You should be able to calculate what the total power draw could be at its most extreme - e.g. headlamp circuit total, wipers maybe, heater, 200W stereo etc.
Watts=Volts x Amps, and you know it's going to be around 12V, so do the sums and you'll get a ballpark figure for current. The alternator will only put out what's asked of it - but if you get a short or fault, and it's a 45A output, it can give that much more than a 25A generator.
Hidden fuse/circuit breaker/fusible link, maybe?