Pete, you've got the math whrong there. A 12V/5A device will need 0.25 Amps at 240V !. However when converted back, it will be the same 12/5. Of course there will be some loss, but these gadgets run at 80-90% efficiency, so you will need no more than 6A input for the inverter.
The main reason you may need one is that some battery driven electronics (laptops, satelite phones, etc) have their own sophisticated charger that only work with 110/240V. Even if available, a 12V charger will cost a fortune (they wanted $360 for a 12V charger for my Mini-M phone). I got a 300W inverter for $42.
For some it may be a good idea to buy a 110V inverter in the 'States (eBay full of them). Most laptop/phone chargers these days are dual rated, and a 110V inverter has a lower conversion ratio (more efficient, less heat) and substantially cheaper. You can get a 300W 110 for the price of a 150W 220.
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Happy Travels,
Andras
http://fjexpeditions.virtualave.net
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