The first step is to calculate how many boat amps you use daily. To do this, you must go through meticulously and calculate amp usage by hours used by anything aboard.
You’ll need this information to determine the size and/or the number of batteries for your house power. This depends a great deal on the amount of electricity the systems on your boat will likely consume over a given period of time.
In order to determine the ampere-hour draw of your vessel, you need to know what electrical equipment you have and, then calculating the current draw of your domestic circuit of the boat electrics over a typical 24 hour period.
How much electricity will your DC loads use? The current, or boat amps, multiplied by the time of operation, or hours, equates to amp-hours (Ah) per day. For example, if you have two running lights that each draw 1.5 amps, and those lights are operating for five hours a night, the Ah per day consumption would be 2 x 1.5 x 5 = 15 Ah/Day.
A small powerboat, usually rigged without a separate generator, generally consumes from 60 to 200 Ah per day. A large cruiser or sportfisherman could easily consume upwards of 400 Ah per day.
To calculate what your boat needs make a chart and list the consumption amounts of all systems, everything from the head, to the bilge pump, to the GPS unit. Now, if your boat has AC power, too, that must be factored in if an inverter is used aboard to turn AC current into DC current. More math is required to translate AC usage into DC usage. AC watts x hours/10 = Ah/Day. Now add all of the consumption figures up to come away with a gross energy consumption Ah/Day.
With this figured out, you can determine the battery capacity that meets your energy needs and begin the process of choosing a marine battery. Many experts say your battery capacity should be three to four times your daily energy consumption. So a boater using an estimated 160 Ah per day should have a house battery bank capacity of 560 Ah of capacity.
If you don’t know how to calculate amp usage for each individual thing that uses electricity, you’re going to have to be a bit of a sleuth – you need to find the watts and voltage for each light bulb, each appliance and so on. Most packaging will contain the information you need, often you can look it up online.
And with that knowledge you can readily calculate the battery charging regime necessary to prevent undue strain on the marine batteries and keep the whole electrical system ticking over.
In order to determine the proper amp hour rating capacity you need for your boat, simply add up the 12-volt accessories you have, multiply by 20; that should give you a very good approximation of your boat’s amp hour battery requirement.
Ratings can usually be found on equipment nameplates or in their manuals, and will be expressed in terms of power (measured in watts) or current draw (measured in amps). The relationship between power and current is expressed as:
Power (W) = Current (A) x System Voltage (V)
To derive amps from watts, simply transpose this equation and divide the wattage by the system voltage.
For example, a 6 watt navigation light bulb in a 12 volt system will draw 0.5 amps - which, if it's switched for ten hour each day when underway will have consumed 5 amp/hours (Ah).
It is usually advised to buy a battery at least 20% over this requirement, as 12-volt capacity varies with usage and as batteries age.
However, many experts say your battery capacity should be three to four times your daily energy consumption