be about double. “For instance, an 1,800-watt inverter can handle a short 3,600-watt power surge requirement,” he explains.

Next, determine how long the inverter can handle the surge. Some inverters can handle only a few milliseconds of surge before the power draw shuts down the inverter, while others can last five seconds or more. That’s what you should look for, Carlson notes. The longer the better.

Sine or Modified Sine Wave?

There are two types of inverters on the market: sine wave and modified sine wave. Both work well in a truck environment, and modified sine wave power is generally fine for most electronics and appliances.

But for those running sensitive electronics (like CPAP machines) or products that are plugged into their own chargers—a drill or a toothbrush—sine wave is the better choice, says Carlson. “The voltage is consistent without spikes or drops,” he explains. The device you’re powering reacts just as it would if you were plugged in at home.

As for the price difference? Carlson says the gap has narrowed and today most higher wattage sine wave inverters cost about 15% to 20% more than a modified inverter.

With a sine wave unit, you’ll notice a slight decrease in the efficiency rating since electronics within the inverter use power to keep electrical levels consistent. “It’s not much—we have an 87% efficiency rating on a Xantrex sine wave unit compared to 92% on a modified inverter,” says Carlson. It’s like the difference between running a 6-cylinder car versus 4-cylinder car. That 4-cylinder car may get a bit better fuel economy, but the 6-cylinder is better in overall performance.

Can We Install It Ourselves?

Generally speaking, yes, says Carlson, noting that Xantrex does offer training for customers. “We certainly would recommend our training program if there are any questions on installation,” he says. “Remember, you’re working with electricity and electricity can bite if you’re not careful. It’s our recommendation that inverters over 300-watts feature hard-wiring and fusing.”

Carlson says there is a bevy of things to consider when installing an inverter, starting with where it should go and making sure there is adequate ventilation to allow heat to dissipate. And you have to be cognizant of wire sizing and the distance between the inverter and plug-ins which can be put in the sleeper; plus the distance between the battery and inverter.

According to Carlson, most fleets want the convenience of a factory-installed and warranted inverter. Xantrex recommends either an OEM-installation when you purchase a new truck or having the job done by an authorized dealer.

“The OEMs have installation down to a science. It’s done on the line to rigid specs,” says Carlson. “Truck and aftermarket dealers also have the experience. It’s worth spending a few extra bucks to have the installation done right, the first time, should your own staff not have the time or expertise.”

Are Inverters Reliable?

You get what you pay for, says Carlson. “An inverter that has a Regulatory Listed approval such as UL or ETL with UL458 rating will cost more,” Carlson says, “but this means the inverter was inspected and approved by an independent agency which safeguards against issues with electricity. UL458 is the listing for inverters and chargers in mobile applications. They must meet strict vibration, environmental, and thermal requirements that non-UL458 units do not.”

Choosing a UL485-listed inverter is a matter of safety. “In fact, TMC’s RP-163 calls out this UL listing for all inverters and chargers installed in a truck,” Carlson says. “What’s more, TMC’s RP-160, which discusses DC and AC wiring in a truck, has requirements that are automatically met by UL458 listed-inverters. The main point being that the neutral and ground are bonded together within the inverter. Inverters that are not UL458 listed do not do this as it allows the inverter to be made at a much lower cost.

“Buying an inverter that is not Regulatory Listed tells you ‘buyer beware.’ We’ve seen these types of products actually shock users. Plus, internally, they often can’t protect themselves against power surges.”

Carlson says that inverters installed by truck manufacturers all are UL approved, but inverters sold at truck stops typically are not.

He suggests you look for how inverters are internally tested for quality control. If the manufacturer you’re considering tests to ensure quality, then it will likely promote that fact in its marketing material or on its web site. And, inverters that have been tested will last longer versus inverters from manufacturers that don’t spend the time and money to ensure quality. A quality inverter should last well beyond its warranty period.

While inverters will occasionally shut down, quality inverters do so without damaging themselves. If dust or cat hair, for instance, gets inside the inverter, it can cause it to overheat. A higher-watt Xantrex inverter, for example, has an error code that lets you know what the problem is—in this case, it will tell you that the unit is overheating and to check the fan. A simple cleaning will correct the problem and get you back up and running. Other inverters could leave you guessing as to what the problem is.

If you overload the inverter, placing more wattage demands on the inverter than it can handle, the inverter will shut down. The difference between a quality inverter and low-end inverter is how they deal with a shut down. A quality inverter is designed to shut down with no ill effects. A low-end inverter can wear out after multiple overloads.

Should I Get an Inverter with a Battery Charger?

Yes, if you can use shore power (electrical outlets at home or on the road at terminals, loading docks, or truckstop). When plugged in, you can run everything you’re running with your inverter for as long as you want, plus you can recharge and top off your batteries. The more you can use shore power, the better, as it prolongs the life of your batteries.

In fact, Carlson says, a shore power option and a charger in the system will add 20 to 30% to the life of the batteries if plugged into grid power whenever possible. It has the potential to eliminate one battery swap-out over a five- or six-year period by keeping batteries fully charged, offsetting parasitic loads, and reducing the number of cycles.

According to Carlson, most installations use the inverter off the truck’s starting batteries, and quality inverters will have a low voltage disconnect (LVD) to shut down when voltage drops to 11.7 volts. This ensures the truck will have enough juice to start.  

Check on the LVD feature before you buy an inverter, cautions Carlson. Many inverters on the market will run the batteries down to 10.5 volts, which will let drivers run electrical devices longer in the cab and sleeper. However, they wont be able to start the truck unless the truck comes equipped with its own LVD.

Another option is to run two dedicated deep-cycle batteries and connect them to the inverter. “Yes, they add weight to the vehicle, and cost,” he says. “But deep-cycle batteries are designed to be drawn down to a 50% state of charge, or 10.5 volts. This gives you double to triple the amount of continuous power to run hotel loads. Something your drivers will appreciate.”

What If Drivers Use Their Own Inverters?

Fleets often have a policy of not allowing drivers to outfit company trucks with their own inverters. If you do allow drivers to bring in their own inverters, Carlson recommends giving them a list of approved devices, with the most important common denominator being UL458-listed. He also suggests you make it mandatory that your shop or outside dealer does the installation.

If I Have an APU, Why Would I Need An Inverter?

An inverter can reduce hours of use on a diesel-fired APU (assuming that the APU does not already have shore power compatibility), and it can reduce maintenance costs and increase APU life. “An inverter can be used for hotel loads in the cab as long as environmental conditions do not require air conditioning,” Carlson says. When those conditions happen, just power up the APU for air conditioning.

With this set-up, the only time the APU would need to come on is if the batteries drop to a low level. “Once the batteries are charged, the APU can shut off again,” Carlson says. “This reduction in APU run-time can produce a quick payback on the cost of the inverter.”