What year is your ST1100? If it's a '96 or newer you'll have the 40A upgraded one from the factory. On pre-'96 models there's several ways to tell. Pop off the left side cover under the seat. Is there a large voltage regulator/rectifier bolted to the aluminum foot peg/muffler carrier plate? If so you probably still have the 28A unit installed, for the 40A there will be a fuse or fusible link installed there. To make sure, look up under the lower left rear of the engine/transmission. You can use a mirror or pop off the small gray panel just inboard of your left foot when placed on the peg. The 40A unit will have a vented case with ribs as it's air cooled, it looks exactly like a small automotive unit, while the older 28A unit will appear as a smooth, closed cover, it's oil cooled.