Well I already have the been there.
I have a nice 4x4 van and I wanted to put a TV and VCR and possibility of more AC powered stuff like my laptop and my dig cam (to charge them I mean) so I di9d some research.
A 14" TV 25-75w depending on brands and quality.
VCR 50-80W
Laptop 10w
cam 5W
I found a couple of options 300w 19.95 all the way to 800w $95.99 and then I found one on sale: 1400W $64.95 so I got this one.
But for your application anything from 300 and up will work.
ANd remember, they have nominal power and peak power, you want the lower number which is the nominal and stay within 80% of that power so if 240w is your max, a 300 watt should be fine.
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