Getting ready to move back to the land and say "Adios" to the local power company? It's an empowering feeling, but it's also a process that takes a ton of planning. As your mind swims in a veritable sea of options for the off-grid solar and wind system you'll be installing for your home, cabin or workshop, you will probably discover that choosing the right power inverter-the magical component that transforms low-voltage DC to high-voltage AC-is the hardest decision you'll have to make. Solar modules are pretty much all the same, and the batteries and charge controller you select will likely be determined as much by your budget as your preferences. But inverters are different; there are several ways to go-from rock-bottom basic to highly sophisticated-and ultimately you may discover that the one you need is not the one you want to pay for.
How do off-grid inverters differ? Primarily in the waveform they produce, and in the range of abilities beyond simple power transformation.
Sine wave or modified sine wave?
The alternating current (AC) produced by your local power company is a pulsing waveform, undulating between a positive peak and a negative trough. In normal house current, the peaks and troughs top (and bottom) out at 170 volts before gliding obliquely through the zero-voltage point. Mathematically, it works out to an effective 120 volts, pulsing at a standardized frequency of 60 cycles per second (Hertz). Except for a few DC appliances and lights, this is the type of electricity used by every common electrical device on the U.S. market.
A pure sine wave is the natural waveform produced when you spin fixed magnets around stationary coils of wire, and this is exactly how power companies do it. Power inverters, however, are solid-state devices. Nothing spins around anything. Instead, electrical engineers use a clever device called an H-bridge to approximate a pulsing current. An H-bridge uses a series of gates, or switches, to ferry direct current (from the battery bank) first one way, then the opposite way, through a coil of wire. The overall effect is a crude, square-ish waveform that rises and falls in discrete, choppy steps. But by using a series of H-bridges, the current can be made to approximate a waveform that is close enough to a true sine wave for most purposes.
The more steps you add to the process, the pricier the inverter becomes. That's why we're given a choice: you can either have a bargain-basement modified sine-wave inverter, or a much more expensive sine-wave model. What difference does it make? It depends on what sort of things you intend to operate with it. If you're only going to use it for lights, power tools, coffee makers, microwaves etc., then you'll probably do fine with a modified sine-wave inverter, though most loads will draw more power than with a sine-wave model.
The real trouble begins when you try to operate things such as dimmer switches, variable-speed drills, sewing machines, battery chargers, or anything else where the current varies. All of these devices use solid-state switches called Silicon Controlled Rectifiers (SCRs) to control how much current is allowed through the circuit. To do this, an SCR needs a point of reference to continually reset its "clock," and the handiest one to use is the point where the slope of the sine wave passes through the zero-voltage point.
A modified sine wave, however, does not pass through the zero-voltage point at a gentle angle. It instead drops abruptly from 150 to zero volts, and then lingers at zero for a moment before dropping abruptly again to the negative side of the waveform. With no distinct zero-voltage point, the SCR cannot effectively reset its clock. The SCR becomes confused and the tool or appliance will either not work at all, or will behave erratically.
In addition, you may discover that fluorescent lights and stereo equipment produce an annoying buzz with a modified sine-wave inverter, and certain computers and peripherals that utilize SCRs may not work properly (though, admittedly, every computer and printer I ever tried with a modified sine-wave inverter worked fine). Nor will electric clocks keep proper time, but that's really a blessing in disguise, since plug-in clocks are so wasteful you'll be doing yourself a favor by switching to battery-powered clocks.
Battery charging, and other goodies
Once you determine which waveform is adequate or preferable, you'll need to decide what else you want the inverter to do. The main dividing line here runs between inverters that can charge the batteries with a gas-powered generator, and those that can't. I should point out here that, generally (though not always), inverters capable of charging batteries are made for home use, while the less expensive, non-charging inverters are designed more for boats, trucks and recreational vehicles. The former have hard-wired AC outputs, while the latter simply have female sockets. Examples of these types of inverters are those made by Aims and Invertech, though there are many others. The point is, if your system requires an electrical inspection, I'd advise you to have a friendly chat with the electrical inspector before you buy, just to make sure the inverter you have your eye on is up to snuff.
The multi-stage battery chargers built into most high-end inverters are powerful and sophisticated, and can add a lot to the price (and, yes, the weight) of the unit. When charging, the inverter will first use the generator's input to run whatever loads may be drawing power at the time, then use the remaining wattage to charge the batteries. For this reason, after a run of cloudy weather it's common for those of us living off-grid to charge our batteries with a generator, while running a clothes washer or dishwasher, or some other large load.
Will you need battery charging? There's really nothing chiseled in stone here, but usually folks who use inverters to power fulltime residences will want the battery-charging option, while those with solar and wind systems in weekend cabins or small workshops that only see occasional use can generally get by fine without it. The theory is that you probably won't waste enough energy in a day or two to deplete a battery bank that has a week or better to recharge from the normal solar and wind sources.
Other features that often come standard with battery-charging models, such as Xantrex's DR Series, include a search function that will allow the inverter to rest in a low-energy state unless a load is running. Most sine-wave models, such as the Xantrex SW Series and most OutBack inverters, include multiple AC inputs for grid and generator power, and computer interface options which will allow you to program and monitor the inverter from your computer.
How much inverter do I need?
Inverters neither make power nor store it. They transform it. But to do this, they also use power, and the bigger they are the more they use-anywhere from two to 20 continuous watts, depending on the model. So buying more inverter than you need is like driving a super-charged V-8 to the corner store for a loaf of bread-besides looking like a showoff, you're wasting energy. On the other hand, you're sure to have a really bad day if you find out too late that your expensive inverter is inadequate for your needs.
Your inverter does not have to run everything in your house at once; it only has to run those things that are likely to be drawing power at the same time. If you are pumping water directly from a well, the inverter will have to handle the startup surge of the well pump (which may be two or three times the power needed to run it), as well as, say, the dishwasher, tv, and a few lights.
Fortunately, all off-grid inverters have the capacity to handle large, brief surges of power. In fact, most have surge ratings at least double the continuous-output rating. So, with rare exception, your inverter will be able to start any load it can comfortably run.
We run our house-and a voracious 1.5 hp well pump-with a Xantrex SW 4024 sine-wave inverter (offering 4,000 watts of continuous output) and a 120/240-volt transformer. Our guest cabin, on the other hand, does nicely with a Xantrex DR 1524 modified sine-wave inverter, capable of 1,500 continuous watts, while the tools in my small workshop work handily with an inexpensive Aims 2,500-watt modified sine-wave unit.
Three different inverters, for three different jobs. It just a matter of knowing what each one will, and won't, do.
Rex Ewing is the author of several renewable energy books, including Power with Nature, Got Sun? Go Solar, and Hydrogen: Hot Stuff-Cool Science. He lives with his wife, LaVonne, in a handcrafted log home powered solely by the sun and wind in the foothills of Colorado. His books can be purchased from the Countryside Bookstore.
