The fact that the inverter is not sinewave is not the real problem here. The problem is the type of transistor bridge doing the switching.
There are two types of AC load, resistive and reactive. For a resistive load, eg filament light bulbs and heaters the load seen by the AC source is the same as the load seen by a DC source.
If a load is reactive however, eg. contains a choke like in a fluorescent lamp, or a capacitor, such as a plug in time-switch or electric shaver, this gives rise to a circulating current which travels between the load and the source, but is not a real quantity in that it does not contribute to the useful output of the device.
This current needs a return path in order to circulate. If the inverter powering the load uses a "full bridge" of transistors, aka an "H-bridge" then there is a natural return path for the reactive current. If however, the inverter uses a half-bridge, which is more common on the "modified sinewave" inverters (which are really more like a square wave than a sinewave) then there is no return path for the reactive current and it has to be dissipated in the transistors themselves, causing them to get hot and much more current is drawn from the battery than should be for the given load. This can burn out the inverter and/or the load.
Unfortunately, manufacturers rarely advertise whether their inverter will handle reactive loads or not. Many even state that they can run microwave ovens and refrigerators etc. A microwave is about as reactive (inductive) a load as you will find at home, because it has a big transformer in it. This means that if the inverter will run it at all, it will do so very inefficiently, drawing many times more current from your battery than is necessary!
to work out roughly whether this is the case or not, run a traditional fluorescent lamp fitting from your inverter (eg. one with a choke ballast) and measure the current drawn from your battery. If the battery current is around (lamp power/battery voltage)+10% then the inverter can handle a reactive load. If it is significantly higher, then you have a problem.
The modified sinewave issue also contributes to inefficiency in the load. The inverter itself is about 90% efficient, but if running a motor or transformer load, then the square edges of the "modified sine" waveform contribute to iron losses in the load. These I am sure you are familiar with as a raspy buzzing noise, lines on the TV picture, etc.
I am using an old APC smart-UPS as my inverter (5kVA, 48VDC input) This is both sinewave and full-bridge and it runs any load in my house up to about 4kW as if it was running from the mains line. In fact the sinewave from the UPS is less distorted than the mains itself!
I hope this has cleared up a few issues with inverters damaging equipment and I hope I have not told you too much of what you already know. If I have, I apologise!
best Wishes.
