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How to Calculate Battery Capacity (Ah, mAh, and Watt-hours Explained!)

๐Ÿ”‹ When you’re building a solar system, sizing a power bank, or choosing a backup battery for your gadgets โ€” understanding battery capacity is key.

This guide will explain what battery capacity means, how to calculate it, and how to convert between units like Ah, mAh, and Wh โ€” with a calculator to make it all easy.

๐Ÿ™‹ What Is Battery Capacity?

Battery capacity tells you how much energy a battery can store and deliver over time. Itโ€™s usually expressed in:

  • Amp-hours (Ah) or Milliamp-hours (mAh) โ€“ common for small electronics
  • Watt-hours (Wh) โ€“ used for total energy storage (power ร— time)

๐Ÿงฎ Formula 1: Convert Ah or mAh to Wh

To calculate how much energy a battery holds in watt-hours, use:

Watt-hours (Wh) = Amp-hours (Ah) ร— Voltage (V)

If your battery capacity is in mAh (milliamps), convert it to Ah first:

Ah = mAh รท 1000

๐Ÿงฎ Formula 2: Estimate How Long a Battery Will Last

Battery Life (hours) = Capacity (Wh) รท Load Power (W)

โšก Example

You have a 12V battery rated at 100Ah.

Wh = 100 ร— 12 = 1200 Wh

So it stores 1200 watt-hours of energy.

If youโ€™re powering a 100-watt device:

Battery Life = 1200 รท 100 = 12 hours

โš™๏ธ Battery Capacity Calculator

Watt-hours: โ€”

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๐Ÿ”‹ Common Battery Examples

Battery TypeTypical Capacity
AA (alkaline)~2,500 mAh
Smartphone battery~3,000โ€“5,000 mAh
12V car battery~50โ€“100 Ah
Power bank (portable)~10,000 mAh
Home battery (Tesla)~13.5 kWh

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