To size a 12V battery bank, first total the watt-hours used by the loads each day. Multiply that result by the required autonomy days, then divide by usable depth of discharge and system efficiency to find the required battery energy. Finally, divide by 12V and round up to a practical amp-hour capacity.
Start with the energy your loads use each day
List each device, its wattage, and the hours it runs per day. Multiply watts by hours for each device, then add the results:
Daily watt-hours = watts × hours per day
For example, a 60-watt fan running for 8 hours uses 480 Wh. A 20-watt light running for 5 hours uses 100 Wh. A 500-watt pump running for 1 hour uses 500 Wh. Together, those loads use 1,080 Wh per day.
Use measured consumption when available. If a device cycles on and off, estimate its average running time rather than multiplying its maximum rating by 24 hours. Include only the loads the battery must actually support during an outage or off-grid period.
Apply autonomy, depth of discharge, and efficiency
Use this sizing formula:
Required battery energy (Wh) = daily load (Wh) × autonomy days ÷ depth of discharge ÷ efficiency
Use decimal values in the calculation. For example, 80% depth of discharge is 0.80, and 90% efficiency is 0.90.
Autonomy is the number of days the battery must supply the load without help from the grid or solar source. The Oregon State University battery-sizing procedures show how system voltage, temperature, aging, maximum depth of discharge, and discharge rate can also be incorporated into a more detailed design.
Depth of discharge, or DoD, is the share of the battery's rated energy that you plan to use. A lower DoD produces a larger bank and leaves more energy in reserve. Select the DoD value specified for the battery chemistry and model, then use that percentage consistently throughout the calculation.
Efficiency accounts for energy used by the inverter, wiring, and other conversion equipment. Use measured or manufacturer-specified system efficiency when available. A documented planning value can be used for the initial calculation and refined when system-specific data becomes available.
Convert the result to 12V amp-hours
After calculating required watt-hours, convert to amp-hours:
Required amp-hours = required watt-hours ÷ nominal system voltage
For a 12V bank, divide by 12. A battery marketed as 12V may have a nominal voltage such as 12.8V, but use the voltage convention required by your system and battery documentation. Keep the calculation consistent when comparing products.
Example: 1,080 Wh per day for two days
Assume the loads use 1,080 Wh per day, the desired autonomy is 2 days, planned DoD is 80%, and system efficiency is 90%.
1,080 × 2 ÷ 0.80 ÷ 0.90 = 3,000 Wh
3,000 Wh ÷ 12V = 250 Ah
The planning result is 250 Ah at 12V, equivalent to a 3,000 Wh target. The Vipboss 12V 210Ah Bluetooth LiFePO4 battery contributes 2,688 Wh of nominal capacity per battery based on its 12.8V, 210Ah specifications. Use that per-battery capacity with the product's supported configuration options to select a bank whose combined nominal energy meets or exceeds the 3,000 Wh sizing target.
