Lower-cost inverters produce a simpler square wave instead of the more complex sine wave. Some sine wave inverters have very visible, coarse steps making up the sine wave, while others have very smooth waves, by using hundreds of steps and more sophisticated AC output filtering.
The AC output wave form is also affected by the load being operated and the DC input voltage level. Many lower-cost inverters cannot regulate the AC output voltage when running more difficult loads such as motors, which can result in voltage spikes that may damage some appliances. More expensive sine wave inverters can operate nearly any load without problems and often have better power quality than a utility grid, since the utility distribution system can be affected by other AC loads operating in your neighborhood, resulting in power-quality issues.
Square Wave Inverters. The easiest AC output wave form to make is a “square wave,” in which the voltage alternates from positive 120 volts to negative 120 V, back and forth. This wave form has a lot of total harmonic distortion (THD) and results in poor operation of nearly all AC loads.
A square wave inverter cannot regulate its AC output voltage when the battery voltage changes significantly. They produce 120 VAC when the battery is at 12 VDC, but also produce 140 VAC when the battery is at 14 VDC and 100 VDC when the battery voltage is pulled down to 10 VDC, like during a motor startup. This can cause even simple AC loads like motors or lightbulbs to fail prematurely.
Because of these severe drawbacks, no square wave inverters are being manufactured today. They still do sometimes turn up used, but they are not worth considering, even if they are free.
Modified Square Wave Inverters. The addition of a small “off” time between the positive and negative pulse of the square wave significantly reduces the THD. And the shape of the wave form also can be controlled to allow regulation of the AC output voltage level as the battery’s voltage changes. Modified-square pulses are tall and narrow when the battery voltage is high, but become short and wide when the battery voltage is low. This results in a consistent average voltage being supplied to the AC loads, and improves load compatibility and performance. However, more sensitive loads, such as variable speed motors on some hand tools and appliances, may still operate incorrectly, overheat, and be damaged from this type of wave form.
All of the inexpensive inverters and even some of the more expensive off-grid and mobile inverters produce this type of AC wave form. This wave form cannot be used for grid-tied inverters as the THD does not meet the utility requirements.
Modified Sine Wave Inverters. Although this term is commonly used, it is really a misnomer—there is no difference between a modified sine wave inverter and a modified square wave inverter, other than some sleight-of-hand marketing.
A stepped sine wave inverter produces another “in between” AC wave form. Instead of having a single positive or negative pulse punctuated by an “off” period between, a stepped sine wave inverter is able to produce a series of different voltage levels which can be arranged to produce what is often described as a “Mayan temple” shape. The number of steps varies as the battery voltage changes. At higher battery voltages, there are fewer, but taller steps; at lower battery voltages, there are many shorter steps.
This approach produces a wave form with a much lower THD than a modified square wave inverter and offers good performance and DC-to-AC conversion efficiencies of more than 90%. Some of these inverters are even able to be grid-tied since the THD is low enough to meet UL and utility requirements.
True sine wave inverters produce a wave form that closely matches what is provided by a utility grid. Some of them are able to provide AC power that is better regulated and even has lower THD than utility power.
To make this wave form, a true sine wave inverter produces hundreds of positive and negative pulses during each AC cycle. These pulses are then filtered into a smooth sine wave shape. Most true sine wave inverters are able to adjust the duration and timing of each pulse by using very fast digital electronic circuits and/or microprocessor control. This allows the voltage and frequency to be well controlled, ensuring that any AC load within the inverter’s power limits will operate properly.
Because the output is a well controlled, very sinusoidal shape, using these inverters for grid-tied applications is possible by the addition of the required safety systems and the additional testing and certification to meet the requirements of UL1741 and utilities.