The alternators marginal capacity was discussed on another thread some time ago (cant recall which one - but I did post on it- and I think its about 290Watts total from memory)
Issue usually raises its head if running big watt halogen spots/headlight (none HID) heated grips, and other heavy current accessories (like heated vest) etc.
GPS/phones are very low current draw devices so dont normally trouble the battery /alternator (assuming thier in good health) .