Will be travelling soon with my Iphone 5S, and was wondering which portable phone chargers I should be considering for purchase? I know it should be a "certified by Apple" brand - but which ones seem to be the most reliable. I'd also like one that would hold a charge for a couple of months if not used - in case of a power outage while at home. It's confusing to see the different power ratings (is that the correct terminology?) - so I sure would appreciate some expert advice before making a purchase! Thanks in advance.