Size Inverter Batteries

Inverters convert direct current (DC) electrical power into alternating current (AC) electrical power. DC comes from batteries, just like the one that is used to start a car. AC power is the same type of electricity that comes from the home's common wall outlet. Batteries that are for inverters are generally of the "deep cycle" type used in marine and RV settings. These batteries look the same as car batteries, but can be discharged and recharged many hundreds of times over the common car battery. Whenever sizing batteries for inverters, they must be of the deep cycle type.
#1. Make a list of all electrical appliances and devices that will be operating under the inverter battery power. Next to this list make another column of the predicted daily operational hours for each device. In the last column, list the total wattage power usage of each appliance.
 #2. Calculate the wattage power of the appliance if it is not labeled on the unit. Wattage is found by multiplying the voltage times the amperage (w = v X a). As an example, a refrigerator that is powered by 120VAC and draws 5 amperes will have a wattage power consumption of 120VAC times 5 amps, which is equal to 600 watts.
#3. Add all of the daily operational hours together on the bottom of the list. Add all the wattage values together as well.
#4. Find the total amperage draw on the inverter electrical system. The inverter system supplies 120VAC for all the electrical devices, you can divide the total wattage value by the voltage to find the amperage draw (a = w /v). The wattage total, from the list in step 2, may add up to 12,000 watts. Divide 12,000 watts by 120VAC and the total amperage draw is equal to 100 amps.
#5Estimate the complete daily amperage hours of the inverter system by multiplying the 100 amps times the list's total operational hours, found in step 3. If the system is going to operate for 10 hours a day, then the inverter will require 100 amps times 10 hours for 1000 amp-hours of battery power.
#6Multiply the 1000 amp-hours by at least 2 for a safety backup, in case the charging system cannot recharge the batteries in a 24-hour period. The total amp-hour requirement is now 2000 amp-hours.
#7Divide the 2000 amp-hours by the individual rating on each battery. Most deep cycle batteries are rated from 100 amp-hours up to 500 amp-hours, depending on type and manufacturer. In other words, the 2000-amp-hour system will require twenty 100 amp-hour batteries or four 500-amp-hour batteries.