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If you are installing a grid-connected wind system, the decision will be made for you by the manufacturer as noted earlier. If you are installing a battery-based system, you’ll need a battery-charging wind turbine and an inverter that’s compatible with batteries.
Most installers carry inverters they have a high degree of confidence in. Consequently, they will make a recommendation that fits your needs from their product line. Unfortunately, there are not many battery-based inverters available in North America.
System Voltage
When shopping for a battery-based inverter, you’ll need to select one with an input voltage that corresponds to the battery voltage of your system. System voltage is the voltage of the electricity produced by the wind turbine. That is, the generators in these turbines are typically wired to produce 12-, 24- or 48-volt electricity. The batteries are wired similarly.
Because all components of an off-grid renewable energy system must operate at the same voltage, the inverter must match the source (wind turbine) and the batteries. If you are installing a 48-volt Bergey XL-R, you’ll need a 48-volt battery-based inverter, and you must wire your battery bank for 48 volts. It is a good idea to talk with the wind turbine manufacturer to obtain their input on the best inverter.
Modified Square Wave vs. Sine Wave
The next inverter selection criterion is the output waveform. Battery-based inverters are available in modified square wave (often called modified sine wave) and sine wave. Grid-connected inverters are all sine wave so their output matches utility power. What does all this mean?
Waveform refers to the voltage of AC electricity as it changes over time (alternates). Modified square wave electricity is a crude approximation of the grid power voltage pattern. It works fairly well in many appliances and electrical devices in our homes. Although most all office and household electronic equipment and appliances can function on modified square wave electricity, they run less efficiently, producing less of what you want — i.e., light, water pumped, etc. — and more waste heat for a given energy input. When operated on modified square wave electricity, microwave
ovens cook slower. Equipment and appliances that run warmer might last fewer years. Computers and other digital devices operate with more errors and crashes. Digital clocks don’t maintain their settings as well. Modified square wave electricity may cause an annoying high-pitched buzz or a hum on TVs and stereos and may also produce annoying lines on TV sets. It can even damage sensitive electronic equipment. Some equipment, like modern washing
machines, may not operate at all on modified sine wave electricity. The computer that controls these units won’t run on it. Unless money is tight, get a sine wave battery-based inverter for an off-grid system.
Output Power, Surge Capacity and Efficiency
When selecting an inverter, even a grid-tied inverter, be sure to pay attention to continuous output, surge capacity and efficiency.
Continuous Output
Continuous output is the power an inverter can produce on a continuous basis. It is measured in watts, although some inverter spec sheets also list continuous output in amps (to convert, use the formula watts = amps x volts). For example, OutBack’s sine wave inverter VFX3524 produces 3,500 watts of continuous power and is designed for use in 24-volt systems. The 35 in the model number stands for 3,500 watts. The 24 indicates it is designed for a 24-volt system.
To determine the continuous output you’ll need, add up the wattages of the common appliances you think will be operating at once. Be reasonable, though. Typically, only two or three large loads operate simultaneously.
Surge Capacity
Electrical devices with motors, such as vacuum cleaners, washing machines and power tools, require a surge of power to start up. It typically lasts only a fraction of a second. Even though the power surge is brief, if an inverter can’t provide the power, the motor won’t start. Moreover, the stalled motor will draw excessive current and could overheat,
unless it is protected by a thermal cutout. If not, it may burn out.
When shopping for an inverter, be sure to check out the surge capacity. All quality inverters are designed to permit a large surge of power over a short period, usually about five seconds. Surge power is listed on spec sheets in watts and/or amps.
Efficiency
Converting one form of energy to another results in a loss of energy. Efficiency is calculated by dividing the energy coming out by the energy going in. Fortunately, efficiency losses in inverters are quite low — usually only 5 to, at the most, 15 percent. It should be noted, however, that inverter efficiency varies with load. Generally, an inverter doesn’t achieve its highest efficiency until output reaches 20 to 30 percent of its rated capacity. A 3,000-watt inverter, for instance, will be most efficient at outputs above 600 to 900 watts. At lower outputs, efficiency is dramatically reduced.
Noise and Other Considerations
Battery-based inverters are typically installed inside, close to the batteries to reduce line loss. Grid-tied inverters are almost always installed near the service entrance — where the utility service enters the house, which is near the breaker box. (Most inverter manufacturers like their equipment to be housed at room temperature.)
If you are planning on installing an inverter inside your home or office, be sure to check out the sound it produces. Inquire about this upfront. Ask to listen to the model you are considering in operation.
Some folks are concerned about the potential health effects of extremely low frequency electromagnetic waves emitted by inverters, electronic equipment and electrical wires. If you are concerned about this, install your inverter away from people. Avoid locations in which people will be spending a lot of time — for example, don’t install the inverter on the other side of a wall from your bedroom or office.
Be sure to add ease of programming to the checklist of features to consider when purchasing an inverter. Find out in advance how easy it is to change settings. Spend some time with the manual.
Stackability
Finally, when buying a battery-based inverter, you may want to select one that can be “stacked” — connected to a second inverter of the same kind. Stacking permits homeowners to produce more electricity if demands increase over time. Two inverters can be wired in parallel, for example, to double the output (amps) of a batterybased wind system.
Stacking may also be needed to supply 240-volt AC electricity to operate appliances such as electric clothes dryers, electric stoves or central air conditioning. We recommend that you avoid such appliances, especially when installing an off-grid system. That’s not because a wind or hybrid wind and solar system can’t meet those needs, but rather because these appliances use lots of electricity and you’ll need a very large and costly system to power them. Welldesigned, energy-efficient homes can usually avoid using 240 VAC. An exception is a deep well pump, which may require 240-volt electricity. In most cases, high efficiency 120-volt AC pumps, or even DC pumps, perform admirably.
If you must have 240-volt AC electricity, purchase an inverter that can be wired in series to produce 240 VAC. Or you can purchase an inverter that produces 120- and 240-volt electricity. Or you can install a step-up transformer that converts 120-volt AC electricity from your inverter to 240-volt AC. Or you can simply install a dedicated 240-volt output inverter for that load.
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