Modified sine wave inverters are fairly cheap, but they can make chargers overheat, or florescent lights buzz or even interference lines on your 240v TV.
OK for microwaves, fridges etc.
Pure sine wave inverters are normally twice the price, and are better quality. with less possible damage to electronic bits and pieces.
I used a 24v 1000w modified sine wave
inverter on 2 security lights on my camper/bus (150w 240v) (I do security work sometimes and use my bus as my base) and after about an hour both IR sensors overheated and buggered up.
Some inverters also include chargers, so when plugged in to the mains, charges the vehicle
battery's and supply's 240v at the same time, but these cost even more!
I now have a 24v 1400v
Pure sine wave
inverter with internal charger, off ebay for £130,
Lorry