A 20,000mAh power bank takes ~4–5 hours with a 20W charger or ~8–9 hours with 10W, assuming ~85% charging efficiency.
You get a clear estimate, a table of real-world ranges by charger power, and the exact setup choices that move the needle.
How Long To Recharge A 20,000mAh Power Bank: Quick Math
Energy is the starting point. A 20,000mAh pack uses a 3.7V lithium cell, so the stored energy is about 74Wh (20Ah × 3.7V). Time comes from energy ÷ effective charging watts. Real devices waste a slice of power as heat and taper current near full, so planning with ~85% efficiency gives realistic ranges.
Formula you can use: Charge time ≈ (Capacity in Wh) ÷ (Charger watts × efficiency). With 74Wh and 0.85 efficiency, a 20W input lands near 74 ÷ (20 × 0.85) ≈ 4.3 hours, while a 10W input sits near 8.7 hours. Many banks top out at 18–30W on the input side, even if your wall charger is stronger, so the bank’s “Input” spec rules the day.
Quick Table Of Estimated Times (0–100%)
This table assumes ~74Wh capacity and ~85% end-to-end efficiency. If your bank accepts only 18W input, times beyond that won’t shrink further.
| Charger/Input (W) | Estimated Time | Notes |
|---|---|---|
| 10W (5V⎓2A) | ~8–9.5 hours | Common with older 5V chargers |
| 12W (5V⎓2.4A) | ~7–8 hours | Often labeled “12W USB-A” |
| 18W (PD/QC) | ~4.5–5.5 hours | Typical “fast” input on many 20k banks |
| 20W (PD) | ~4–5 hours | Common with modern phone bricks |
| 30W (PD) | ~3–3.5 hours | Works only if bank accepts 30W input |
| 45W (PD) | ~2–2.5 hours | Only if bank accepts 45W input |
What Actually Controls The Timing
The Bank’s Input Rating
Flip the pack and find “Input” near the ports (often printed as “USB-C IN/OUT, 5V⎓3A, 9V⎓2A, 12V⎓1.5A” etc.). Those numbers map to watts (volts × amps). If the label caps at 18W, a 30W wall brick won’t make it faster, since the bank will sip only up to its limit.
The Charger’s Real Output
The brick must speak the same fast-charge language and deliver the watts the bank asks for. USB Power Delivery is the common standard over USB-C and can deliver much higher power levels on capable gear. The spec now reaches up to 240W for laptops and docks, but your pack’s input still sets the ceiling.
The Cable’s Current Rating
A weak cable can choke current. With USB-C to USB-C, certified 60W or 240W cables signal the right power path. With USB-A to USB-C, the best you’ll get is legacy 5V modes that rarely exceed 12W.
Charge Curve And Taper
Cells accept high current when low, then taper near full to protect longevity and safety. That last 10–15% can add noticeable minutes. Many published “fast” times refer to ~0–80% fill; a full 100% takes longer.
Fast-Charge Standards And Why They Matter
Most modern banks and chargers use USB-C with USB Power Delivery (PD). PD negotiates voltage and current in steps, so a bank might ask for 9V⎓2A (18W) or 12V⎓2.5A (30W) when the cells are low, then step down later. The PD 3.1 update introduces higher power levels for bigger devices, yet small packs still hover around 18–45W on input. You’ll also see proprietary modes like Quick Charge on older USB-A ports; those usually trail PD for speed on current gear.
Want the standard from the source? See the USB-IF page on USB Power Delivery for the power ranges and naming.
How To Check Your Setup And Predict Time
Step 1: Read The Bank’s Label Or Specs Page
Find the exact “Input” line. If it lists 5V⎓3A and 9V⎓2A, that’s up to 15W or 18W through USB-C. If it lists 12V⎓2.5A, that’s up to 30W. Some premium models accept 45W. That single line is the most practical clue to your ceiling.
Step 2: Match A Charger That Can Feed It
Pick a PD charger with a wattage rating at or above the bank’s input spec. A 30W PD brick covers an 18W bank easily; a 20W phone brick matches a 20W-capable bank. Multi-port chargers share total output, so plug into a port that can deliver the full rate solo.
Step 3: Use A Cable That Carries The Current
With USB-C, a certified 60W cable suits phone-class power banks. For anything above, look for the 240W mark. Labels printed on plugs or cable sleeves help, and certified marks listed by the USB-IF reduce guesswork.
Step 4: Run The Math
Take 74Wh, divide by the lower of charger watts or bank input watts, then divide by 0.85. Round up to allow for taper and room temperature. That gives a range you can trust in daily use.
Do Capacity And Voltage Complicate The Numbers?
Pack boxes list mAh at the cell’s nominal 3.7V, while USB ports move energy at 5V, 9V, or 12V. Compare power in watt-hours, not mAh. Aviation rules use the same math: Ah × V = Wh. Using 20Ah × 3.7V yields ~74Wh, which is the right basis for time estimates.
For the underlying Wh formula, see the IATA guidance on watt-hour calculation used for safety and transport.
Real-World Scenarios
20W PD Charger With Midrange Bank
Many midrange banks accept 18–20W on USB-C. Pairing one with a 20W PD brick lands in the 4–5 hour range, which feels like an overnight top-off or a half-day desk refill.
30W PD Brick With 30W-Capable Bank
Step up to a 30W input and the window tightens to about 3–3.5 hours, starting from near empty. Good cables matter here; a tired cable can drop voltage and stall the negotiation.
Second Table: Factors That Shift Time In Daily Use
These aren’t edge cases—they show up in homes and offices every day.
| Factor | What Changes | Impact Range |
|---|---|---|
| Cable Quality | Voltage drop at higher current | +10–40 minutes |
| Shared Charger | Ports split total output | +30–90 minutes |
| Bank Input Limit | 18W vs 30W vs 45W | Hours swing by 1–2+ |
| Room Temperature | Taper starts sooner when hot or cold | +10–60 minutes |
| Battery Age | Higher resistance over time | +10–45 minutes |
| Pass-Through Use | Charging phone while filling bank | Can double time |
| Starting Level | From 20% vs from 0% | Saves 30–90 minutes |
| Top-Off Window | 90–100% current taper | +15–30 minutes |
Safety And Care Tips That Also Save Time
Keep It Cool
Heat slows charging and ages cells. Set the pack on a hard surface with airflow. Skip stacking devices on top.
Charge In One Hop
Daisy-chaining through hubs or dongles adds loss and can confuse power negotiation. Plug the charger straight into the bank; add gadgets after it’s done.
Mind The Last 10%
If you only need tomorrow covered, stopping near 80–90% saves time and heat. Many people do a full refill only before travel days.
Buy With The Input In Mind
Specs often hype output watts for fast phone charging, but the input line is what sets refill speed. For a 20k pack that you top up often, 30W input is a sweet spot with today’s chargers.
Wrap-Up: Your Action Plan
Use This Three-Step Recipe
- Confirm the bank’s input spec (18W, 20W, 30W, or 45W).
- Match a PD charger at that wattage or higher, with a certified cable.
- Use the 74Wh ÷ (watts × 0.85) math to set expectations, then check the table.
With those pieces aligned, a midrange 20k pack lands near 4–5 hours on a 20W setup, closer to 3–3.5 hours on a 30W-capable model, and around 8–9 hours on older 10W bricks. That’s the whole story in plain terms—no mystery, just the math plus the limits printed on the case.