You have a boat and you are considering installing some solar panels on it to generate 40Aday electricity to power a 12V battery.
But you kinda confuse how many solar panels you need to get.
Think electricity like a river, it flows down.
A solar panel for charging 12 volts battery, the solar panel itself needs to generate a voltage which is higher than 12 volts to ensure the electricity could flow down into the 12V battery. The voltage of the solar panel itself usually will be 18Volt.
Now you want to have 40Aday electricity on a very clear day.
On a clear day, the average efficient charging time for a solar panel is around 4hours.
So it will be 180W in a perfect scenario.
But things can't be perfect in reality.
So you will need a 225Wh solar panel to make sure you have 40A per day for your 12V battery.
And you need a controller for your solar panels and battery, the controller will consume a bit of power, which could be ignored because it's so little.
And you need to make sure your battery could save some power for cloudy days or rainy days.
In conclusion, you need to get a solar panel bigger than 225Wh.
If you wire two 100Wh solar panels in parralel, you will get
Each 100W solar panel produce about 5.5A per peak sun hour.
Assume you have 4 peak sun hours
Each 100W solar panel produce about 22 amp-hours per day.
And your two 100W solar panel produce about 44 amp per day.
In reality, you two 100W solar panel probably produce about 35.2 amp per day.
Correct me if I'm wrong, leave your comments, please. Your advice only makes us better!
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