Kirrbby is right. It's available current output from the alternator, minus total current draw (entire system) equals amount of current left to charge the battery. IF the remainder is a positive number, that's how many watts are available to charge the battery. (As long as that's at least 5-8W, you should be okay, although values below 10-15W are going to mean very slow charging capability, with 12v.) If the number is a negative value, the battery will discharge...and the rate of discharge can be calculated. The basic formula: volts x amps = watts...a manipulation of Ohm's law...analagous to horsepower = torque x rpm/5252. With some basic algebra, any missing value can be calculated.
LEDs don't like AC, so battery feed is ideal...as long as you can supply enough DC charging voltage to keep the battery from discharging. How's that 75W alternator outputting that power? If it's the full 75w, you could power an H4...or later 65W automotive version. If it's 100% available as DC, then your biggest concern would be a regulator that controls output well enough to prevent the battery from being cooked. OTOH, if it's a typical half-wave system...with a split/"balanced" output...then AT MOST, 50% of alternator output is fed to a diode, leaving less than half of that as DC voltage. That'd be somewhere around 15-18W, max. It could be a 5-pole radial array stator...in which case the coil arrays would be split 3/2, with the smaller, 2-coil array feeding the charging circuit, unless you swap the feeds (white for yellow). The thing to remember is that a diode, with a half-wave system, not only transforms half of the AC waveform into waste heat...it's not very efficient, so you'll get less than 50% of the AC input as DC output.
FYI, I ran everything off of battery power (5.0ah/12v) for 15 years...knowing that the system ran at a partial loss. With a 35W incandescent HL bulb, 1157 array LED TL rated at 1W/5W, that left the system with -18W, while the HL was powered. That's 1.5a @ 12v... 1.5 x 3 = 4.5, thus I estimated 2-3 hours of usable HL power, before the 12v/5.0ah battery was completely discharged. I've since gone to an 18W/24W LED headlight...and a full-wave DC system, with an estimated 70W (a conservative estimate) available output. I probably could have gotten by (just) with the old system, leaving one 3-coil array unused and it would have been a big improvement. An 18W LED can be impressively bright. However, there are sound reasons why running a partial-loss electrical system can be a bad idea. I've a horrible tendency to leave the HL "ON" and that's very easily overlooked on a sunny afternoon. Having enough electrical output to power everything and charge a drained battery can be more than just convenient...especially if you're riding back home around sunset.