The biggest issue is you are all over the place with your units of measurement:
- Watts = Instantaneous power usage regardless of voltage
- Amps = Instantaneous power usage at a particular voltage (12V in this case)
- Amp-hr = Total power usage at a particular voltage (in theory you could use watt-hr but it's not commonly used when talking about 12V boat systems)
(feel free to look of the official definitions, I've simplified them a bit for ease of understanding).
Batteries: They have two key measurements.
- Cranking Amps: This is really only of concern for starting the motor
. It's almost unheard of for anything else to use even half the cranking amps the starter needs.
- Amp-Hr Rating: This is the total storage
capacity (think gal of gas) except you can't use all of it. If you empty the tank on lead acid batteries you damage them. Also, how fast you discharge them (amp draw) also impacts how many amp-hrs you get out of the batteries. A high amperage will result in less amp-hrs available.
: Divide the rated wattage by 12V and multiply by 5 or 6hr to get an estimate of amp-hrs produced in a clear day.
: The 40amps will do well until the batteries get near full and then the batteries simply won't take all the amps available.
Usage: Take the amps (or wattage/12) and multiply by how many hours the device will be in operation. Add up all the devices to get your total amp-hr usage.
Now you get to the fuzzy math:
- I would figure on using only 30-40% of the battery amp-hr rating.
- The Cranking Amps must meet the starter requirements not much room to play with there but small sailboat engines usually don't take a lot so your deep cycle bank may have plenty.
- Are you willing to run the motor
regularly? If not, you need to decide how many days of overcast before you are willing to run the motor and then multiply your battery bank amp-hr by that amount.
- Do you regularly stay at docks with shore power
? If so, you should be good.
I suggest starting with a spreadsheet to list out and calculate the amp-hr loads as your starting point.