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Battery Capacity Calculator (Ah / Wh)

Calculate the required battery capacity in Amp-hours (Ah) and Watt-hours (Wh) for a specified load and runtime.

1. Enter your numbers  →  2. Check the answer  →  3. Read the steps before you trust it

W
hours

Required Battery Bank Capacity

Required Battery Capacity
115.74 Ah

@ 12V Nominal (1.39 kWh nominal bank)

Usable Energy Delivered
1,000 Wh
Nominal Total Energy Required
1,388.89 Wh

Step-by-Step Engineering Calculation

  1. 1Total Energy Consumed: E=200 W×5 h=1000.0 WhE = 200\text{ W} \times 5\text{ h} = 1000.0\text{ Wh}
  2. 2Total Battery Bank Energy (accounting for 80% DoD & 90% Eff): Ebank=1388.9 WhE_{\text{bank}} = \mathbf{1388.9\text{ Wh}}
  3. 3Required Capacity: C=1388.9 Wh12 V=115.74 AhC = \frac{1388.9\text{ Wh}}{12\text{ V}} = \mathbf{115.74\text{ Ah}}

Formula & Theory

Capacity (Ah) = (Load W × Hours) / (Voltage × DoD × η)

Computes total nominal battery bank capacity required to sustain target runtimes without over-discharging.

Variable Definitions
P_load:
Load Power (Watts (W)) – Operating power draw
T:
Desired Runtime (Hours (h)) – Target runtime between charges
Ah:
Required Capacity (Amp-hours (Ah)) – Calculated nominal capacity

Worked Engineering Example

Off-Grid Security Camera System

A 40W camera and router running for 24 hours on a 12V battery system with 80% DoD and 90% efficiency.

Inputs:
  • Load: 40 W
  • Runtime: 24 hours
  • Voltage: 12 V
  • DoD: 80%
  • Efficiency: 90%
Result: Required Capacity = 111.11 Ah (Total Bank: 1,333 Wh)

Key Assumptions

  • •Constant average power draw.

Limitations & Safety Scope

  • •Temperature derating during cold weather requires sizing adjustments for outdoor installations.

Frequently Asked Questions

How do I convert Wh to Ah?

Divide Watt-hours by nominal system voltage: Ah = Wh / Volts.

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Educational & Reference Notice: This calculator provides technical estimates based on standard physical formulas. Actual electrical installation, protective device sizing, and conductor selection must be verified against the applicable electrical code (NEC, IEC, CEC) by a licensed electrician or qualified professional engineer.