how much juice?

[Danimal1706]

Ive got a 73-78 55hp how much juice will the motor put out and at what rpm or basically how can i tell how many gadgets i can run on my boat and still maintain the charge in my batt. i dont want to have to buy a gauge if i dont have too. i think most electronic show draw on thier info plates or stickers ect.. most of the time maybe if i know what my engines putting out i can estimate conservativly what i can run all at one time and what not thank for your replies