Join Date: Apr 2010
Location: Wayne, PA
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Re: I have had it with Outboards, Time to go Electric
Figuring how much solar you'll need or can generate (between room for solar panels and batteries to store it) has a lot of variables, the biggest being clouds. On the other hand many of us here are using solar so there is a lot of advice available.
Solar panels refer to watts and most of the larger panels output 17-18 volts from the panel before that power gets to the charge controller, so the amperage calculation is watts divided by volts = amps. 100 watts divided by 17 volts is an output of 5.8 amps when conditions are just right. Most calculations seem to figure an average of about 5-6 hours of good solar exposure per day. Clouds, shadows, shifting axis, all effect that output. Wire size, distances, battery condition effect what gets to the batteries and charging.
The bottom line is 100 watts of solar panels will put about 32 amps into your battery per day, 200 watts about 64 amps, etc....
From Boat US - " To use an example, a 55 lb thrust motor unit has a peak power draw of 40 amps. If you use the trolling motor for 6 hours at average 50% load of 20 amps, the trolling motor will then require a battery load of 120 Ah. (6 hours X 20 amps=120 amp hours)."
Second issue is the size of the battery or batteries -
from Calculating Motor Run Time Ė TrollingMotors.net
The larger the rating, the more power the battery can hold, and the longer it will be able to power a motor at a given speed. For example, a 100 amperage hour battery is one that would be able to supply 25 amps of power for 4 hours (25 amps * 4 hours = 100 amp hours) before running out of energy.
Running a battery too low shortens its life, to consistently run 100 amps from the battery bank you'd probably want 400-600 amp capacity, like 2 Trojan T 105s in series (great batteries but 64 pounds each). Unfortunately if you use the motor during the daylight hours it won't charge at night, so the next morning you have depleted or dead batteries.
All this is why I really appreciate my little Suzuki 2hp 2 stroke (and use the solar system to keep the refrigerator cold).