Power bank runtime equals usable watt-hours divided by your device’s watts; most 10,000 mAh units power a 5 W phone for 5–7 hours of active use.
Shoppers want a straight answer: how long will a portable battery actually run your gear? The short version is a clean formula. Usable energy in watt-hours, divided by your device’s average watts, gives hours. The longer story is where real gains live. Capacity labels hide conversion losses, phone behavior shifts minute to minute, and cables or fast-charge modes change the math.
How Long A Portable Charger Runs: Real Numbers
Start with the pack’s printed capacity in milliamp-hours (mAh) and the cell voltage inside the pack, usually around 3.6–3.7 V for lithium-ion cells. Convert to watt-hours (Wh) with this: Wh = (mAh × 3.7) ÷ 1000. Then account for efficiency losses inside the power bank and cable. A safe planning range is 70–85% efficiency for phone-level loads, lower for high-voltage laptop outputs. Last, divide by your device’s draw in watts.
Rule of thumb: Hours ≈ (Wh × efficiency) ÷ device_watts.
Quick Capacity Benchmarks (Phone At 5 W)
The table below uses a 75% efficiency assumption and a steady 5 W draw, which is a realistic mid-use phone load while the screen stays on.
| Labeled Capacity (mAh) | Usable Energy (Wh) | Phone At 5 W (hrs) |
|---|---|---|
| 5,000 | ≈ 18.5 | ≈ 2.8 |
| 10,000 | ≈ 37 | ≈ 5.6 |
| 15,000 | ≈ 55.5 | ≈ 8.3 |
| 20,000 | ≈ 74 | ≈ 11.1 |
| 26,800 | ≈ 99 | ≈ 14.9 |
These hours describe active powering time at a steady 5 W. Real sessions float. A phone idling with the screen off sips 1–2 W. Shooting video, gaming, or tethering can push 7–10 W. That swing explains why two owners of the same pack report different results.
The Simple Equation For Power Bank Hours
Work through one concrete scenario. A 10,000 mAh pack uses a single 3.7 V cell group inside, so it stores about 37 Wh. Set efficiency at 0.8. Your wireless earbuds case pulls roughly 2 W while topping up, a mid-range phone draws around 5 W during maps or streaming, and a compact camera might ask for 7–8 W while charging through USB-C.
- Earbuds case at 2 W:
(37 × 0.8) ÷ 2 ≈ 14.8 hoursof active powering. - Phone at 5 W:
(37 × 0.8) ÷ 5 ≈ 5.9 hours. - Camera at 8 W:
(37 × 0.8) ÷ 8 ≈ 3.7 hours.
Swap in a 20,000 mAh unit and you just double the usable energy, so you double the hours at the same load. That’s the clean benefit of moving up a size.
Why Labeled mAh Never Equals Hours
Three levers change real-world run time: voltage conversion, efficiency, and device behavior.
Voltage Conversion
Power banks store energy at the cell’s native voltage, around 3.6–3.7 V, then boost it to 5 V, 9 V, 12 V, or higher for USB-C fast charge. Watt-hours handle that change cleanly since watts already account for volts × amps. Converting from mAh to Wh first keeps the math honest. Learn more about nominal Li-ion cell voltage from Battery University.
Efficiency Losses
Every boost step wastes a slice of energy as heat. Light loads through 5 V USB-A sit near the high end of the efficiency range. High-voltage PPS or 20 V laptop profiles drop efficiency. Cable resistance costs a little more, especially with thin or long cords. A practical explanation of conversion loss appears in Anker’s rated capacity article.
Device Behavior
Phones vary their draw constantly. Screen brightness, 5G activity, GPS, and camera use raise the watts. Trickle near 100% slows charging on purpose. Laptops shift between idle and turbo clocks. These swings make a single “hours” claim misleading unless you anchor it to an average load.
Finding Your Device’s Watts
You have three practical options. First, read your device’s charge profile on the spec sheet; many phones list up to 15–30 W, while thin laptops show 45–65 W chargers. Second, plug a USB power meter inline to see live volts and amps, then multiply to get watts. Third, time a charge: if a 37 Wh pack sends your phone from 20% to 90% twice, you can back-solve an average draw that day.
Fast Charging Changes The Picture
With USB-C Power Delivery, chargers can supply far more than 5 V × 2 A. Modern adapters and power banks can negotiate 9–20 V and several current steps, so phones hit high rates early, then taper. The peak does not last for hours, but it raises short-term watts, which shortens the active powering time while that peak runs. See the USB-IF overview for power levels and profiles.
Realistic Expectations By Device Type
Phones
At a steady 5 W, a mid-size pack lasts many hours of screen-on navigation or streaming. Gaming loads climb, so budget fewer hours. If you only top up during breaks with the screen off, your pack’s energy stretches far longer than the table predicts.
Tablets And Handhelds
Small tablets pull 10–18 W while charging; gaming handhelds can draw similar power during play. A 20,000 mAh pack still gives several hours of active powering, but not an entire day of heavy use unless you step up to 26.8 Ah or carry two units.
Ultrabooks
Many thin laptops accept 45–65 W over USB-C. Only high-capacity PD banks can keep up at those levels. At 60 W, a 74 Wh bank with 80% efficiency yields around (74 × 0.8) ÷ 60 ≈ 1.0 hour of full-tilt charging. Real usage is nicer because laptops idle and cycle the load.
Estimating With Confidence (And Two Handy Charts)
Use the next table to translate popular bank sizes into hours across common loads. It assumes 80% efficiency. Your numbers may land a bit higher with gentle loads, or lower with high-voltage fast charge.
| Bank Size | Load (W) | Estimated Hours |
|---|---|---|
| 10,000 mAh (≈37 Wh) | 2 W (earbuds/case) | ≈ 14.8 |
| 10,000 mAh (≈37 Wh) | 5 W (phone mid-use) | ≈ 5.9 |
| 10,000 mAh (≈37 Wh) | 8 W (camera/console) | ≈ 3.7 |
| 20,000 mAh (≈74 Wh) | 5 W | ≈ 11.8 |
| 20,000 mAh (≈74 Wh) | 15 W (tablet) | ≈ 3.9 |
| 26,800 mAh (≈99 Wh) | 30 W (big tablet) | ≈ 2.6 |
| 26,800 mAh (≈99 Wh) | 60 W (laptop burst) | ≈ 1.3 |
How To Read A Label And Avoid Guesswork
Spot The Real Energy
Look for watt-hours on the back of the bank. If it only lists mAh, multiply by 3.7 and divide by 1000 to get Wh. Some brands list 3.6 V or 3.7 V as the nominal cell voltage; the difference is small for this purpose.
Match The Output To The Device
USB-A tops out around 12 W. USB-C with Power Delivery negotiates much higher steps, which you need for tablets and laptops. Check the printed output profiles: 5 V, 9 V, 12 V, 15 V, 20 V, along with current limits like 3 A or 5 A.
Watch Efficiency Clues
Vendors sometimes publish “rated capacity” at 5 V that already bakes in conversion loss. If your 10,000 mAh unit lists ~6,000–7,000 mAh “rated,” that reflects normal losses through the 5 V regulator. It is not a defect; it is honest math.
Factors That Shorten Or Extend Runtime
Cable And Accessories
Low-resistance, certified USB-C cables reduce waste and heat. Dumb adapters or worn connectors cause extra drop. Magnetic tips and extenders add a tiny loss; not a deal-breaker, but they shave minutes off long sessions.
Temperature
Cold packs deliver less current. Hot packs throttle for safety. Keep your bank in the shade and give it airflow while fast charging.
Pass-Through And Multi-Port Use
Feeding two devices raises total watts, so hours fall. Pass-through (charging the bank while it charges your phone) adds heat and conversion steps, which lowers efficiency again. Use it when you must, not as a daily habit.
Worked Examples You Can Copy
Maps-Heavy Day With A Mid-Size Phone
Trip plan: screen at medium brightness, 5G on, location services active. Assume 6 W average while navigating, 2 W idle during stops, and a 60/40 split between moving and idle. Average load ≈ 4.4 W. A 10,000 mAh pack at 80% efficiency gives (37 × 0.8) ÷ 4.4 ≈ 6.7 hours of active powering.
Tablet Streaming And Emails
Assume 12 W average. A 20,000 mAh bank at 80% efficiency gives (74 × 0.8) ÷ 12 ≈ 4.9 hours.
Airline Work Session On A 60 W Laptop
Assume peaks at 60 W but a 50% duty cycle thanks to idle stretches. Effective average ≈ 30 W. A 26,800 mAh PD bank gives (99 × 0.75) ÷ 30 ≈ 2.5 hours. If the CPU stays pegged, cut that number in half.
Common Mistakes When Estimating
- Using mAh directly without converting to watt-hours. mAh only makes sense with a stated voltage.
- Assuming 100% efficiency. Add a real-world factor between 0.7 and 0.85 for phone-level use.
- Sizing by peak charger wattage alone. Your device rarely pulls peak power for hours.
- Ignoring cable quality. Thin or damaged cables drop voltage, which lowers charging speed and wastes energy.
- Forgetting airline rules. Keep spare lithium banks under 100 Wh for smoother check-in.
When A Bigger Bank Makes Sense
Move up a class if you often drain below 20% before you can plug in, carry a tablet, or rely on a USB-C laptop. The weight jump from 10,000 to 20,000 mAh is modest next to the doubled energy. For fliers, stay at or below 100 Wh to match common airline limits for carry-on spare batteries.
Quick Checklist Before You Buy
- Watt-hours listed on the label.
- USB-C with PD profiles that match your device.
- Published efficiency or rated capacity at 5 V.
- Good cable included; certified for the current you need.
- Thermal design that stays cool at high output.
Takeaway
Runtime is math, not mystery. Turn mAh into Wh, pick a realistic efficiency, pick your device’s watts, and divide. With those three numbers, you can size a bank that lasts through your day without guesswork.