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Selecting the right power inverter

Whether portable or permanently installed, the result is the same: AC power without running the generator

Whether portable or permanently installed, the result is the same: AC power without running the generator

Read the other story in this package: It’s not just an inverter/charger

There’s no doubt that today’s boaters are looking to retain many of their shoreside amenities as they cruise and anchor away from the dock. Television, air conditioning, refrigeration, stereo systems, computers and an array of electronics are becoming standard accessories on all but the smallest of boats.

As such, we are increasingly dependent on outfitting our boats with alternative energy sources, which has prompted boatbuilders to offer more generator options in smaller vessels, and power inverters aren’t far behind. While generators can provide the muscle to run air conditioners and charge batteries, they also require maintenance, can be noisy and produce unwanted exhaust. Where AC power requirements are smaller and more intermittent, a power inverter should be the equipment of choice.

The beauty of inverters is that they can deliver this power silently. With a modern non-intrusive inverter on board you can avoid running the generator at the anchorage yet still brew coffee, microwave popcorn and watch television. And no longer are they stand-alone devices. They can be integrated into a complete on-board power management system controlling both AC and DC systems.

What is a power inverter?

Power inverters are complex pieces of electrical hardware that provide a method of changing, or inverting, the 12-volt direct current (DC) from your batteries to 120-volt alternating current (AC). AC is the same type of power provided by utility companies through residential grids and is similar to the power provided by on-board generators.

Today’s inverters are compact, relatively light and generally easy to install. They are available as portable units to handle the smallest AC requirements, such as charging a cell phone or VHF radio with a desktop charger, or as much larger units that can be permanently installed/hard-wired to provide in excess of 4,000 watts of AC power to run microwaves and dishwashers, depending on the size of your battery bank.

How does it work?

All inverters are not created equal. There are basically two types available, with subtle but significant differences in the way they operate. They are distinguishable by the AC wave form that each produces, one being “true sine wave”; the other is either “modified sine wave,” “quasi sine wave” or “modified square wave.” Each has its pros and cons, depending on the types of loads that are being powered.

True sine wave

A true sine wave inverter’s output voltage wave form is considered “pure,” with very low harmonic distortion and clean power similar to utility company-supplied 120-volt, 60-cycle AC. These units are well suited for sensitive electronic equipment such as laptops and laser printers. Inductive loads like microwave ovens and electric motors run faster, quieter and cooler with true sine wave inverters compared to modified sine wave inverters. True sign wave units also can reduce audible and electrical noise in fans, fluorescent lamps, audio amplifiers and televisions. But these inverters come with a cost. Due to more sophisticated design and manufacturing requirements, along with more expensive components, true sine wave inverters can be two to three times more expensive per watt than modified sine wave units.

Modified sine wave

Although there are differences within this group, for this article I will categorize modified sine, quasi sine and square wave inverters together, referring to them simply as modified sine wave. The AC output of modified sine wave inverters is designed to have characteristics similar to the sine wave shape of utility power. Some of the inverters produce a square wave, while other wave forms are considered stepped, composed of a series of small steps, depending on the load placed on the inverter. The more steps in a modified wave form, the closer the output voltage is to a normal AC sine wave. Charles Industries of Rolling Meadows, Ill., states that its quasi sine wave inverter has more than 19,000 steps to form a complete single wave form.

Some modified sine wave inverters provide comparatively “choppy” power, which presents compromises with some loads, can tend to overheat items like surge protectors and often makes some appliances unreliable. Problems can occur with portable devices that don’t include a transformer between the power source and the device, such as some low-cost rechargeable tools, electric shavers and emergency flashlights. (Portable transformers can be identified by a small black box attached to the power cord.) One well-known inverter manufacturer states in its literature: “Always check with the manufacturer of an AC device as to whether or not it will run with a modified sine wave inverter.”

Modified wave form inverters are less costly than true sine wave units and have a high surge capacity that allows them to start large motors with high current demands for very short durations. In addition, modified sine wave inverters typically run at a higher efficiency level than true sine wave inverters. (Please note that different inverter manufacturers have varying opinions on what will and will not operate properly with their modified sine wave inverters. The information I have provided here is generally accepted by technicians in the field and is not allied to a specific company.)

In some cases, based on cost, it may be practical to purchase a small true sine wave inverter to power those “special” loads, while using a less-expensive modified sine wave unit as your primary inverter.


Inverters are available in different sizes or ratings. The size of an inverter is measured by its maximum continuous output in watts. To determine the inverter size you’ll need, decide which AC loads you would like to run at the same time and then add a safety factor of 10 to 20 percent. The rating of the inverter must be larger than the total wattage of the AC loads you plan to run, plus the safety factor.

The wattage of most AC loads typically is included on a tag or label on the appliance or in its owner’s manual. In addition, there are load calculations and formulas that will assist you in determining the appropriate inverter size. For example, the tags sometimes indicate amps, not watts. To calculate the wattage when amps are provided, use the formula P = V x I, where P is watts, V is voltage, and I is current. A tag may indicate voltage is 120VAC and current 1.5 amps. The calculation would be: watts = 120 x 1.5, or 180 watts. If you plan to use an inverter to power induction motors, like those of large power tools, dishwashers, etc., it must be designed with surge capability that will deliver short bursts of power that far exceed the continuous rating.

Inverter manufacturers will assist you with any load calculations and sizing questions. There also is a section in the American Boat and Yacht Council’s Standards and Technical Information Reports for Small Craft that provides load calculation information. (Log on to the ABYC Web site at and type “inverter” in the search window.)


Basic low-wattage inverters aren’t difficult to install. However, as inverters increase in output and complexity they become an integral part of a boat’s AC and DC power system. As such, there are very specific guidelines that must be followed.

ABYC states: “All marine power inverters shall meet the applicable requirements of UL 458.” There are numerous engineering and design criteria required to meet these requirements, most of which are for your safety, and manufacturers that meet them typically advertise their products as such.

If you plan on installing an inverter yourself or are considering purchasing a boat that has one, here are a few items to consider:

• The inverter should be as close to the batteries as practical to minimize DC input voltage drop.

• Inverters shouldn’t be installed directly above the batteries (to avoid corrosive fumes).

• Use the cable (gauge and length) recommended by the inverter manufacturer, which usually is in excess of the minimum wiring standards but is required for efficient operation. It is more electrically efficient to run lower-current AC wiring longer distances.

• Most inverters aren’t ignition-protected and, as such, shouldn’t be installed in a gasoline-engine compartment.

• Inverters must be installed in a dry, well-ventilated area. They produce a lot of heat, which serves to derate output, and require a fresh air supply to operate efficiently.

• A visible means (volt meter or lamp) of determining that the inverter is “online” and /or in “standby” mode shall be provided at the AC main electrical distribution panel (ABYC

• Overcurrent protection (Class T fuse) should be installed in the DC input side of the inverter.

Combination units

Inverters and battery chargers share certain electrical components, and many inverter models are available as combination inverter/chargers. In addition to providing AC power for your batteries, inverter/battery chargers typically provide temperature-controlled, intelligent, three-stage charging (bulk, absorption, float) and allow you to accurately monitor battery condition through unit-mounted controls or remote accessory panels. An added feature of combination units is an integrated automatic transfer switch that forms a reliable connection separating incoming and outgoing AC power sources.

When shore power is connected or an on-board generator is providing AC power, the inverter/charger automatically switches to charger mode, taking the batteries through all three charging stages while monitoring them for temperature and state of charge. In the event that there is an interruption of AC power, the unit automatically switches to inverter mode, providing AC power to the circuits. The automatic switching modes can be also disabled, allowing manual control of the transfer as well.

Options and features

Most inverter/chargers incorporate features that protect the unit and shut down the systems before damaging any connected accessories and wiring. The safety systems address over- and under-voltage conditions, overload and short-circuit protection, temperature compensation and reverse polarity.

Load sensing is a feature that’s increasingly found on today’s inverters. The inverter will operate in a sleep mode, consuming only small amounts of current, until a load is applied, such as turning on a light, at which time the inverter senses the demand and begins to provide the required AC power. Load sensing typically activates the inverter mode when it sees a 5- to 10-watt load demand.

Most manufacturers offer remote panels that not only allow inverter control but also can monitor battery condition and state of charge while providing actual AC and DC power consumption information in real time. The remote panels can provide low battery alarm, overload warnings and the total number of amp hours consumed.

Current technology

Switch-mode technology is replacing the heavy, bulky transformers that inverters once relied upon. This technology has led to more compact and lighter-weight units, providing increased portability with greater power output and smaller size, as with the recently introduced Mastervolt AC Master Power Inverter (350-watt, 120-volt model).

Mastervolt of Hanover, Md., also offers the Mass Combi, which combines several functions into one remotely controlled and programmable unit. The inverter/charger has a three-stage battery charger, pure sine wave power inverter, and a sophisticated AC transfer system, which offers several options for transferring AC power to supplement shore power, combined with generator power. The optional remote control panels allow complete operation and configuration of the system. (For an operational review of the Mass Combi, see the accompanying story on Page 56.)

Inverter technology is constantly improving and is being combined with other related systems to enhance your on-board lifestyle.

The major players


Hanover, Md.

Phone: 443-459-5370

Charles Industries

Rolling Meadows, Ill.

Phone: 847-806-6300


Burnaby, British Columbia

Phone: 604-422-8595