The pure sine wave, as we can see below in the diagram, has a curved peak transitioning smoothly into a curved valley.
The modified sine wave above has more of a stepped platform structure.
As we can see there is a “rest” on the central zero line for a few milliseconds. Why is this relevant? Well, some processors in sensitive equipment count the zero crosses & use this to measure time.The prolonged rest of a modified sine wave could be interpreted by the processor as multiple zero crosses & therefore upset the delicate calibration of some electronic devices. Motors that have a speed control, such as washing machines, and light dimmers contain sensitive components that can be upset by the use of modified sine wave inverters. If you are installing a washing machine, it is good practice to go for the pure sine wave unit.
That said, a modified sine wave inverter is quite capable of running a wide variety of loads produced by household equipment and tend to be less expensive than pure sine wave units. In some cases, when running low power equipment such as computers or televisions for long periods of time, it is even preferable to run equipment from a modified sine wave inverter. They tend to require lower power to run themselves and in these cases are more power efficient.
As always when specifying equipment, please take the advice of your local specialist marine electrician, BSS examiner