You bought a 20,000 mAh power bank. Your phone has a 4,000 mAh battery. Simple math says you should get five full charges, right? Wrong. You'll get about three — maybe three and a half if you're lucky. And that's from a good power bank. From a bad one, you might get two.
Where did the other 8,000 mAh go? The answer involves voltage conversion, heat loss, and in some cases, outright dishonesty from manufacturers. Understanding why power banks never deliver their advertised capacity is the difference between buying a device that meets your needs and one that leaves you stranded with a dead phone.
The mAh Lie: Understanding Voltage Conversion
Here's the core issue: power bank batteries operate at 3.7 volts, but USB output is 5 volts. To deliver power through USB, the power bank has to step up the voltage — and that conversion isn't free. The formula is simple:
Usable capacity = Advertised capacity × (3.7V / 5V) × efficiency factor
For a 20,000 mAh power bank with a typical 85% efficiency:
20,000 × (3.7/5) × 0.85 = 12,580 mAh usable
That's the real capacity. Not 20,000 — 12,580. And some power banks are even less efficient, especially cheap ones with poor-quality circuitry. A budget 20,000 mAh bank with 70% efficiency gives you just 10,360 mAh — barely half the advertised number.
Why Cheap Power Banks Are Cheap
The cost difference between a $15 power bank and a $35 one isn't just brand markup. It's in the components you can't see:
- Battery cells: Reputable brands use cells from manufacturers like LG, Samsung, or Panasonic. Cheap brands use generic cells that degrade faster and deliver less capacity over time.
- Conversion circuitry: Better circuitry means higher efficiency (90%+ vs 70%) and less heat generation. Heat is wasted energy — and wasted capacity.
- Safety features: Overcharge protection, temperature monitoring, and short-circuit prevention. Cutting corners here isn't just about capacity — it's about safety.
- Actual capacity honesty: Some no-name brands literally print a larger number than the battery inside can deliver. We've tested "20,000 mAh" banks that contained 8,000 mAh cells.
How to Spot an Honest Power Bank
✓ Signs of Quality
- Published watt-hour (Wh) rating
- Lists actual cell manufacturer
- Lists efficiency percentage
- USB-IF certification
- Proper safety certifications (UL, CE, FCC)
- Brand has a real website and support
✗ Red Flags
- Only mAh listed, no Wh
- Unrealally low price for capacity
- No safety certifications listed
- Sold only on Amazon, no brand website
- Suspiciously high review count, recent
- Multiple capacity options, all same price
Watt-Hours: The Number That Actually Matters
If you want to compare power banks fairly, ignore mAh and look at watt-hours (Wh). Wh accounts for voltage, so it's a true measure of total energy storage. A power bank listing 74 Wh is more informative than one listing 20,000 mAh — and honest brands publish both.
To convert: Wh = (mAh × 3.7) / 1000. So a 20,000 mAh bank should be 74 Wh. If a brand lists 20,000 mAh but only 55 Wh, the math doesn't add up — they're being dishonest about something.
Charging Speed: Another Numbers Game
Power bank output speed is measured in watts (W). A 10W charger is slow. 18W is adequate. 30W+ is fast. But there's a catch: the listed wattage is often the maximum output from a single port, and it drops when you charge multiple devices simultaneously.
Also, your device has to support the charging protocol. A 65W power bank won't charge your phone at 65W unless the phone supports Power Delivery (PD) at that level. Check out our USB-C explained guide for a deep dive on charging protocols and why not all cables support fast charging.
| Capacity (Advertised) | Realistic Usable | Phone Charges (4,000 mAh) | Fair Price |
|---|---|---|---|
| 10,000 mAh | ~6,300 mAh | 1.5 charges | $15-25 |
| 20,000 mAh | ~12,600 mAh | 3 charges | $25-40 |
| 30,000 mAh | ~18,900 mAh | 4.5 charges | $35-55 |
| 40,000+ mAh | ~25,200+ mAh | 6+ charges | $50-80 |
Size vs Capacity: The Travel Dilemma
Higher capacity means more cells, which means more weight and bulk. A 10,000 mAh power bank fits in a pocket. A 30,000 mAh bank is a brick. If you're carrying it daily, 10,000-15,000 mAh is the sweet spot — enough for 2-3 phone charges without weighing you down.
For travel, 20,000 mAh is the sweet spot, and there's an airline regulation reason: the FAA limits lithium-ion batteries on flights to 100 Wh (approximately 27,000 mAh) without airline approval. A 20,000 mAh bank (74 Wh) flies without issues. A 30,000 mAh bank (111 Wh) requires airline approval. Know this before you fly.
The Built-In Cable Trap
Some power banks come with built-in charging cables. Convenient? Yes. But they're often low-quality cables that don't support fast charging, and if the cable breaks, the whole power bank becomes less useful. We prefer power banks with standard USB-C ports — you can use your own quality cable, and replace it if needed. See our phone protection guide for a similar take on why bundled accessories are often a false economy.
Pass-Through Charging: Does It Matter?
Pass-through charging lets you charge the power bank while it simultaneously charges your device — useful for traveling with one outlet. Not all power banks support this, and some that claim to do it poorly (generating excess heat). If this matters to you, verify it's explicitly supported, not just listed as a feature.
The Verdict
Buy power banks from brands that publish watt-hour ratings and efficiency numbers — Anker, Ugreen, and Baseus are reliable in the value tier. Expect 60-65% of the advertised capacity as real-world usable charge. A 20,000 mAh bank for $30-35 from a reputable brand is the sweet spot for most people: enough for 3 phone charges, flies without airline approval, and fits in a bag without being a burden.
Need a cable to go with that power bank? Our USB-C cable guide explains why the cable you use affects charging speed as much as the power bank itself. And if you're setting up a charging station at home, our smart plug guide can help you monitor energy usage.