The primary job of an inverter is to convert the DC (Direct Current) power from the battery bank or solar panels to AC (Alternating Current) power needed for most appliances. To do that, it must take the constant DC voltage and change it to a sine wave curve that goes above and below 0 volts. When inverters first came out, the most common way to do this was to make the voltage go straight up and down, creating a blocky signal. This is called modified sine wave, seen in orange in the image above. More advanced modified sine waves make multiple steps in an attempt to approximate a pure sine wave.

A modified sine wave inverter can be used for simple systems that don’t have any delicate electronics or audio equipment that may pick up the choppy wave and produce a hum. Old tube TVs and motors with brushes are usually ok with modified sine wave. Your digital clock will likely act funky, and battery rechargers quite often just plain won’t work. Some equipment may seem to be working fine, but may run hotter than with a pure sine wave and reduce the life of it.

In general, because the total harmonic distortion is higher in modified square wave inverters, motors will run hotter (less efficiently, consuming up to 30% more energy than with pure sine wave inverters), and likely not last as long. Additionally, a modified square wave inverter will often cause a “buzz” to be heard from audio devices and sometimes other appliances like ceiling fans and microwave ovens.

We liken using a modified sine wave inverter to running a car with square wheels versus a pure sine wave inverter like running a car with round wheels. In the first case the ride is going to be awful rough and depending on the sensitivity of the car’s occupants they may not survive the ride.

Generally, modified sine wave inverters are less expensive than a pure sine wave inverter, so they are still used in simple systems. But as technology advances, the cost of pure sine wave inverters is coming down, making them much more affordable and the favorite option.