Alternator size

I recently went through this exercise. It may be a case unfortunately of dropping down to a autoelectrician and first seeing whats the biggest alternator you can get fitted. I could only upgrade my dead 55amp alternator to a 70 amp unit. If I wanted anything bigger it was going to start costing me $$$$.

The old high school physics formula Power (Watts) = Volts * Amps should help here. i.e. Most spotlights run 100 Watt globes so... one spotlight globe is 100 W / 12 V = 8.33 amps. Do this for all your lights, etc and estimate anything your not sure about.

If you are still unsure if you've estimated correctly, think about buying a amps guage for a car. Hook it up to your battery so when you can see when you are drawing amps from your battery with the engine on. I would think (im not sure??)that you could estimate by turning everything on in the car and then checking the amps reading....

i.e. 50 amp alternator + 20 amps drain from battery = i need at least a 70 amp alternator.

Note that I think the alternator may produce less amps on idle than say with a few rpm - im not sure tho cause im no auto electrics expert! :)

Just my 2 cents. Hope this helps!
Cheers.
Dave.

Dave.
Your standard alternator will be fine, 50amps is plenty for your vehicle, the battery requires about 8 amps maximum to charge it. your lights are not on all the time but when they are they will draw 16amps, the normal head lights draw about 10 amps. Your radio and other bits will be under 5 amps average. The most common problem with older vehicles is slipping fan belts. Modern vehicles use a moltyvee belt to solve this problem, if you decide to put a large alternator on you must change all the pulleys or you will not get any extra amps just more belt slip. Eric.