Battery Bank Amp Hours Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

One battery voltage (V): The nominal voltage printed on one battery, such as 12 V, 12.8 V, 24 V, or 48 V.

One battery capacity (Ah): The amp-hour rating of one battery. Amp-hours describe charge capacity, but energy also depends on voltage.

Series string: Batteries wired end-to-end so their voltages add. Amp-hours stay the same within one string.

Parallel strings: Matching series strings connected side-by-side so amp-hours add. Voltage stays the same as one string.

Rated stored energy: The full battery-bank energy in watt-hours before usable-limit and efficiency adjustments.

Planned usable energy: The energy you plan to deliver after applying Usable battery limit (%) and System efficiency (%).

Wh and kWh: Watt-hours and kilowatt-hours measure energy. 1 kWh equals 1,000 Wh.

Load power (W): The average power used by the device or group of devices for the runtime estimate.

Mistake: Comparing Battery bank capacity in Ah between banks with different Battery bank voltage values.
Fix: Compare Rated stored energy in Wh or kWh when voltages are different.

Mistake: Putting the total number of batteries into Batteries in each series string (count) instead of the number in one series chain.
Fix: Enter only the count wired end-to-end in one string, then put the number of matching strings in Number of parallel strings (count) .

Mistake: Treating Planned usable energy as the same as Rated stored energy .
Fix: Use realistic values for Usable battery limit (%) and System efficiency (%) before planning loads.

Mistake: Entering a peak surge value in Load power (W) when you want Estimated runtime .
Fix: Use the average watts the load uses over time, not the short startup surge.

Mistake: Mixing old and new batteries while trusting Total batteries needed as a full wiring plan.
Fix: Use identical batteries with the same voltage, capacity, chemistry, age, and state of charge, and check fuses and cable ratings before building.

• The calculator assumes all batteries are identical in voltage, capacity, chemistry, age, health, and state of charge.
• Battery voltage changes during discharge, so Wh estimates based on nominal voltage are planning estimates, not lab-test measurements.
• Estimated runtime uses average Load power (W). Real loads that cycle on and off, surge, or change speed can run for a different time.
• System efficiency (%) is a single user-entered estimate. Inverters, cables, charge controllers, and high current can all change delivered energy.
• The calculator does not size fuses, breakers, bus bars, wire gauge, battery management systems, chargers, or ventilation.
• Large series or parallel counts need extra design checks for current sharing, fault current, balance, and equipment voltage limits.

How the bank is calculated

The calculator treats the bank as identical batteries arranged into series strings, then connected in parallel. Series wiring raises voltage. Parallel wiring raises amp-hours. Watt-hours are used because Ah alone can be misleading when battery voltages differ [2].

total_batteries = series_count * parallel_count

bank_voltage_v = battery_voltage * series_count

bank_capacity_ah = battery_capacity_ah * parallel_count

rated_energy_wh = bank_voltage_v * bank_capacity_ah

rated_energy_kwh = rated_energy_wh / 1000

Usable energy and runtime

For planning, the calculator applies the user-entered battery-use limit and system efficiency. This keeps the rated size separate from the energy you expect to actually use.

usable_energy_wh = rated_energy_wh * (usable_battery_limit_percent / 100) * (system_efficiency_percent / 100)

usable_energy_kwh = usable_energy_wh / 1000

If Load power (W) is entered, runtime is found by dividing usable watt-hours by average watts.

estimated_runtime_hours = usable_energy_wh / load_power_watts

Worked example

For eight 12.8 V, 100 Ah batteries wired as 4 in series and 2 parallel strings, the bank voltage is 12.8 * 4 = 51.2 V and the bank capacity is 100 * 2 = 200 Ah. Rated stored energy is 51.2 * 200 = 10,240 Wh, or 10.24 kWh. With an 80% usable battery limit and 90% system efficiency, planned usable energy is 10,240 * 0.80 * 0.90 = 7,372.8 Wh. With a 500 W load, estimated runtime is 7,372.8 / 500 = 14.7456 hours.

Input checks

Voltage, capacity, series count, parallel count, usable battery limit, and system efficiency must be positive valid numbers. Series and parallel counts must be whole numbers. Percent settings must be greater than 0 and no more than 100. Load power (W) may be blank, but if entered it must be greater than 0.