Power bank runtime ranges from a few hours to days of phone use, based on capacity, device watt draw, charging losses, and battery age.
A portable pack stores energy in a 3.6–3.8 V cell, then boosts it to USB output. Some energy is lost on that boost and again when your phone or laptop takes the charge. Tally the usable watt-hours, divide by your device’s watts, and you’ll have a grounded estimate. The sections below make that painless with a chart, a quick formula, and clear examples.
What Actually Sets The Runtime
Four levers decide how long a single charge from a portable pack will last:
- Capacity on the label (mAh) and the cell’s nominal voltage (about 3.7 V) define stored energy.
- Conversion efficiency on the step-up converter and your device’s charge circuit. Real-world packs deliver only part of the labeled energy to the device.
- Device power draw in watts while you use it. A phone idling at 1–2 W sips; gaming at 6–8 W gulps.
- Age, temperature, and cable quality nudge the result up or down.
The Quick Formula
Usable Wh ≈ (mAh × 3.7 ÷ 1000) × efficiency
Estimated hours ≈ Usable Wh ÷ device watts
For most packs, an efficiency band of 0.70–0.85 covers real use. You’ll see this play out in the chart below and in the worked examples later.
Capacity-To-Use Cheat Sheet (Phones)
This broad table lands within the first third so you can act fast. It assumes a common phone battery of 11–19 Wh (about 3,000–5,000 mAh at 3.7 V) and real-world efficiency between 70–85%. Recharges are rounded.
| Pack Size (mAh) | Usable Energy (Wh) | Typical Phone Recharges |
|---|---|---|
| 5,000 | 13–16 | ~1–1.3 |
| 10,000 | 26–31 | ~2–2.7 |
| 20,000 | 52–63 | ~4–5 |
| 27,000 (airline limit ~100 Wh) | 70–85 | ~5–7 |
| 30,000+ | 78–94 | ~6–8 |
How Long A Portable Charger Lasts In Practice
Use the formula once, and you’ll never guess again. Three quick scenarios show the range you can expect.
10,000 mAh Pack → Phone
Stored energy ≈ 10,000 × 3.7 ÷ 1000 = 37 Wh. With 80% real-world delivery, usable energy ≈ 29.6 Wh. A mid-range phone drawing 3 W during mixed use gets ≈ 9–10 hours of screen-on time from the pack; a heavier 6 W load lands near 4–5 hours. Topping up a 4,500 mAh phone while it sleeps yields roughly two full refills.
20,000 mAh Pack → Tablet
Stored ≈ 74 Wh; usable ≈ 52–63 Wh. A tablet at 7–10 W adds ≈ 5–8 hours of live use.
Large Pack With USB-C PD → Laptop
Many modern packs offer USB-C PD. At 25 W average, a ~63 Wh usable pack adds about 2–2.5 hours; at 45 W for edits, about 1–1.4 hours.
USB-IF lists PD levels up to 240 W for compatible gear; that sets the ceiling on what a pack and cable can negotiate rather than a promise of runtime. See the official USB Power Delivery overview for the current fixed voltages and limits.
Why Your Pack Never Feels Like “Full Label”
Packs are labeled in milliamp-hours at the cell’s voltage, not at the USB output. A single Li-ion cell sits near 3.7 V, while USB starts at 5 V. A step-up converter and the phone’s own charge circuit both waste some energy as heat. That’s why a 10,000 mAh unit rarely delivers a full 10,000 mAh at 5 V to a phone.
Major brands publish the math behind this. Anker explains that boosting from ~3.7 V to 5 V and device-side losses mean the “rated” output is smaller than the printed capacity. Their guide cites an efficiency near 82% for one model and gives the exact calculation that converts cell capacity to 5 V output. Read the note titled Why the rated capacity looks smaller.
Typical Efficiency Bands
- Lean designs or heavy loads: around 70% end-to-end.
- Good mainstream packs: near 80–85%.
- Quality cells and tidy thermals: up to the mid-80s in light use.
Estimate Your Own Runtime (Step-By-Step)
- Find the mAh on the pack.
- Convert to watt-hours: mAh × 3.7 ÷ 1000.
- Apply a fair efficiency: pick 0.75 for a safe estimate, 0.85 for best case.
- Divide by your device’s watts during the task you care about.
Example: a 20,000 mAh pack → 20,000 × 3.7 ÷ 1000 = 74 Wh. With 0.8 efficiency, usable ≈ 59 Wh. If your handheld gaming device averages 10 W, expect ≈ 5–6 hours.
Device Power Draw Reference
Numbers reflect common loads. Your exact draw varies with screen brightness, radios, and workload.
| Device / Task | Approx Watts | Hours On A ~60 Wh Usable Pack |
|---|---|---|
| Phone, idle music (screen off) | 0.5–1 | 60–120 |
| Phone, web + maps | 3–6 | 10–20 |
| Tablet, video streaming | 7–10 | 6–8.5 |
| Switch-style handheld gaming | 8–12 | 5–7.5 |
| Laptop, light work | 20–30 | 2–3 |
| Laptop, photo edits | 35–50 | 1.2–1.7 |
| Mirrorless camera via USB | 3–7 | 8.5–20 |
| Bluetooth earbuds case | 0.2–0.5 | 120–300 |
Factors That Shorten Or Stretch Runtime
Fast-Charge Overhead
Higher voltage modes move energy faster but waste a little more as heat. You trade minutes for a small hit to total hours.
Brightness, Refresh Rate, And Radios
Drop screen brightness, set a lower refresh rate, and pause 5G tethering when you don’t need it. Those three alone can double phone hours on the same pack.
Temperature And Aging
Cold cells sag. Packs kept near full charge at high heat also age faster. For the pack and the device, partial charges and moderate temps tend to preserve capacity, a point echoed by long-running battery guides from industry educators.
Cables And Connectors
Loose connectors and skinny cables waste energy. Use short, certified cables, and keep the USB-C contacts clean.
Picking The Right Size For Your Use
If You Carry Only A Phone
Choose 5,000–10,000 mAh. That’s a single day’s peace of mind or a full weekend with light use. Slim packs slip into a pocket and charge a phone fast at 18–30 W.
If You Pack A Tablet Or Handheld Console
Pick 15,000–20,000 mAh with 30–45 W USB-C. You’ll add 5–8 hours of real use or two tablet refills, and you can top off a phone at the same time.
If You Need Laptop Power
Go 20,000–27,000 mAh with 60–100 W USB-C PD and the right cable. Check your laptop’s required profile first. The USB-IF summary linked above lays out the voltage steps packs and notebooks can negotiate.
Simple Tips To Stretch Every Charge
- Start charging near 20–40% to avoid heat spikes at the ends.
- Keep the pack and phone out of direct sun while charging.
- Use airplane mode in dead zones; radios burn watts while hunting.
- Disable high refresh on long travel days.
- Charge small items first (earbuds, watch), then the big device.
FAQ-Free Bottom Line
Runtime isn’t a mystery. Convert mAh to watt-hours, apply a fair efficiency, then divide by your device’s watts. Phones get hours to days from a single pack charge. Tablets land in the mid-hours. Laptops gain one to a few hours depending on draw. With the quick math above and the two charts, you can pick the right pack today and know exactly what to expect from the next full charge.