Charging System Requirements
Your charging system should be capable of delivering a charging current equal to 25% of the amp-hour capacity of your battery. For example, if you have a 200 Ah battery, the charging system should be able to deliver 50 amps. The charging system must also be able to charge each 12 volt battery up to approximately 14.4 volts and then drop back to a “float” voltage of 13.5–14 volts (or shut off). Source: Xantrex
Battery Capacity
Amp-hour capacity is a measure of how many amps a battery can deliver for a specified length of time—usually 20 hours. For example, a typical marine or RV battery rated for 100 Ah can deliver 5 amps for 20 hours (5A x 20 hours = 100 Ah).
This same battery can deliver a higher or lower current for less or more time, limited approximately by the 100 Ah figure (for example, 50 A for 2 hours, or 200 A for 1/2 hour), but usually the capacity figure given is only accurate at the specified rate (20 hours). Source: Xantrex
Formula:
Amps * Hours = Amp Hours
Example:
5 amps * Hours = 300 Ah (my battery bank capacity)
Hours = 300 Ah / 5
Hours = 60 (my battery bank can last for 60 hours with a 5 amp draw)
How about idle inverter draw? Lets say I have an inverter that draws .3 amps (300 milliamps) at idle. How much does it drain my battery?
.3 amps * Hours = 300 Ah
Hours = 300 Ah / .3
Hours = 1000
It will take 1000 hours to drain my batteries. How much does it drain in one day? 24/1000 = 2.4%. So over a 3 day weekend it would draw 7.2% of by battery's banks power..if it was left on the whole time.
Battery State of Charge
You can measure battery state of charge with a hydrometer or approximate state of charge with a voltmeter. Use a digital voltmeter that can display tenths or hundredths of a volt when measuring 10 to 30 volts. Make your measurements when the battery has not been charged or discharged for several hours. For a deep-cycle battery at 77º F (25º C), use the following table:
Battery Voltage State of Charge
12.7–13.0 V 100%
12.5–12.6 V 80%
12.3–12.4 V 60%
12.1–12.2 V 40%
11.9–12.0 V 20%
Source: Xantrex
Battery Sizing Worksheet
Use the following worksheet to calculate your battery needs. To ensure sufficient battery capacity, be generous when estimating the operating time per day for each of the loads you will run.