A 20,000mAh power bank stores ~74Wh; expect 3–5 phone refills, with per-charge time set by the device’s intake wattage.
Quick Math: What 20,000mAh Means In Energy
Most portable packs use lithium-ion cells rated near 3.7 volts. Converting milliamp-hours to watt-hours is simple: capacity × voltage ÷ 1000. That puts a 20,000mAh pack at about 74Wh. Energy matters because charge time depends on how fast that energy can flow into your gadget and how much is lost in the process.
Charging Time Basics (So You Can Predict It)
Time is energy divided by charging power, adjusted for efficiency. A phone that can draw 20 watts will fill faster than one that only takes 10 watts. Real devices also slow down near the top to protect the battery, so the last stretch takes longer than the first half; Battery University’s overview of charging stages shows that taper clearly.
Fast Reference Table: Typical Phone Fill Times
The table below groups common battery sizes with likely intake power levels from a quality USB-C pack. It assumes wired charging with reliable cables and healthy batteries.
| Phone Battery Size | Intake Power | Time To Full (Range) |
|---|---|---|
| 3,000–3,500mAh | 10–18W | 1.0–1.8 hours |
| 4,000–4,500mAh | 18–25W | 1.1–2.0 hours |
| 5,000–5,500mAh | 20–30W | 1.2–2.3 hours |
| Small tablets (7–10Ah) | 18–30W | 2.0–4.0 hours |
How Long A 20,000mAh Power Bank Lasts In Practice
A pack near 74Wh rarely delivers all of that to devices. Voltage conversion, cable resistance, and heat shave the total. Good packs land near 80–90% round-trip efficiency. With that in mind, count on roughly 60–65Wh reaching your phone or tablet. That equates to three to five refills for many phones, or one small tablet refill, before the pack taps out.
Why Two Identical Packs Can Perform Differently
Label capacity is only part of the story. Peak output, PPS or fixed profiles, cable gauge, and even ambient heat shape outcomes. If your phone requests only 9 volts at 2 amps, you’ll see one set of times; if it can negotiate 20 volts at higher current, times drop. Wireless charging trims efficiency even more, so wired wins for speed and endurance.
What Sets The Pace: Device Intake, Profiles, And Cables
USB-C charging negotiates a voltage and current the two sides agree on. Older phones often stick to 5 volts. Newer ones accept higher voltages and steadier current with PPS, which keeps heat in check and shortens the overall session. Thick, well-made cables keep voltage sag in line and preserve speed.
Realistic Scenarios You Can Use
Let’s put numbers on it. Assume the pack can deliver 30 watts and your phone can take that rate for the first half, then slows. A 5,000mAh phone may reach 50% in roughly 25–35 minutes, 80% in about 50–70 minutes, and full in 70–110 minutes. Swap in a 10-watt intake phone and those figures stretch to roughly double.
Does A Bigger Wattage Label Always Charge Faster?
Not always. If the handset limits intake to 18 watts, a 45-watt port won’t speed it up. The ceiling is set by the smaller of two numbers: the port’s output and the device’s intake. Heat also trims speed under heavy use, so gaming while charging can slow everything down.
How Many Full Phone Refills Can You Expect?
Work from usable energy, not the label. With around 60–65Wh delivered, divide by your phone’s battery energy. A 12Wh phone battery yields about five refills; a 17Wh unit lands near three. Real life varies with screen-on time during charging, background tasks, and cable quality.
Understanding The Standards (So Specs Make Sense)
USB Power Delivery set common voltage steps like 5V, 9V, 15V, and 20V for up to 100W on Standard Power Range designs, and later added an Extended mode that allows up to 240W on capable chargers and cables. That doesn’t change phone limits, but it explains why modern packs can run small laptops and handheld consoles.
Two Handy Links For Deeper Detail
You can check the official USB-IF page on USB Power Delivery for the voltage steps and power ceilings, and use a clear formula reference like Goal Zero’s mAh→Wh conversion when you want to double-check energy math.
Recharging The Pack Itself: How Long It Takes
Filling a 20,000mAh pack depends on input power and taper at the top. A basic 10-watt input can take most of a workday. A 30-watt USB-C input trims that to a long lunch. Top-tier inputs near 45 watts go quicker, but the last few percent still slow down to keep cells healthy.
Input Power And Full-Charge Time For The Pack
Use the figures below as a clean planning guide. Times assume about 74Wh total energy and a practical charging efficiency near 85%, with a modest top-off period.
| Charge Input To Pack | 0–100% Time | Good Use Case |
|---|---|---|
| 10W USB-A or 5V/2A | 8–10 hours | Overnight refill |
| 18–20W USB-C | 4–6 hours | Daytime top-up |
| 30W USB-C | 3–4 hours | Fast turnaround |
| 45W USB-C (PPS/PD) | 2.5–3.5 hours | Quick recycle |
Tips That Cut Charge Time Without Extra Stress
Plug in with a short, well-made USB-C cable. Keep the phone screen off while charging. Take off thick cases that trap heat. If the device offers a charging limit at 80% for daily use, enable it when speed doesn’t matter; turn it off when you need a full tank before a long day.
Can This Size Run A Laptop?
For short spells, yes, if both the pack and the laptop accept the same profiles and the cable is rated for higher current. Expect a light ultrabook to sip 25–40 watts on light work, more under load. That draws down a 74Wh pack within two to three hours of use, not counting conversion losses. Gaming laptops draw far more and will drain the pack fast.
Travel Notes For Flyers
Most 20Ah packs sit under 100Wh, which fits airline rules for carry-on only. Check the watt-hour label on the case. If the math isn’t printed, multiply capacity by 3.7V and divide by 1000. Keep terminals covered, and pack the unit in your cabin bag, not in checked baggage.
Worked Examples (Use These As Templates)
Case A: 5,000mAh Phone, 25W Intake
The handset reaches 50% in about 25–35 minutes, 80% in about 50–70, and full in 70–110. From a fresh 20Ah pack you’ll see around three full refills with screen-off charging and good cables.
Case B: 4,000mAh Phone, 10W Intake
Half fill in 35–45 minutes, 80% in 70–90, and full in 95–130. Expect four to five refills from the pack since the phone battery is smaller.
Case C: Small Tablet, 7Ah Battery
At 18–30 watts, plan on 2–4 hours to full. The pack will often manage one complete refill with some energy left for a phone top-up.
Common Bottlenecks And Simple Fixes
Cable Limits
Thin or worn cables drop voltage under load. Swap to a cable rated for the current your device draws. If the plug feels loose, replace it.
Charger Handshake
Some phones only take higher voltage from brands that match their fast-charge scheme. If your times seem slow, switch to plain PD or PPS on a good USB-C port and skip proprietary bricks when charging from a pack.
Heat
Heat slows charging and wastes energy. Keep the pack and phone on a hard surface with airflow. Avoid stacking them under blankets or pillows.
Quick Formulas You Can Trust
Energy
Watt-hours = milliamp-hours × 3.7 ÷ 1000 for most packs.
Time To Charge A Device
Hours ≈ (battery Wh ÷ charger watts) ÷ efficiency. With wired charging, a simple 0.8–0.9 factor keeps estimates honest.
Bottom Line: What To Expect From This Size
A 20Ah pack is a sweet spot for phones and small tablets. With wired charging and a decent cable you’ll see brisk top-offs, three to five full phone refills, and sensible refill times for the pack itself. Stick to known brands, labeled cables, and the simple math here, and you’ll plan charges with confidence.
Safety note for air travel: see the FAA’s guidance on lithium batteries in carry-ons; packs near 100Wh stay within the usual carry-on limit. Also, for charger and cable capabilities, the USB-IF’s page on USB PD explains the power ranges and voltage steps.