12 v inverter

The Gonzis

I am considering purchasing a 12v inverter to be able to power up a small tv and dvd player so our son with ASD can use while on the road in the car, has anybody got experience with this gadget? the tv shows 20 w and the dvd player 10 w, does that mean that I would need at least 30 w output right? and what impact it has on the car's battery, considering that we would have the tom tom and a electric box both plugged in simultaneaously, I am not technically minded so i am not sure if this is possible. He usually uses portable dvd players but we would like to have a back up plan for when they go wrong as they did on a recent trip.

thank you