Quick Answer
To size batteries in off-grid solar systems, calculate the daily energy load, determine the required battery bank capacity, and select deep cycle batteries with a sufficient depth of discharge (DOD) rating.
Load Calculation
To calculate the daily energy load, add up the energy requirements of all devices connected to the system. Convert the energy requirements of each device to watt-hours (Wh) for a 24-hour period. For example, a 100W refrigerator operating for 12 hours a day requires 1200Wh (100W x 12h). Add up the energy requirements of all devices and round up to the nearest kilowatt-hour (kWh) to determine the daily energy load.
Battery Bank Capacity
Determine the required battery bank capacity by dividing the daily energy load by the desired DOD rating. A 50% DOD rating is a common standard for off-grid solar systems. For example, if the daily energy load is 5kWh and the desired DOD rating is 50%, the required battery bank capacity is 10kWh (5kWh / 0.5).
Panel, Battery, and Inverter Sizing Worksheet
Use a worksheet to determine the required panel, battery, and inverter sizes. Start by entering the daily energy load and desired DOD rating. Then, calculate the required battery bank capacity as described above. Next, select a deep cycle battery type and enter its DOD rating. Finally, select a solar panel type and enter its wattage rating. Use a solar panel sizing worksheet or online calculator to determine the required panel array size. Once the panel array size is determined, select an inverter type and enter its maximum power rating.
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