A power inverter is simple. It lets you plug in household appliances and run them off the battery system in your, RV, truck, or boat. Selecting the right one, however, is not always so simple. There are many brands and models made by dozens of companies and offered at wildly different prices. Many inverters look alike, sharing the same shapes and colors, even though they have very different ratings. To complicate matters, power inverters are sometimes called voltage adapters, power adapters, power converters, or power supplies.
| From Black & Decker comes this 750-watt power inverter with dual AC and USB outlets. Low-battery shutdown prevents total battery drain.
To be specific, a power converter changes 120V AC to 12V DC, supplying power to 12V circuits and appliances and charging the batteries. Most RVs have power converters as standard equipment to run 12V appliances in the RV using electricity from a 120V campground connection.
A power inverter does the opposite, changing 12V DC from a battery source into 120V AC to run household electric items using battery power. Inverters make it possible to run household items like laptops, TVs, DVD players, game consoles, and blenders. Even high-wattage items like toasters, microwave ovens, and vacuum cleaners can be run from battery power using an inverter, given the right capacity. Some RVs come with inverters, but most of the time they are not standard.
Generally, the more self-sufficient you want to be, the more you need a power inverter. So if you camp in the boondocks without electrical hookups, if you don't like to run a generator a lot, or if you have equipment that is not converted to run on direct 12V current, you would make good use of a power inverter.
Power Capacity
Power inverters vary considerably in capacity, with different features and significantly different engineering accounting for a wide variation in price range. They are available with capacities ranging from 150 watts all the way up to 3,000 and more, with prices rising accordingly. Unfortunately, many look alike, even when they have very different ratings. Like any electronic product, you need to be aware of the wattage rating, input voltage requirements, features, and manufacturer's warranty.
| If all you need is a simple power inverter, you might buy an inexpensive 100-watt product that plugs into your standard 12V power outlet, like this one from Black & Decker.
Smaller power inverters can be simply plugged into a cigarette lighter or attached to the battery with small clips. This kind of inverter is certainly better than nothing, but for larger, more current-hungry appliances, the inverter will need to be installed in a permanent location and properly wired. Temporary installations for larger-wattage inverters-using jumper cables to connect to a deep-cycle battery-are simple and easy but vulnerable to wind, weather, and nearby foot traffic.
Ideally, a larger inverter should be permanently installed in a cool, dry place close to the battery or batteries it draws from. Inverters get hot when they operate, so many are fitted with cooling fans for safety, but even so they should not be placed in locations near sources of heat.
Underhood locations are generally not recommended because of heat and moisture, so the best location would be in the box containing the auxiliary battery or batteries, connected to a deep-cycle battery. Ideal wiring means using the thickest wire available in the shortest length possible. Inverters in the neighborhood of 3,000 watts are usually about the size of a shoebox or laptop computer and have multiple plug sockets available.
How Much Capacity Do I Need?
Inverters are rated in both continuous watts and surge power. Surge power is the output an inverter can supply for a minute or two in order to start up certain tools, pumps, and air conditioners.
| Pure sine wave inverters can deliver power as clean as your utility company, so there is less risk of interference patterns and humming noises. They cost more but are a better choice for running sensitive equipment, such as laptop computers, oxygen generators, and microprocessor-controlled laser printers. This is a 300-watt unit made by Samlex.
As for continuous wattage, it is always wise to get an inverter capable of delivering more wattage than you expect to need at one time, with a capacity 15-20 percent greater than your largest load. A light-duty inverter that plugs into the vehicle's cigarette lighter socket might deliver 180 watts of continuous power, good enough to power a laptop (50-75 watts) and a small inkjet printer (75 watts). However, a workstation with a PC, a monitor (400 watts), and a printer might consume 475 watts, requiring at least a 500-watt inverter. It might take as much as 1,100 watts to run a vacuum cleaner, 2,000 watts to run a microwave oven, or 1,500 watts to heat up a toaster. A 1hp air compressor might need 2,000 watts to run and require a startup surge of 5,000 watts.
Many appliances will have a label on the bottom showing their wattage draw, but if not, tables showing average current draws for various appliances are on the Internet, and most manufacturers list current draw specs on their websites. (If the draw is given in amps, multiply by 120 to get watts.)
As a rule, startup voltage is generally about twice the continuous load for any given appliance, but there are notable exceptions. Induction motors, such as air conditioners and refrigerators, may require steep startup surge voltages, sometimes three to seven times the continuous operating voltage, so the peak-surge capacity of an inverter is critical when powering that type of appliance.
Pure Sine Wave vs. Modified Sine Wave
It doesn't take much shopping to notice that prices vary widely. One reason is the quality of the electricity an inverter is capable of delivering. Inverters known as pure sine wave or true sine wave cost more but deliver electrical output with very low harmonic distortion, much like the electricity we get from the utility company at home. Pure sine wave inverters reduce the audible humming ordinary inverters (called modified sine wave) can cause in appliances like fans, fluorescent lights, answering machines, and audio amplifiers.
| A strong performer for larger loads that demand a clean, pure source of AC power, the Go Power! GP-SW1500 can run appliances and sensitive electronic equipment like TVs, stereos, and computers.
Modified sine wave inverters, also known as square wave inverters, are much cheaper but can create visual interference patterns in plasma and LCD TVs and game consoles. Some devices, such as laser printers, certain laptop computers, fluorescent lights with ballasts, or anything with microprocessor control (digital clocks, variable-speed tools) may not work with modified sine wave inverters.
However, modified sine wave inverters are about five times cheaper and work fine to run lights, coffee pots, microwaves, vacuums, and other devices that are not controlled by microprocessors. For example, we saw prices for 400-watt modified sine inverters as low as $29, while a 300-watt Samlex pure sine inverter was priced around $160. The difference may be steep, but the benefits of a pure sine wave inverter justify the higher price, where one is truly needed.
A workable strategy would be to use a larger-output modified sine wave inverter for most appliances and keep a smaller, true sine wave inverter on hand for sensitive appliances like laptops and medical equipment.
Other desirable features worth paying are cooling fans, low battery alarms, battery discharge protection features, and visible LED display ladders that track input voltage and output wattage.
A Word About Batteries
Power inverters are potentially hard on batteries. The length of time an inverter can supply power depends on the size of the battery or batteries and the number of appliances. A single, weak battery under heavy load can quickly discharge, perhaps in an hour or less, shortening the life of the battery and potentially stranding your vehicle. To avoid this, when the inverter is used with the engine off, it is best to start the engine periodically and let it run for 15-20 minutes to recharge the battery.
| The GP-SW3000 from Go Power! is the ultimate choice for large loads that demand an exact replica of household AC power. The GP-SW3000 can run any tool or appliance within its wattage range.
Deep-cycle batteries tolerate discharge better than ordinary car batteries, so they are a good choice for use with power inverters. A better solution is to connect one or more deep-cycle auxiliary batteries in parallel configuration and power the inverter from those.
Some power inverter manufacturers also make solar panels that can supplement the capacity of the vehicle's alternator and help recharge the battery over time.
Terminology
Learning a little bit of the lingo goes a long way. Here are some terms that may make your research more enjoyable.
Converter: A device that converts AC to DC power, sometimes called a power supply.
Continuous Power Rating: Power supplied continuously once loaded. The higher the wattage rating, the more appliances can be run.
Input Voltage: The voltage required to operate the inverter. Since most vehicles run on 12V battery power, most inverters are designed to operate on 12V. However, some vehicles, such as trucks and military vehicles, operate on 24V electrical systems, requiring an inverter specifically developed for 24V sources. Just make sure the inverter you choose is consistent with the electrical system on the vehicle you have.
Inverter: A device that converts DC (12V) to AC (120V).
Modified Sine Wave Inverter: Also known as a square sine wave inverter, an inverter that produces current with waves that look squared off when viewed on an oscilloscope. Should cost less.
No-Load Power Drain: The amount of power an inverter drains from the battery when it is connected but without an appliance drawing current. Less amperage is better.
Power Efficiency: The amount of 120V power delivered for every unit of 12V power taken from the battery, as expressed in a percentage. Higher percentage is better; a good inverter should pass through more than 90 percent overall, with minimal variation at all load levels.
Pure Sine Wave Inverter: Also called a true sine wave inverter, an inverter that reproduces the grid power supplied by local utility companies without spikes or jumps. Costs more.
Surge Power Rating: Usually about twice the continuous power rating, surge power is the output supplied to start an appliance. Higher wattage is better.
