Modified sine wave is a square wave that approximates a sine wave by jumping between positive, zero, and negative in steps. Pure sine wave replicates the smooth curve that the grid delivers. The difference sounds technical, but it shows up in real appliances in ways your customers will notice.
The first thing that breaks under modified sine is anything with a motor. Fridge compressors, fans, water pumps — they run hotter, vibrate more, and fail years earlier. We have had customers return units after two seasons because the freezer compressor burned out. The second is audio equipment. A modified sine wave introduces a buzz that any decent speaker will pick up. The third is anything with a sensitive power supply: laptops, phone chargers, medical devices. They usually work but run warmer, and their power factor correction circuits do not love the harmonics.
What modified sine is fine for: incandescent bulbs, basic resistive loads, simple phone chargers, and any device with a brushed motor that does not run for long. A rural solar home with one light and a radio will not notice the difference.
The price gap has narrowed. A 1 kW pure sine inverter costs maybe 15 to 20 percent more than a modified sine one. For any system above a basic kit, the upgrade pays for itself in reduced warranty claims. We stopped shipping modified sine for any commercial application in 2024 and have not looked back.