Pure sine vs modified sine: what actually breaks
A pure sine wave inverter puts out the same smooth AC waveform as the grid, so anything that runs at home will run on it. A modified sine wave inverter puts out a square stepped simple approximation of it: simple resistive loads tolerate it, motors run hot on it, audio hums on it, some chargers overheat on it and some electronics die early on it. That's why pure sine is the default, and modified sine only makes sense when you know exactly what will plug into it.
What's actually happening: mains appliances are designed for a 230V pure sine wave. A modified sine inverter delivers a square block of voltage that averages out right but changes abruptly, and those abrupt edges are the biggest source of the problems. Induction motors, fridge compressors, most pumps and power tools, draw extra current on the square stepped waveform and run measurably hotter, which shortens their life rather than killing them on day one, so the damage gets blamed on the appliance. Anything with a thyristor or triac speed or temperature control, a variable-speed drill, some heaters, dimmers, can misbehave immediately. Audio and sensitive electronics pick the steps up as hum. And some switch-mode chargers and medical devices, CPAP machines are the classic, need pure sine input, full stop.
What modified sine still does fine: purely resistive loads. Basic incandescent lighting, a simple jug, a toaster, a soldering iron, and it does them at a lower price. That makes a modified sine unit a reasonable buy for a work trailer running lights and a jug, and not much else. Modified sine was a temporary trend 20 years ago when electronics was expensive to make, and the price gap has closed to the point where it rarely pays for a mixed household of appliances: one cooked fridge compressor costs more than the difference.
Our advice at the counter is simple. If you can't list exactly what will ever plug into the inverter, buy pure sine. If you can list it and it's all resistive, modified sine will save you money. And if the list includes a fridge, a CPAP, anything medical, anything audio or anything with a motor you care about, it has to be pure sine. In practice we steer everyone to pure sine, and everything we sell for a fixed install is pure sine: the NOMAD NPS range throughout, and the inverter-chargers. The only modified sine units we carry are the two small CARSPA car inverters, 120W and 400W, plug-in units for light loads off a cigarette socket, and they're sold as exactly that.
Not sure what size? The sizing guide you need is next, and the appliance guide has the numbers for coffee machines, microwaves and induction. More in our Inverters FAQ.
Common questions
Will a modified sine inverter damage my fridge?
It makes the compressor motor run hotter and draw more current, which shortens its life rather than killing it outright. The damage shows up as a fridge that dies years early, and gets blamed on the fridge. Compressor fridges belong on pure sine wave.
Can I run a CPAP machine on modified sine?
No. CPAP and other medical devices only run on pure sine wave power, and most manufacturers say so on the machine. This is one of the few hard rules in inverter selection; there's no saving worth the risk. Confirm it's a 230V unit, and remember the humidifier will roughly double the draw.
Is pure sine worth it for occasional use?
Usually yes. The price gap has closed, and an inverter tends to outlive its first job and get asked to run new things. Pure sine gives you the option of running every future appliance; modified sine locks you to a list.