A 20,000 mAh power bank yields about 60–70 Wh usable, which means 3–5 phone refills or hours of runtime that depend on the device’s watt draw.
A 20,000 mAh pack looks huge on the label, yet the real question is how much time it can keep your gear running. The short answer comes from two ideas: energy in watt-hours (Wh) and the power your device pulls in watts (W). Once you convert milliamp-hours to watt-hours and estimate efficiency losses, you can map that energy to hours. This guide shows clear math, ranges you can trust, and quick tables you can use before a trip or a busy workday.
What 20,000 mAh Means In Practice
Most power banks use one or more lithium-ion cells rated around 3.6–3.7 volts. Capacity on the box is measured at that cell voltage, not the 5V, 9V, 12V, 15V, or 20V you see on the USB port. To compare across devices, convert to energy: Wh = (mAh × V) ÷ 1000. For a 20,000 mAh pack at 3.7V, that’s roughly 74 Wh on paper.
Usable Energy After Conversion Losses
Converters inside the bank step cell voltage up to your device’s request. That step burns some energy as heat. Quality banks land near 85–90% under light to medium loads; older or budget units may sit closer to 70–80%. Applying an 85% midline to 74 Wh yields about 63 Wh of usable energy. That’s the number to use for planning.
20,000 mAh Power Bank Runtime: Real-World Ranges
Once you have usable energy, time is a simple division: hours ≈ usable Wh ÷ device W. Phones sip between 3–6 W during screen-on web use; tablets hover near 6–12 W; ultraportable laptops can draw 15–30 W when browsing and much more during heavy work. Game consoles and cameras sit in the middle. The table below shows common cases so you can ballpark quickly.
| Device Type | Typical Battery (Wh) | Full Charges From ~63 Wh |
|---|---|---|
| Smartphone (4,500–5,000 mAh) | 17–19 Wh | 3–4x |
| Small Phone (3,000–3,500 mAh) | 11–13 Wh | 4–5x |
| Tablet 8–11″ | 25–40 Wh | 1–2x |
| Handheld Console | 15–20 Wh | 3–4x |
| Mirrorless Camera | 7–16 Wh | 4–8x |
| Ultrabook Laptop | 40–60 Wh | ~0.8–1.5x (when charging off) |
Why Ranges Beat Single Numbers
Two phones with the same battery can run for very different times because screen size, refresh rate, radios, and chip efficiency all change the draw. A laptop at idle browsing may sip 10–15 W; streaming with high brightness might double that. That’s why planning with ranges sets more realistic expectations.
Step-By-Step: Estimate Your Runtime
Here’s a quick method you can reuse for any device. Grab the device’s typical watt draw or battery size. Do the math once, then keep a sticky note inside your bag.
1) Convert Capacity To Watt-Hours
Multiply the label capacity by 3.7V and divide by 1000. A 20,000 mAh unit becomes 74 Wh. If you want a refresher on energy units, see Battery University on energy terms.
2) Apply Efficiency
Use 80–90% for a solid bank and short, quality cables. Take 85% as a middle case unless you know the brand and tests. That turns 74 Wh into about 63 Wh usable.
3) Map To Your Device
If your phone battery is 5,000 mAh at ~3.85V (about 19 Wh), expect around three full refills from a healthy 20,000 mAh bank. If your tablet is 30 Wh, expect two refills. If your laptop runs near 20 W during light work, 63 Wh buys a little over three hours; at 30 W, just a bit above two hours.
What Changes The Result
Cable Quality And Length
Thin or long leads add resistance. That raises heat and wastes energy. Keep cables short and rated for the current you need, especially for fast-charge profiles.
Converter Efficiency And Heat
High output voltage and high current stress step-up circuits. Under those loads, many banks fall toward the low-80s or high-70s for efficiency. In a hot car or direct sun, losses grow further.
Device Behavior Near Full
Phones and laptops taper charge near 80–100% to protect the battery. That taper stretches time without adding many watt-hours. Topping off from 85% to 100% can eat minutes with little net energy moved.
Background Tasks
Navigation, gaming, uploads, or video calls can double a phone’s draw. If you want the bank to refill instead of hold steady, flip Airplane Mode or close heavy apps while charging.
Math Behind The Label
Why does a 20,000 mAh pack not yield four times the runtime of a 5,000 mAh phone? Two reasons: different voltages and conversion losses. Capacity in mAh is measured at the cell level (~3.7V). Your phone’s battery is also a ~3.8V pack, yet the charge happens at 5–9V on the USB line. The bank boosts voltage, then the phone steps it back down to charge the internal cell. Each step wastes a slice. Looking at energy in Wh lines up both sides and keeps expectations realistic.
Fast Charging, USB-C PD, And What You Can Power
USB-C Power Delivery negotiates voltage and current between the bank and your device. Modern PD adds fixed steps like 5V, 9V, 15V, and 20V and goes far beyond old 5V-only bricks. Some banks also offer PPS, which fine-tunes voltage to reduce heat at high current. A bank that can deliver 45–65W opens the door to many thin-and-light laptops, while 18–30W keeps phones and tablets happy. The spec overview is published by the USB-IF here: USB Power Delivery.
For a quick check, read the print near the USB-C port or the spec sheet. If the bank lists 20V profiles, it can top up a laptop that accepts USB-C charging. If it tops out at 12V or 15V, you may still run many ultrabooks at reduced speed, but not power-hungry models.
| PD Output | Max Watts | Typical Uses |
|---|---|---|
| 18–20W (9V/2A, 12V/1.5A) | ~20 W | Phones, small tablets, cameras |
| 30W (15V/2A or PPS) | 30 W | Tablets, handheld consoles, light laptops idle |
| 45–65W (20V/2.25–3.25A) | 45–65 W | Ultrabooks, compact Chromebooks |
Worked Examples You Can Copy
Phone With 5,000 mAh Battery
Battery energy ~5,000 mAh × 3.85V ÷ 1000 ≈ 19 Wh. From a bank with ~63 Wh usable, expect about 3 full refills. With screen-on charging at 6 W, you might see the bank hold the phone near 100% for 10 hours instead.
Tablet With 30 Wh Battery
Two full refills are realistic. During video playback at 8–10 W, the same bank can keep the tablet running for 6–7 hours without dipping into the tablet’s battery.
Laptop Drawing 25 W Over USB-C
63 Wh ÷ 25 W ≈ 2.5 hours of extra light-work time if the bank supports 20V PD. If the laptop spikes to 45 W during installs or compiling, runtime drops toward 1.3 hours.
Rules Of Thumb For Planning
- Assume usable energy is 80–90% of the printed Wh. Pick 85% unless you have test data.
- If a device lists only mAh, convert to Wh with its own voltage (often ~3.8V for phones, 7.6V for small tablets, 11.4V or 15.2V for larger tablets).
- For time estimates, match the bank’s usable Wh to your device’s average watt draw, not peak.
- Bright screens, 5G hotspots, and gaming chew through energy quickly. Dim the screen and close heavy apps to stretch time.
- Short, thick USB-C cables waste less power at high current.
When A 20,000 mAh Bank Is Enough
For weekend travel with one phone and a small tablet, a 20,000 class pack is roomy. It covers a couple of full refills and leaves headroom for earbuds or a watch. Photographers who rotate camera batteries in the field will also be covered. Students with thin laptops are fine if the bank can deliver 45–65W PD and the work is light.
When You Need More
If you run power-hungry laptops, stream while tethered, or charge several phones at once, move up to 26,800–30,000 mAh models with higher PD wattage. The extra capacity bumps usable energy to ~85–100 Wh, which doubles laptop time and gives more ports to share.
Care Tips That Protect Runtime Over Months
A power bank that stays cool and lives between ~20–80% during storage keeps its cells healthier. Avoid deep depletion, avoid leaving it on a hot dashboard, and give it a light top-up monthly if it sits in a drawer. Gentle habits preserve both capacity and peak output.
Source Notes
Energy math uses the standard relation between charge, voltage, and energy: Wh ≈ mAh × V ÷ 1000, which matches electronics references and calculators. USB-C PD capabilities and voltage steps come from the official standard. Battery care guidelines reflect widely cited lithium-ion behavior under partial charge and moderate temperatures.