I was looking for the inverters for my laptop and came across these people. I could not beleive how cheap they were and I can vouch as to how good they are. If any of you are looking give them a try www.tlc-direct.co.uk
I thought I did well paying £30 pounds for the 150 watt 24v converter.
Sane modell as this one . And I’m happy with it. It runs from the cig lighter my 14 inch telly and dvd player without problem.
Sine wave ■■? perhaps that causes the slight horizontal lineas moving up the screen but you dont notice them after a few minutes 
Well, cheaper inverters will make a square wave, and 250v AC is a pure(ish) sine wave, smoother. Some appliances will cope ok with the rougher current produced by a square wave device, and others will show up little problems.
For a laptop, I’d say, if you have a nice one, invest in a higher quality inverter, because you’ll save the vast cost of a replacement PSU.
So what do you look for when buying an inverter - say I want to run a laptop off it.