15W is basically 5V * 3A, there's plenty of PSUs that offer that and more, even some of the phone chargers. My 30K mAh powerbank offers a 6A output, so there sh

15W is basically 5V * 3A, there's plenty of PSUs that offer that and more, even some of the phone chargers. My 30K mAh powerbank offers a 6A output, so there shouldn't be any problems with running RPi on batts.