Correction: the battery was about 60% before, so my wife noticed. I've since tested the drain and it's more than the camera app, but then scannable must be doing a lot of processing to detect the document. It doesn't seem to drain when emailing. I think I've just misjudged what the battery was to begin with and how's he was used for scanning. Apologies. *embarrased*
Dear Evernote, The scannable app is great and I recommend it to our clients for emailing papers in to us. But it can seriously cane the battery. I used it today and emailed the scan to a friend. I took about 10 minutes composing the email and got a 20% battery warning, but to start with I had 99%. iPhone 5 on iOS 8. I thought you'd better know.