A 20,000 mAh power bank holds about 74 watt-hours at 3.7V cells, or roughly 77 Wh with 3.85V cells.
You’re weighing a 20,000 mAh pack and want the energy in watt-hours. The quick takeaway is around seventy-four watt-hours, because most packs use 3.7-volt lithium-ion cells. If the cells are the 3.85-volt type, the number lands near seventy-seven watt-hours. That single figure helps with airline limits, device-charging estimates, and apples-to-apples comparisons across brands.
At-A-Glance Wh For Popular Bank Sizes
The table below converts common labeled capacities to estimated watt-hours using the simple formula Wh = (mAh × V) ÷ 1000. The first numeric column uses 3.7V cells; the second uses 3.85V high-voltage cells.
| Labeled Capacity (mAh) | Est. Wh @ 3.7V | Est. Wh @ 3.85V |
|---|---|---|
| 5,000 | 18.5 Wh | 19.3 Wh |
| 10,000 | 37.0 Wh | 38.5 Wh |
| 20,000 | 74.0 Wh | 77.0 Wh |
| 26,800 | 99.2 Wh | 103.2 Wh |
| 30,000 | 111.0 Wh | 115.5 Wh |
What Watt-Hours Mean In Plain English
Milliamp-hours describe charge. Watt-hours describe stored energy. To translate between the two, you need the cell voltage. Most compact banks use 3.7V nominal lithium-ion cells, while some newer cells run at 3.85V nominal. Use the right voltage in the math and the energy figure will align with the value printed on many housings and spec sheets.
The Simple Formula You’ll Use
The conversion is straightforward: Wh = (mAh × V) ÷ 1000. For a 20,000 mAh bank built from 3.7V cells: 20,000 × 3.7 ÷ 1000 = 74 Wh. If the cells are 3.85V: 20,000 × 3.85 ÷ 1000 ≈ 77 Wh. The reverse works too: mAh = (Wh × 1000) ÷ V. That’s all there is to it.
Watt-Hours For A 20,000 mAh Battery Pack — The Math
Start with the printed capacity, identify the cell voltage, and run the numbers. If the box or the spec sheet lists the energy directly, it should match the formula within rounding. Multiple outputs, PPS support, and a bright display don’t change the energy inside; they only change how quickly the pack can deliver it to your devices.
Why Your Phone Sees Less Than 74 Wh
The energy sits at the cell voltage, but your gadgets sip power at their input voltage, usually 5V or a higher USB-PD profile. Stepping the voltage up wastes a slice of energy as heat. Real-world efficiency often lands between 80% and 90% depending on load, cable, and the pack’s converters. That means a 74 Wh bank might deliver something closer to 60–66 Wh to your device. Cold weather, worn cells, and fast-charge bursts trim a little more.
Air Travel Rules You Should Know
Why does the energy number matter so much? Airlines use watt-hours to set limits. In the U.S., TSA’s power bank page says portable chargers must ride in carry-on, not checked bags. The FAA battery chart sets thresholds: up to 100 Wh is allowed in carry-on without airline approval, 101–160 Wh needs approval, and anything above 160 Wh is not permitted for regular passengers. A 20,000 mAh class bank at ~74 Wh sits well within the easiest bracket.
Packing Tips For Smooth Screening
- Keep the bank in your personal item so you can show it if an officer asks.
- Look for the Wh rating on the housing; many brands print both mAh and Wh.
- Tape over exposed contacts only if the model uses removable cells. Sealed packs don’t need it.
- Never check a spare lithium pack. Cabin only.
Real-World Recharges From A 20,000 mAh Class Bank
The table below uses a practical delivered-energy band of 60–66 Wh. Battery sizes vary by model, so treat these as planning ranges rather than promises.
| Device Type | Typical Battery (Wh) | Estimated Full Recharges |
|---|---|---|
| Smartphone (4,000–5,000 mAh) | 15–19 Wh | 3–4× |
| Small Tablet (7,000–8,000 mAh) | 26–30 Wh | 2× |
| Nintendo Switch | ≈16 Wh | 3–4× |
| Noise-Canceling Headphones | ≈4 Wh | 15×+ |
| Ultrabook Laptop | 40–55 Wh | ~1× |
| Mirrorless Camera Battery | 8–16 Wh | 4–8× |
Cell Count And Voltage Basics
Most compact banks use a single-series pack layout (often written as 1S, sometimes 1S2P or 1S3P when cells are paralleled). That’s why the nominal voltage is near 3.7V or 3.85V across the whole pack. A two-series layout (2S) would double the voltage, add cost and weight, and complicate the output stage. It’s rare in pocket-friendly models. Knowing that 1S is the norm helps you pick the right voltage for the conversion even when the box is light on detail.
Why Product Pages Mention 5V mAh
Some listings show a larger “at 5V” figure and a smaller cell-side figure. The larger number multiplies the 5V output by hours delivered in a gentle test, which inflates the value because the pack had to step up voltage from the lower cell level. The smaller figure in watt-hours reflects actual stored energy inside the cells and lines up with airline rules. When you see both, trust watt-hours for travel and for real-world comparisons.
How Manufacturers Print The Rating
Brands often print a combined line near the ports, such as “20,000 mAh / 74 Wh (3.7V).” That parenthetical voltage tells you which number to use in the formula. If only mAh appears, assume 3.7V unless the maker clearly calls out a 3.85V chemistry. Some models add a separate line that lists output modes (for example, 5V⎓3A, 9V⎓2.22A). Those modes describe speed, not stored energy.
Estimate Laptop Runtime With A Quick Rule
Find your laptop battery’s watt-hours in the system report or on the pack label. Divide the bank’s delivered energy by that number. A 13-inch ultrabook with a 50 Wh battery will get about one top-off from a 20,000 mAh class bank that delivers ~60–66 Wh. High-power work drains faster because the pack’s converters run less efficiently at big loads. Light web use with the lid dimmed stretches runtime.
Quick Calculator: Do Your Own Conversion
Steps
- Find the printed mAh on the label.
- Identify the cell voltage (3.7V is common; some list 3.85V).
- Multiply mAh by the voltage.
- Divide by 1000 to get Wh.
- To estimate delivered energy, multiply Wh by 0.8–0.9.
Worked Examples
- 20,000 mAh at 3.7V → 74 Wh → ~60–66 Wh delivered.
- 26,800 mAh at 3.7V → 99.2 Wh → ~79–89 Wh delivered.
- 10,000 mAh at 3.85V → 38.5 Wh → ~31–35 Wh delivered.
Buying Tips If You Need More Runtime
- Fly a lot? Stay under the 100 Wh line for easy boarding.
- Need laptop charging? Pick a bank with USB-PD 45–65W output and a cable rated for that wattage.
- Cold trips? Keep the pack near your body; low temps trim effective capacity.
- Long events? Two midsize banks beat one large brick for flexibility and sharing.
Care And Safety For Long Service
Store the pack around half charge if it will sit unused for weeks. Avoid hot dashboards and tight cases that trap heat. Use a reputable charger that meets the pack’s input rating. Replace frayed cables. If the housing swells, gets oily, or smells odd, retire the unit. These are simple habits that keep charge cycles healthy and reduce waste.
Reading Spec Sheets With Confidence
When a brand lists both mAh and Wh, you can check the math on the spot. If a listing shows a huge “output mAh” but a smaller cell-side figure, scale expectations to the cell-side energy. If a model claims near-instant charging at very high wattage yet quotes a tiny input rating, that’s a mismatch. Good spec sheets feel consistent: a clear energy figure in watt-hours, sensible input numbers, and output modes that match the advertised use cases.
What About The 26,800 mAh Ceiling?
Travel-friendly models often top out near 26,800 mAh because 26,800 × 3.7 ÷ 1000 ≈ 99.2 Wh, which hugs the 100 Wh line in the FAA guidance. Packs above that line can still fly with airline approval up to 160 Wh, yet they’re less handy for frequent trips. Many travelers carry two smaller banks instead of one heavy unit. That approach spreads wear, adds redundancy, and keeps each pack within the easiest ruleset.
Fast-Charge Modes Don’t Change Stored Energy
USB Power Delivery and PPS step voltage up to meet your device’s request. They don’t add energy to the tank. The converters pull from the same 3.7V or 3.85V cells and translate to the needed profile. If you want faster laptop charging, look for higher sustained output wattage and a cable that can handle the current. The watt-hours stay the same.
Final Take: The Number You Can Trust
A 20,000 mAh class bank stores about seventy-four watt-hours when built from 3.7V cells, or around seventy-seven watt-hours with 3.85V cells. That energy sits well under airline thresholds and covers several phone refills, a tablet boost or two, and a light laptop top-off. Use the simple formula whenever a spec feels fuzzy, and you’ll pick the right size for trips, events, and daily carry.