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→Total Daily Load
Use whichever number is the highest for all future calculations. Let’s call this the <i>Total Daily Load</i>.
== Capacity of House Bank Required ==
When you have determined the <i>Total Daily Load</i> in AH, multiply it by the desired <i>Charging Interval</i> in days to determine the <i>Battery Drain Between Charges</i>.
<pre>Battery Drain Between Charges = Total Daily Load * Charging Interval</pre>
Once a day seems like a common-sense choice. With less than a day, there will be a tendency for charging cycles to run into each other, along with all the extra fuss for your neighbours at the anchorage. With more than a day, you will need an ever bigger and more expensive house bank and alternator to carry over. With once a day, you exercise the system every day, keep the engine from rusting out, produce minimal fuss, and keep battery and alternator costs in a reasonable range.
There are several approaches to determine the <i>House Bank Required</i>. A common one is to size the bank so that it cycles between 50% and 80% charged. Using this approach, you would simply multiply the <i>Battery Drain Between Charges</i> by 333% and throw in a 15% fudge factor for good measure, i.e., multiply the <i>Battery Drain Between Charges</i> by 350% to determine the size of the house bank.
However, batteries are constrained by their discharge/charge rate. For example, flooded-cell batteries cannot discharge at a rate more than 25% of their capacity. A better way is to base the size on the discharge/charge rate of the selected [[BatteryType|battery type]]. For a flooded cell, you would apply a factor of 400% to determine the total capacity required.
For gel and AGM cells, you could go as low as 300%; although in all cases more battery is better than less. The resultant is the <i>House Bank Required</i>.
Divide this number by the AH rating of your chosen battery type, to determine the number of batteries in the house bank. Typically, for a boat under 65 ft, the house bank will have four to ten 8D deep-discharge batteries with a capacity of 1,100-2,800 AH.
