A power bank usually needs 2–8 hours to recharge, shaped by capacity, charger wattage, and whether it supports USB-C PD or QC input.
This guide shows you how to predict power bank refill time with plain math, real-world ranges, and setup tips that cut hours off the wait. You’ll get estimates for common sizes, learn the variables that matter, and pick the right wall charger and cable for faster, safer top-ups.
Power Bank Charge Time: Factors And Math
Charging speed comes down to energy stored versus power going in. Capacity is expressed in milliamp-hours (mAh), which converts to watt-hours (Wh) using Wh = (mAh × V) ÷ 1000. Most packs use cells around 3.7V nominal, so a 20,000mAh unit holds about 74Wh. Input power is the charger’s watt rating multiplied by what the bank actually negotiates over USB-C PD or Quick Charge. Real devices often run a little below the brick’s label due to protocol limits, cable loss, and heat management.
Use this back-of-the-envelope equation to estimate refill time:
Charge time (hours) ≈ Battery energy (Wh) ÷ Effective input power (W).
Effective power usually lands below the port’s headline number. With smart charging, the bank adjusts voltage and current across the session, tapering near full. Expect high power early, then a slower finish to protect the cells.
Typical Ranges You Can Expect
Small packs (5,000–10,000mAh) often finish in 2–6 hours with a decent charger. Mid-sizes (15,000–20,000mAh) take 4–9 hours. Large packs above 25,000mAh can need 8 hours or more unless you feed them high-watt USB-C PD input. Some high-end models accept 45W, 65W, or even higher bidirectional USB-C input, cutting time sharply with the right brick.
Estimated Times By Size And Charger
The table below uses a 90% efficiency factor and 3.7V nominal cell voltage. Times assume you’re not using the bank while it recharges and that the charger sustains the listed power.
| Bank Size | Charger Power | Estimated Full Charge |
|---|---|---|
| 10,000mAh (~37Wh) | 10W (5V⎓2A) | ≈ 4–5 hours |
| 10,000mAh (~37Wh) | 18–20W (USB-C PD/QC) | ≈ 2–3 hours |
| 20,000mAh (~74Wh) | 10W (5V⎓2A) | ≈ 8–9 hours |
| 20,000mAh (~74Wh) | 18–20W (USB-C PD/QC) | ≈ 4–5 hours |
| 20,000mAh (~74Wh) | 30W USB-C PD | ≈ 2.5–3 hours |
| 26,800mAh (~99Wh) | 20W USB-C PD | ≈ 5–6 hours |
| 26,800mAh (~99Wh) | 45W USB-C PD | ≈ 2–3 hours |
| 30,000mAh (~111Wh) | 20W USB-C PD | ≈ 6–7 hours |
| 30,000mAh (~111Wh) | 65W USB-C PD | ≈ 2–3 hours |
What Counts As “Fast Input” On A Power Bank?
Two names pop up the most: USB-C Power Delivery and Qualcomm Quick Charge. USB-C PD is the open standard that now reaches very high watt levels, while many QC-branded inputs also negotiate higher voltages and currents. Plenty of modern banks accept PD, and many accept both. Look for USB-C input with PD 20W or higher for a clear speed bump over old 5V-only micro-USB inputs.
USB-C PD In Plain Terms
USB-C PD lets devices and chargers agree on a voltage/current combo rather than staying fixed at 5V. Common steps include 9V, 12V, 15V, and 20V, with newer PD versions adding even higher levels. That flexibility delivers more watts through the same cable, which shortens refill time when the bank supports it. See the USB-IF Power Delivery overview for the official outline of how PD scales.
Quick Charge Basics
Quick Charge is a proprietary set of profiles, widely supported on older phones and many accessories. Later versions moved closer to PD behavior. Many current chargers and banks advertise both PD and QC for broad compatibility.
How To Find Your Real-World Time
You can turn the specs on the label into a good estimate in a minute or two. Grab three numbers: the bank’s capacity in mAh (or Wh), the input spec on the bank (often printed near the port), and your charger’s max wattage on the matching port. Use the math from earlier to turn capacity into Wh and divide by input watts. Add a small buffer because charging slows near the top.
Step-By-Step Mini Method
- Convert capacity to energy: Wh = (mAh × 3.7) ÷ 1000.
- Take the lower of your charger’s watt rating and the bank’s input rating.
- Divide Wh by that watt number.
- Pad the result by 10–20% to account for taper and overhead.
Example Walkthrough
Say your pack is 20,000mAh. Energy ≈ (20,000 × 3.7) ÷ 1000 ≈ 74Wh. Your bank lists 20W USB-C input, and your brick can deliver 30W. The lower number rules, so 74Wh ÷ 20W ≈ 3.7 hours. Add a modest buffer and you’re looking at roughly 4 hours when conditions are good.
Why Your Result Might Differ
Lab math is tidy; charging is not. Temperature, cable quality, and whether you’re using the bank at the same time all shift the outcome. Some packs also limit input power once the cells warm up. Others ship with slower micro-USB inputs that cap at 10W. The quickest gains usually come from pairing a PD-capable bank with a PD brick that meets or slightly exceeds the input rating on the label. Cold packs charge slowly; hot packs throttle. Keep it at room temp and in the open while it fills.
Fast-Track Setups That Save Hours
These pairings produce the biggest time wins for common sizes.
| Setup | When It Helps | Time On A 20,000mAh Bank |
|---|---|---|
| USB-A 5V⎓2A (10W) | Old chargers and micro-USB inputs | ~8–9 hours |
| USB-C PD 20W | Most modern mid-size banks | ~4–5 hours |
| USB-C PD 30–45W | Newer banks with higher input ratings | ~2–3 hours |
| USB-C PD 65W+ | High-end models that accept high input | ~2–3 hours, sometimes less |
Safety And Cell Health While Charging
Lithium-ion chemistry prefers moderate charge rates and avoids extreme heat. Fast sessions are fine when the product supports them, but you’ll get better longevity if you don’t push the pack to the edge every time. Aim for well-matched gear, keep the bank on a hard surface while filling, and avoid pass-through use on small packs unless the maker says it’s OK. For reference, the Wh figure used in airline rules comes from capacity × voltage ÷ 1000; see the IATA guidance.
Why Your Charger’s Watt Number Doesn’t Always Show Up
Chargers print the total watts across all ports, but the port you use may cap lower. A 65W brick can devote only 20–30W to one USB-C when other ports are active. Many multi-port models follow fixed split charts, so check the label near the outlets or the product page. If a test run seems slow, unplug other devices and move the cable to the highest-power Type-C port.
Quick Troubleshooting For Slow Refill
- Wrong cable: swap in a known good USB-C cable rated for 60W or 100W. Thin or damaged leads kill negotiation.
- Shared power: try a single-port session on the charger.
- Low input spec: some banks only accept 10W on micro-USB. Use the USB-C input if present.
- Hot pack: move it to a cool, shaded spot and give it airflow.
- Pass-through use: if it’s charging a phone while filling, expect longer times or cycling.
Bottom Line On Picking A Charger
Match or exceed the bank’s listed input with a quality USB-C PD wall charger. For small to mid sizes, 20W is a sweet spot. For large packs, 30–45W pays off. Bring the right cable and keep things cool, and your refill window shrinks from a long wait to a short break. For the formal overview of PD scaling, see the USB-IF page.