Features February 15, 1998 Issue

7 Inverters Tested: Ace is Statpower

If the prospect of consistently available 120-volt AC power on your boat sounds enticing, you might be in the market for an inverter. An inverter is a device that acts something like a battery charger in reverse, converting 12-volt DC power to usable AC power in a two-step process. The first is the conversion of direct current to alternating current, the second the adjustment of the AC to the required voltage. The reference to 120 volts is actually an average called RMS, or root mean squared, used for making power comparisons between AC and DC: 120 volts AC is roughly equivalent to 120 volts DC in terms of the work it can do.

There’s a wide variety of inverters on the market. Here, we look at some of the higher-output models—from 1,000 to 2,500 watts—that can be used to power major appliances.

Inverter Basics
The best approach to inverter selection involves balancing four requirements. First, you need to determine what size you need for your specific application, and whether you want a fixed or portable installation. Then, you must determine the size of battery supply required to support your usage. Last, you have to decide on the most efficient means of recharging your batteries. Any time you plan to use an inverter of more than, say, 300 watts, it’s really necessary to take this systematic approach to optimize results.

There are three types of modern inverters. True sine wave models are the most expensive (although prices are coming down) and produce output virtually identical to standard house current. The main advantage of this type is that it can be used for the most sensitive electronic circuits, such as SSB radios, fax machines, laser printers, and sophisticated battery chargers.

The most prevalent type of inverter is the modified sine wave variety, which uses a dynamic stair-step wave to approximate a sine wave. These are fine for powering higher-quality gear, which often contain effective filters in the power supply. TVs, computers, and resistive loads generally run fine on this type of inverter. Cheaper gear tends to be more sensistive. The last category is the square wave inverter, which has not been included in this test because of its limited use with today’s sensitive electronic equipment.

You may have noticed that some inverters are much larger and heavier than others yet have the same power rating. This is because there are two methods of producing the AC wave form. One is called line-frequency switching, and the other high-frequency switching. It’s important to know the difference Gn making an informed selection, because each has its advantages and disadvantages.

In line frequency inverters, a time-tested technology with fewer parts, banks of transistors convert DC to AC. The frequency of the AC is controlled by a crystal oscillator. The AC voltage is then stepped up by a transformer to approximately 120 volts AC RMS. As battery voltage falls, the output wave form changes to maintain 60 or 50 cycles, but becomes less and less an approximation of a true sine wave. The term used for this compensation is pulse width modulation. A problem with pulse width modulation is that as battery voltage declines, hum and noise interference become more apparent on such equipment as TVs. If the interference occurs at only low battery voltage, more battery power may reduce the problem. Line frequency inverters are about four times heavier than a high-frequency unit for a given power output.

In high-frequency inverters (a newer technology), transistors also are used, but here they switch on and off thousands of times a second. This high-frequency AC is then fed to another bank of transistors, which are switching at line frequency (i.e., 50 or 60 cycles). The higher the frequency of the initial switching, the smaller the inverter for a given power output.

High-frequency inverters may cause interference with SSB radios or weatherfax receiving equipment, although there are filters available to minimize this potential problem.

Inverter Selection
Which is best? To quote an oft-used phrase “it depends.” High-frequency inverters, while much smaller and lighter, are more complex than line- frequency models. As noted, they also are prone to generating interference with some equipment.

True sine wave versions are generally 5% to 10% percent less efficient than modified sine wave models, and most have a substantially higher standby current while idle, which can be particularly important on smaller boats with limited 12-volt reserves (unless you can remember to shut it off when not in use). The biggest advantage to line-frequency models is the built-in high-capacity battery charger, worth at least $300 if purchased separately, and found in most models. The PROsine we tested is an exception because it is a high-frequency model with a sophisticated four-stage charger.

An inverter has to be able to handle peak loads. This means doing some planning, since an undersized model will leave you frustrated, and an oversized model, in addition to added expense, will not operate in its most efficient load range, which is usually at half to three-quarters capacity. For example, induction motors such as pumps can take up to six times the rated load on start-up. Even resistive loads like toasters can momentarily draw several times their rated load at start-up. Lastly, the AC power factor has to be considered. Certain items such as induction motors use more power than their apparent rating. In some cases this may be as much as 50%. That’s why it’s advantageous to sit down with a chart to come up with as accurate an assessment as possible.

Fortunately, inverters typically have a surge capacity (temporary overload) of 100% or more to handle motor starting requirements.

Batteries are your inverter’s fuel tank. Remember, it takes roughly 10 times more 12-volt DC current to equal AC house current plus conversion losses; for example, it takes .83 amp to run a 100-watt bulb at home, where a 12-volt inverter will require 9 DC amps for the same bulb when running off battery/inverter power.

Heat and moisture are the two biggest enemies of electronics. Inverters are no exception. In fact, inverters are rated based on being cool and ventilated. Heat degrades their performance. Different types of metal are used in inverters and are subject to corrosion unless kept in a dry environment. They should be located as close to the battery compartment as possible, but not in it, due to the spark danger of inverters and the potential for explosive gasses in the battery or engine compartment. Careful attention must be paid particularly to the DC side of the installation due to the requirement for very large capacity cables connecting to the battery. Pay extremely close attention to the recommended sizes and lengths of cable, otherwise the installation will badly fail to meet specifications. You are probably best off having these cables fabricated whenever required to assure proper terminal attachment. Welding cable is the wire of choice due to its high flexibility. (Heart Freedom inverters come with 6-foot cables already attached).

How We Tested
We used a number of meters in the empirical side of the evaluation. The two primary instruments were the Fluke 36 clamp-on multimeter, which can accurately measure up to 600 amps DC or 1000 AC without breaking into the circuit. Electrical wave forms were checked with a Fluke 867 graphical display meter. This portable tool provides a simultaneous display of the AC voltage, frequency, and similar to a digital oscilloscope it provides either a running or snapshot display of the wave form. With this meter we were able to track real-time displays under various loads as well as sample and recall a series of recordings over time.

The instrument was capable of attachment to a computer via a serial cable for extensive recording capabilities. Additionally, the Fluke 867 features a “glitch mode” that can detect transients as short as one microsecond. This instrument was very valuable in finding potential problems with any of the inverters. We tested each inverter to its listed capacities in terms of watts as well as “tough” loads such as induction motors to see how well they handled the surge requirements these motors require to start. We also used a variable speed drill, since this type of variable speed motor with a SCR (silicon controlled rectifier) can be difficult for modified sine wave inverters to handle, sometimes affecting frequency or voltage stability when the drill is used at partial power.

Additionally, we used typical products from hair dryers to a microwave. A new, group 27, 115 amp-hour, lead acid, deep- cycle battery was used and recharged with a deep-cycle charger to optimize the charge. (Note, a group 27 battery is not really practical for full-sized microwave use, but was OK for short tests). DC connections were made with 1/0 copper cable.

What We Found
We were pleased to see that all the inverters met both their continuous and surge rated load capacities, though with some differences in voltage and wave form. All the modified sine wave models experienced some erratic wave forms and frequency or voltage instability when the variable speed electric drill was used. Steady readings of up to 140 volts were observed on some inverters, as well as frequency jitters when the drill was used at partial power. This has no real adverse effect on the drill, but if there were a frequency- or voltage-sensitive piece of equipment also plugged in, such as a VCR, it would probably not work properly while the drill was operating (particularly at partial power). One hundred and forty volts RMS is on the high side of acceptability and is hard on equipment.

The bottom line is that one should be careful when using multiple items with a modified sine wave inverter if one of them is frequency-sensitive, and the other has a motor speed control. Conversely, the two pure sine wave models were rock solid regardless of the nature of the load. The only penalty was the 5% to 10% efficiency loss in the AC conversion process.

Modified Sine Wave Models

Heart Freedom 10 & Trace M 1512
We’ll discuss Heart and Trace together since they are essentially head-to-head competitors in the ultra-sophisticated, “total solution” modified sine wave market. They are designed to be totally integrated into a boat’s electrical system, requiring hard wiring into the power system and the addition of AC receptacles. With built-in, sophisticated battery charging circuits, auto-transfer switches, and all forms of overload protection, they can be essentially “transparent” once installed and set up.

None of the inverters tested is immune to reverse polarity at the DC battery connection. They have fuses, but this does not guarantee safety of the inverter. If you make a reverse polarity connection, you void the warranty on most models, so be careful. Special idle circuits that drop the standby current to milliamps allow them to be left on all the time. They automatically switch between shore power and battery power almost instantly to provide continuous AC. The table gives specific numbers for each brand. Trace is superior in the idle power arena, using almost zero power in the standby mode. Both units have remote switches or complete remote controller circuitry available as an option. The new Heart Link 1000 controller ($300 at discount) came with our test unit and provides total system customization and battery equalization to the multi-charge cycle—things which are not adjustable or available in the base unit. Installation requires five wires, a supplied shunt and phone-type cable for integration into the inverter power system. The display is comprehensive and uses LEDs. These remote units really should be considered almost mandatory in order to take full advantage of the Heart inverter.

Trace has a similar remote controller unit called the RC-5 ($200 discount). The difference is that the inverter essentially has all the adjustments built into the basic unit and the RC-5 is an extension of that capability to a remote LCD, two-line display that adds a few extra adjustments. The RC-5 is much easier to install, requiring only the connection of a phone-type cable to the inverter.

The battery equalization cycle is not currently available in the Trace unit, but it has a separate remote temperature sensor included for the battery to promote more effective charging. Each brand seems to offset each other’s advantages with special features. In short, they are both very sophisticated, highly evolved units in the modified sine wave arena.

As far as marine adaptability, both units have powder-coated paint finishes, stainless steel hardware and coated circuit boards. Both would do well in a marine environment given proper installation. Both brands have remote indicator units that are designed for a marine environment.

Which to choose? We liked the Trace unit a bit better, especially if you only want a remote switch with the inverter, as the Trace has full adjustability built in as well as volt and amp indicators. A remote switch (RC-4 with no indicator) is about $50 extra.

Heart, on the other hand, is far more widely available in the catalogs (only the Defender catalog carries Trace) and you may be able to get a better price on a per watt basis. Heart has models from 750 to 2500 watts in the Freedom series. Trace is available from 1,500 to 3,000 watts in the Marine series. You need to add the price of the Link 1000 (or 2000 for multiple battery banks) to get true customization. Heart has a longer warranty.

In our last evaluation of inverters (April 1 and May 1, 1993), we said that Trace “affords finer control over more parameters of its operation than the Heart.” But “if you have little interest in ‘getting into’ your inverter, go for Heart.” You'll probably be happy with either brand.

Tripp Lite Model 1200 FC
The model does not have a battery charger built in and accordingly is less expensive. It performed well, but did not have the idle circuits of the Heart or Trace, so you would want to turn the unit off when not in use. It did have built-in AC receptacles and the ability to add a remote switch to make turning the unit on or off easy. As a line-frequency unit, it weighed in at a hefty 38 lbs., so you wouldn't want to move it often. It employed auto-type fuses for AC short circuit protection, as well as internal automatic circuits for over temperature or overload conditions.

Statpower PROwatt 1500 & PORTAwattz 1750
In contrast, the Statpower high-frequency, modified sine wave units were light and easy to move around. They offer a different dimension of utility. They do not have adjustable remote controller units or transfer switches, and since they do not have battery chargers built in you will have to plan on a separate, multi-stage charger unit unless your needs are very intermittent or light duty. There is a lot to recommend them for temporary or semi-permanent installations.

Besides light weight, they have built- in volt and amp meters, AC receptacles, over temp and overload indicators and auto resetting circuit breakers if subjected to power overloads (but not DC short circuits). They also have a provision for a remote on/off switch. The lower cost PORTAwattz model is designed primarily as a portable installation with no provision for hardwiring AC receptacles as does its electrical brother the PROwatt, which also offers GFI AC receptacles built in and a slightly higher surge rating.

It also comes with a remote switch. We also found the Statpower unit’s wave forms and frequency to be less affected by our nasty variable speed drill motor test. On the other hand, high frequency inverters are known for causing RFI (radio frequency interference). We only had VHF and AM radios to use for RFI detection, and experienced no real problems with any of the inverters. These units are made in China for Statpower, but examination of the circuitry indicated good quality workmanship and materials. All the other inverters were made in North America.

Sine Wave Inverters

Exeltech XP 1100
Sine wave models, we believe, represent the future of inverters. As electronics become more sophisticated and prevalent on our boats, the need for carefree true sine wave power will increase.

The unit from Exeltech demonstrates that true sine wave power is possible at a price lower than ever (albeit with no frills or remote adjustments other than a capability for remote off and on). It is a high-frequency unit, and very lightweight. It was amazing to watch the sine wave hold solid regardless of the loads we threw at it. You will require a separate charger. Further, because there is no idle circuit the unit draws 2 amps when it is on with no loads; thus you will want to turn it on and off as needed. Its 87% peak efficiency rating at one-third power puts it about 9% behind the best modified sine wave units. You also would probably want to set up your own volt and amp meters to monitor battery charge and usage. AC receptacles are built into the unit to give it portability, as well as a provision for hard wiring additional AC receptacles if you desire to mount it permanently.

Exeltech’s manual was decidedly thin, essentially giving basic specifications and DC wire recommendations. This is a sharp contrast from Heart, Trace and Statpower, which had extremely well-done and comprehensive manuals. The Exeltech unit itself is a small marvel even in its basic form, and a more comprehensive manual would better help to exploit its maximum capabilities.

Statpower PROsine 2.5
We saved the best for last. This unit is nothing less than phenomenal. The exterior case is all aluminum, with all mounting hardware made of stainless steel. The powder-coated side panels are 1/8" thick aluminum for extreme ruggedness. While the price sounds high at first, looking at what you get actually makes it an outstanding value. Included is an external remote controller that is worth $200 to $300 dollars. This unit looks like Statpower took a clean sheet of paper and decided to include every good feature from of all of the other inverters.

Statpower has covered all the details well. For example, toroid RFI filters were installed in the AC outlet and inlet circuits, which are very effective in preventing interference. Also, captive wiring terminals (as recommended by ABYC) are used for the most secure connection of AC wiring. Being high frequency, it is light for the power capability. It has an extremely sophisticated four-stage battery charger, adjustable up to 100 amps DC output. The charger is also more efficient and cleaner than line-frequency inverters with chargers, allowing more AC shore power to be available for other circuits. Its standby circuit uses only 3 watts at idle power (adjustable), so it can be left on without undue drain on the battery.

The AC transfer switch is fast enough to allow a transfer of shore power to inverter power and back without an operating computer missing a beat. Trace claims this same capability) It can be totally customizd via dip switches for each individual installation. Its 88% peak efficiency at 1,500 watts is only 8% behind the best modified sine wave models. Lastly, there are onboard diagnostics which flash an error code if there is a problem of any sort. The codes are listed in the manual. The AC sine wave power is so clean as to be indistinguishable from house power. It has a surge capacity of 4,000 watts, so there is almost nothing this unit can’t handle. This unit clearly eclipses anything else we know of.

True sine wave power at reasonable prices is here. But modified sine wave inverters are still a viable option, providing you assess your needs and equipment carefully.

Heart and Trace are both outstanding “total” solutions, while the high-frequency Statpower modified sine wave units are excellent values.

The Exeltech unit offers just the basics, but the low price for true sine wave output is very compelling. The fact that it’s portable and weighs only 10 lbs. is a plus. We just wish it had a better manual.

We were most impressed by the state-of-the-art PROsine inverter from Statpower, and highly recommend it as the best, most complete true sine wave version we have seen. A modified sine wave model of equal power with all the options (if you could find one) would be nearly as expensive. Put PROsine on your short list.

Contacts— Exeltech, 2225 E. Loop N, Ft. Worth, TX 76118; 817/795-8969; Heart Interface, 21440 68th Ave., Kent, WA 98032; 206/872-7225; Statpower Technologies, 7725 Lougheed Hwy., Burnaby, BC V5A 4V8, Canada; 604/420-1585; Trace Engineering, 5916 195th NE, Arlington, WA 98223; 360/435-8826; Tripp-Lite, 500 North Orleans, Chicago, IL 60610; 312/755-5401; West Marine, P.O. Box 50050, Watsonville, CA 95077; 800/538-0775.

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