In the February 15, 1998 issue we reviewed large inverters capable of producing up to 2500 watts. For those of you with smaller AC power needs, a small or pocket inverter may do for running such things as a 13″ television, a VCR, computer or small rechargeable devices. There are quite a few low power output models on the market, but most are high-frequency modified sine wave devices. There are also a few older-style designs, called line frequency inverters, which we tested. Finally, true sine wave inverters, with output virtually indistinguishable from house power, are now available in this compact size. One is included in this test.
What We Tested
For this comparison, we included a representative sample of small inverters with continuous power output range of 125 to 300 watts, and ranging in price from $40 to $225. With rare exceptions, most models in this power range are from the Far East. We found that quality and true power output does vary considerably, particularly in surge power needed to start devices such as televisions. Electronics, which are not a pure resistive load, can require a momentary surge current far greater than the rating printed on the side of the particular device. A 60-watt TV, for example, can easily need 300 momentary watts to start up (particularly older models). Inductive motors also require more starting power-up to six times their rated load. However, this information usually is not reflected on data plates of appliances. We tested these capabilities in our hands-on tests to see just how well the inverters handled the loads.
Inverter Basics
Inverters use several basic approaches to produce power. Older types make use of transistors that operate at line frequency-i.e., 60 cycles-switching on and off to convert DC to AC electricity. The AC is then stepped up via a transformer to approximate 120-volt house power. However, this output is only a rough approximation, and the output waveform is called a modified sine wave (see diagram on next page). Because most AC-powered electronics are designed for sine wave power, this older technology can result in lower efficiency and occasional operating problems with some devices, especially those with sensitive transistor or integrated circuit components. Also, be aware that only a true RMS (root mean square) voltmeter will accurately measure the AC output voltage of modified sine wave inverters. This is because the vast majority of voltmeters are designed to assume that they are measuring a sine wave representative of house power. The concept of RMS is a way to equate AC and DC in terms of formulas for power output.
A newer method of DC conversion is called high frequency switching, which produces a modified sine wave. Here, DC input is converted via banks of transistors from low to high voltage-12 volts to 145 volts DC, for example. Another bank of power transistors converts the 145 volts DC to AC . The advantage to this method is that it requires a much smaller and lighter package for a given power output. The disadvantage is a tendency to generate radio frequency interference unless the unit has effective filter circuitry built in.
The latest development is the true sine wave inverter. This circuit is usually a high frequency design that employs additional transistor circuitry to produce true sine wave output virtually identical to house power. This means you can run even the most sensitive electronics without worry of operational problems or damage. The primary disadvantage is higher price and a bit less efficiency in the conversion process, although no inverter is 100% efficient at converting DC to AC.
What to Look For
The basic attributes in an inverter that a buyer should look for are convenience and safety. If the unit has a switch, the inverter should have a power-on indicator so you can tell at a glance whether the power is on. Over-current protection will shut the inverter down if the AC demand exceeds the inverters capability, and automatically reset when the load is removed.
Overheat shutdown protects the inverter in the event there is a sustained load that exceeds the continuous rating, or if the inverter is located where there is inadequate circulation or a hotter than normal environment. Low battery voltage shutdown is designed primarily to protect the inverter. But you have to consider your batteries as well. Many inverters have too low a shutdown voltage, that is, below 10.5 volts which is very hard on a battery (a battery is considered completely discharged at 10.5 volts, under moderate load). To maximize battery life even deep-cycle batteries should not be repeatedly discharged below 50% capacity, which is about 11.6 volts under moderate load. (Dont confuse open circuit voltages that are higher). Also, if you are running a device off a starting battery from an inverter, low battery shutdown is only going to alert you after its too late to start your engine!
There should be adequate wiring size for the DC side of the inverter. Fourteen gauge is appropriate for up to 300 watts continuous. Some inverters are very marginal in this respect, which wastes power in the form of heating the input wires. Be aware that the practical limit for using a cigarette lighter socket with an inverter is about 110 watts or 10 amps DC input (allowing for inverter conversion losses). If you need more power, a more direct connection to the battery is needed. Some inverters come with a set of additional short cables that have temporary clamps for direct connection to the battery. This should be considered a short-term expedient. If you end up needing a direct connection, it would be highly desirable to have a separate fuse for battery short circuit protection with appropriately sized wire to a point convenient to inverter use.
As with house circuits, you should never replace the fuse in your cigarette lighter circuit with a larger one to increase the amount of power the lighter circuit can handle-no matter what a manufacturer might say. The manual for one of our test units, Power to Go, recommends substituting a 30-amp fuse in place of the existing 10- to 15-amp fuses found in most typical cigarette lighter sockets if you experience problems blowing the original lighter fuse. This recommendation could be very dangerous as the wiring in your lighter is sized and designed for the original fuse. Replacing it with a larger fuse is both a potential fire hazard and a waste of power as heat in the DC wiring. We don’t recommend it. Instead, run a separate, appropriately sized, fused wire from the battery.
You should know that it takes more than 10 times the DC current of a 12-volt system to equal the given power in watts of a 120-volt AC appliance. (If the supply voltage goes down by a factor of 10, then the current must increase by a factor of 10 to compensate and keep the power equation balanced). The basic formula for DC and RMS AC is: volts x amps = watts. For example, a 100-watt light bulb uses only .87 amps AC power in a 120-volt home system (120 x .87 = 104 watts), whereas in an inverter setup the same bulb will require more than 9 amps DC supply voltage to provide that 100-watt output. There are additional losses in converting DC to AC in the inverter circuits, on the order of 10% or even more in less efficient inverters. The point is that a great deal of current is necessary with inverters because of low voltage input.
If you are planning to use a pocket inverter to recharge nicad batteries, its best to check your charger manual to make sure the inverter will not harm the charger. Some of the cordless 12-volt drills we reviewed in the October 15, 1997 issue came with a warning against charging off an inverter. If the manual does not have any guidance, then check with the manufacturer. If you damage a charger with an inverter, theres a good chance the manufacturer will not honor the warranty. The problem is that the modified sine wave of most inverters is not compatible with some charger designs, mainly those with SCRs (silicon controlled rectifiers).
True sine wave inverters, on the other hand, are just the ticket for very sensitive circuits. They are available in a variety sizes from 125 to 1100 watts (Our report includes a 125-watt unit from Exeltech). The downside is a much higher cost for the inverter, as well as a bit less efficiency in the conversion process.
How We Tested
When you have low-cost (like some of the models tested) devices with multiple electronic components, there is some concern about high failure rates (weve heard as much from one of the large marine products retailers). And customers are just not as likely to pursue warranty repairs on low-cost equipment (and the equipment makers know this), so there may be less effort to assure quality control or long term reliability in the design. Most of these units look solid enough from the outside. but you cannot easily tell the quality of these devices from their external appearance. So we took them apart to see if there were meaningful differences.
We hooked up the inverters with 14-gauge wire to a group 27 deep-cycle battery, which was fully charged between tests. Waveforms and output frequency were checked with a Fluke 867 graphical display meter, an RMS device that is functionally similar to a digital oscilloscope. We tested each inverters ability to run a variety of appliances from 60 to 250 watts continuous power, as well as overload capacity. The test items included a fan, small electric tools, such as a jig saw, a stereo system, and a new 13″ color TV.
To check performance and idle current, we attached an ammeter in line with the DC side of the power input, as well as the AC side of the power output. This also allowed us to see how much power was being used up in the inverter circuit making the conversion. To see how well the waveform held up under load, we placed the Fluke 876 digital display meter on the AC side of the inverter to monitor the nature of the modified sine wave under varying loads. (There is a tendency for the waveform to become more square and less efficient as the load increases or the supply voltage decreases). Lastly, we checked to see how well the inverters operated the TV and stereo and whether there were any problems with radio interference or noise on the inverter-powered devices or on nearby radio devices.
Radio frequency interference (RFI) in TV and radios can be the bane of high frequency inverters, but it can sometimes be cured by following some simple procedures. First, make sure you use shielded coax cable with TVs. Next, experiment with the location and position of the inverter, placing it as far from the device as possible. Finally, experiment with the position of the cables and power wires, as sometimes a slight change of position can help a great deal. These procedures will not cure the basic design problem of all high-frequency, modified sine wave inverters, which are more electrically noisy than house power. The quality of the power supply and filtering of the device you are powering plays a large role here, as does the filtering circuitry added to the basic inverter design. Generally, higher quality equipment is more tolerant of modified sine wave inverters.
What We Found
All models ran the test items within their ratings, and the stereo and TV worked acceptably with all inverters- somewhat to our surprise. Beauty is only skin deep, however. In spite of the very similar appearances among the high frequency models, there were significant differences in internal construction and components.
These differences had both a qualitative and quantitative component-not only what electronic components were used, but how they were put together. As an example, the Power to Go models were not as well made as the Statpower devices, using cheap composite circuit boards instead of highly dielectric phenolic material, and no-name integrated circuits. Our lone low-frequency inverter model from Tripp Lite presented no radio frequency suppression advantages over the high-frequency units which were properly shielded. Low-frequency models have fewer parts which may or may not contribute to longevity, but their greater weight is a negative.
Power to Go PC140
The Power to Go PC140 is a low- priced model, with virtually no surge capability. It does have all the needed safety features mentioned in our earlier guidelines, including overheat shutdown and overcurrent protection. The separate power switch is more convenient than having to pull the plug to disconnect the unit.
There are some big negatives, however. For one, the unit has an annoying buzz. Surge capacity, as mentioned, is negligible, limiting the devices this unit will start. This unit has little in the way of frequency suppression other than the basic aluminum case. It emitted sufficient interference to blank out a portable AM radio 10′ away. Construction was poor; the power transistors have only a paper wedge for secure contact with the case for heat sink purposes, which is less than reliable for long-term use. And it has but a three-month warranty. Not recommended.
Statpower Prowatt 150
The Statpower Prowatt 150 unit did not have a separate power-on switch, but it did have an LED power-on indicator. It also had all the safety features, but no external indicator of shutdown status. If the unit is overloaded, it will shut down and automatically reset, but theres no indicator to display this fact. On the plus side, the circuit board is of good quality, and there is an excellent complement of power capacitors to give a substantial surge capability (400 watts) and a smoothing effect to the output power. Additionally, there is an RF filter capacitor on the AC outlet, as well as a ferrite RF suppressor on the AC wiring to further reduce interference problems. Our tests showed this unit to be free of radio frequency interference problems. It easily started our 13″ TV. Power transistors were firmly secured to their respective heat sinks for more reliable heat dissipation and longevity-heat kills electronics more than any other factor. Recommended.
Statpower also markets a cheaper line of inverters under the name Portawattz, which we didnt test.
Power to Go PC 300HS
The Power to Go PC 300HS is a substantial step up in capacity and surge capability over the PC 140, athough hardly any bigger physically. It is better built and designed than its sibling, but it still lacks any substantial RF suppression components. The RF signal it transmitted to the AM radio 10′ away sounded like a helicopters whup-whup, probably caused by the cooling fan that runs continuously. The fan does contribute to its high surge rating, however. This inverter has the basic safety features, a power on switch, and a short accessory cord for attaching the unit directly to the battery with temporary clamps, providing the battery is convenient to the short cord. This would be a very awkward setup for anything other than a see if it works test.
This unit works as advertised, but RF interference is a problem. The fact that the TV and stereo worked OK was more a function of the quality of the filtering on the TV and stereo. It carries a three-month warranty. Marginally recommended, if only because of its 600 watts of surge power, highest of the units tested.
Statpower Prowatt 250
The Statpower Prowatt 250 is constructed similarly to the Prowatt 150, with the same quality components and RF suppression devices. It includes an audible alarm for low battery at 10.7 volts, before auto shutdown at 10 volts. Surge capacity is up to 500 watts. Statpower claims that this unit will power a 25″ TV and it probably will, although we tried it only on a 13″ set. Physically, its only a bit larger than the 150. Interestingly, Statpower uses no cooling fan (saving on power and the potential RF interference). Instead, the manufacturer uses a very secure and efficient heat sink connection as well as a one-piece, extruded aluminum case of very rugged construction. We were able to hold our AM radio within a few feet of the Statpower units with no interference, showing that it is possible to successfully filter high-frequency inverters. We do wish there was an on/off switch. Highly recommended.
Tripp Lite PV 200
The Tripp Lite PV 200, our lone line frequency entrant, was a disappointment. We ordered it from a catalog, assuming it was a modified sine wave model, only to find out it was a square wave inverter (the companys latest model). Square wave technology predates modified sine wave and is less efficient (it used 1.7 amps at idle), as well as harder on electronics because of its poor similarity to a sine wave. And while there are fewer components, the old technology requires much bigger components, including a massive transformer. Additionally, there was no frequency control circuitry of the output AC, further narrowing the items this unit would be electrically friendly toward. And because the plastic-cased device weighed 6 pounds (compared to about 2 pounds for the other units), buzzed, and generated RF out to about 3′, well keep it off our recommended list.
Exeltech XP 125
Our true sine wave entry, the Exeltech XP 125, was as delightful as its big brother the XP 1100, which we tested last February. At $225, its certainly not cheap, but it performs just like house power. It has all the safety circuits, and the specs predict a mean time between failure rating of over 20 years-impressive.
Examination of the circuitry revealed very high quality components and construction throughout. There is nothing we would hesitate to run with this inverter, within the output power constraints. There will be a higher rated 250-watt version due out by the time you read this. If there is something we would like to see added it would be a power-on indicator and an on/off switch, but Exeltech recommends unplugging the unit when not in use-probably to preclude accidentally draining your battery. When you consider the cost of electronics, and the cost to repair them, the price of this inverter is not too high.
Conclusions/Recommendations
As is the case with so many other products, with pocket inverters you tend to get what you pay for-although that is hardly a guarantee anymore. Among the modified sine wave models, the Statpower Prowatt series are clearly the best buys when you consider the combination of warranty (four times as great as the Power to Go units), construction, and performance. Of the two Prowatt models, we would recommend going with the higher capacity 250-watt model, as its physically only a bit bigger with nearly twice the capacity, will run cooler at reduced loads, and has the low-battery warning buzzer at 10.7 volts.
If you will be running expensive or sensitive electronics, particularly on a frequent or extended basis, then the Exeltech XP 125 or 250 is your best guarantee for trouble-free operation. We believe sine wave power will become more common because of the increasing prevalence of sophisticated electronics requiring an equally sophisticated inverter. Lower prices are bound to follow.
Contacts- Exeltech, 2225 East Loop N., Ft. Worth, TX 76118; 817/795-8969. Power to Go, 85 Fulton St., Boonton, NJ 07005; 201/316-2400. Statpower Technologies, 7725 Lougheed Hwy., Burnaby, British Columbia V5A 4V8, Canada; 604/420-1585. Tripp Lite, 500 N. Orleans, Chicago, IL 60610; 312/775-5401.
