How to calculate the size of battery bank needed for off-grid?

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Calculating Power Requirements When determining the size of a battery bank, start by calculating the total power required by your off-grid system. For a 12V system, this means adding up the wattage ratings of all appliances, such as lights, pumps, and refrigerators. For example, if you have a 50W refrigerator, a 20W LED light, and a 10W fan, your total power requirement is 80W. Since most appliances are used for 8-12 hours per day, multiply the total power requirement by the number of hours of use to get the daily energy consumption in watt-hours (Wh). For our example, this would be 80W x 10 hours = 800Wh per day.
Battery Bank Sizing Next, consider the depth of discharge (DOD) of your batteries. A DOD of 50% means that your batteries can be safely discharged to half of their total capacity without causing damage. For a 12V system, a safe DOD is typically between 50% and 75%. To calculate the battery bank size, divide the daily energy consumption by the battery voltage and multiply by the DOD percentage. Using our previous example, let’s assume a 75% DOD and a 12V system. The required battery capacity would be 800Wh / 12V x 0.75 = 50Ah (ampere-hours). For a 12V system, this translates to a battery bank of approximately 4-6 12V batteries, depending on the specific battery model and type.
Charging Capacity Finally, ensure that your solar panel or generator array can recharge your battery bank within a reasonable timeframe. A good rule of thumb is to size your charging system to recharge the battery bank within 4-6 hours of direct sunlight or generator operation. For a 12V system, this means calculating the total wattage required to recharge the battery bank in 4-6 hours. Using our previous example, let’s assume a 4-hour recharge time and a 12V system. The required charging capacity would be 800Wh / 4 hours = 200W. This can be achieved with a 200W solar panel or a 200W generator.