12volt to 240 inverter

[oneandonly]

we have a 240v TV in our caravan, also a satelite dish and 240v set top box. We want to be able to run them on 12 volt battery power,(for a short time at least) what size inverter do we need to run them (at the same time).
I am not into electrics at all so I hope someone can help. I have seen 3 different sizes at the shop,what size is best please.

[minnie]

At least 1000 watts.