Pure sine wave vs modified sine wave inverters
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Inverter adverts often mention “pure sine wave” or “modified sine wave”. The difference affects which appliances run properly, and for how long.
What the waveform is
Mains electricity alternates direction many times a second (50 or 60 times, depending on the country), and its voltage rises and falls in a smooth curve: a sine wave.
- A pure sine wave inverter reproduces that smooth curve closely, like grid power.
- A modified sine wave inverter (sometimes called quasi-sine) approximates it with abrupt steps. It is simpler and cheaper to build.
What runs fine on modified sine wave
- Simple resistive loads: kettles, irons, incandescent bulbs, basic heaters
- Many phone and laptop chargers, though some run warmer or buzz
What can struggle on modified sine wave
- Motors and compressors: fridges, freezers, fans, pumps and air conditioners can run hotter, noisier and less efficiently, which can shorten their life
- Some electronics: certain TVs, audio equipment, printers and devices with sensitive power supplies may buzz, show interference or fail to work
- Devices with timers or clocks that count the mains cycles, which may run fast or slow
- Some dimmers, LED drivers and battery chargers
- Medical equipment: check the manufacturer’s guidance before running a CPAP or oxygen concentrator on any inverter
Which should you buy?
For a home backup system, choose pure sine wave. Almost every home system runs a fridge, fans or a pump, and most hybrid and solar inverters sold for homes are pure sine wave anyway. The price difference has narrowed a lot.
A modified sine wave inverter can still make sense for a small, cheap setup that only runs lights and simple chargers, such as a camping or vehicle inverter.
Signs your inverter’s waveform is a problem
- appliances humming or buzzing on battery but not on grid power
- motors running hot
- a fridge that struggles to start on the inverter
- interference lines on a TV or noise from speakers
If you see these, check the inverter’s data sheet. If it is modified sine wave, avoid running motors and sensitive electronics on it.
Size a pure sine wave inverter for your appliances, including the starting surge from fridges and pumps.