The Inner Monologue

Introduction

You’ve probably seen power banks (portable chargers) with big numbers like “10,000 mAh” or even “20,000 mAh” on them. They promise to charge your phone multiple times, but have you ever noticed they don’t actually last as long as you expected?

Turns out, that mAh number is kind of a trick. Let’s break down how power banks really work—and how to know how much energy they actually hold.

The Problem with mAh (Milliamp-Hours)

What Does mAh Even Mean?

• mAh = milliamp-hours, a measure of charge (how much electric current flows over time).
• Example: A 5,000 mAh phone battery can theoretically provide 5,000 milliamps (5 amps) for 1 hour.

The Catch: Voltage Matters!

• The mAh rating on power banks is measured at the battery’s internal voltage (~3.7V).
• But your phone charges at 5V, and laptops often need 9V, 12V, or even 20V.
• Higher voltage = fewer mAh (because energy is conserved, but the way it’s delivered changes).

Example: A “20,000 mAh” Power Bank

So no, your “20,000 mAh” power bank won’t charge your laptop 5 times—maybe once or twice.

Watt-Hours (Wh): The Real Measure of Energy

Since mAh changes with voltage, scientists and engineers use watt-hours (Wh) instead.

What’s a Watt-Hour?

• Wh = Volts × Amp-Hours → measures total energy (not just charge).
• Example:
• A 20,000 mAh battery at 3.7V = 74 Wh (20 Ah × 3.7V).
• That same energy at 5V = still 74 Wh, but only ~13,000 mAh.

Why Don’t Companies Use Wh?

• Bigger numbers sell better. “20,000 mAh” sounds way more impressive than “74 Wh.”
• Most people don’t understand Wh (yet!).

Why You Should Care

• Airlines use Wh to decide if you can bring a battery on a plane (because it’s the real measure of energy).
• If you see both mAh and Wh, trust the Wh number more.

How to Calculate Real-World Charging

Want to know how many times a power bank can charge your phone?

Step 1: Find Your Phone’s Battery Capacity

• iPhone 15: ~3,877 mAh at ~3.86V → ~14.96 Wh
• Samsung Galaxy S23: ~3,900 mAh at ~3.85V → ~15 Wh

Step 2: Check the Power Bank’s Wh

• If it’s 74 Wh, divide by your phone’s Wh:
• 74 Wh / 15 Wh ≈ 4.9 full charges (in a perfect world).

Step 3: Account for Efficiency Loss

• Real-world charging loses 10–20% energy as heat.
• So 74 Wh → ~60 Wh usable → ~4 full charges (not 5+ like ads claim).

How to Avoid Being Tricked

1. Look for the Wh rating (if listed).
2. Assume real mAh is lower than advertised (because of voltage conversion).
3. Bigger number ≠ better—compare Wh, not just mAh.

Conclusion

• mAh is misleading because it changes with voltage.
• Watt-hours (Wh) tell the real story—always check if possible.
• Your “20,000 mAh” power bank isn’t lying… it’s just not telling the whole truth.

Now you know how to spot the marketing tricks and figure out how much energy a power bank really holds!

Quick Quiz (Test Your Knowledge!)

1. If a power bank says “20,000 mAh at 3.7V,” what’s its Wh?
2. Why does a 20,000 mAh power bank give fewer mAh at 5V?
3. How many times can a 74 Wh power bank charge a 15 Wh phone?