Power bank runtime depends on energy (Wh), device watts, and losses; a 10,000 mAh unit runs a 5 W load for about 4–5 hours.
Figuring out how long a portable battery will keep your gear running comes down to simple math and a few real-world variables. You only need three things: the bank’s stored energy, the power your device draws, and a realistic efficiency number. This guide turns those into clear hours you can count on.
Power Bank Runtime: Hours You Can Expect
Start with the energy in the battery. Most packs list capacity in milliamp-hours (mAh) at a cell voltage around 3.7 V. Convert that to watt-hours (Wh) so you can divide by the watts your device uses. Then apply an efficiency factor to account for voltage conversion and heat.
Formula You Can Trust
Wh = (mAh × 3.7) ÷ 1000 → Hours ≈ (Wh × efficiency) ÷ device watts
Many brands acknowledge that only about 60–70% of the rated energy reaches your phone, tablet, or laptop because the pack must boost voltage to USB levels and your device then steps it down again. A 65% default keeps estimates honest. Anker’s support page explains this practical 60–70% delivery range in plain language; it’s a handy benchmark for sizing a bank for trips and outages.
Capacity To Real Runtime At A Glance
| Power Bank Label (mAh) | Effective Energy (Wh @ 65%) | Rough Phone Recharges* |
|---|---|---|
| 5,000 | 12 Wh | ~1–1.5× |
| 10,000 | 24 Wh | ~2–3× |
| 20,000 | 48 Wh | ~4–6× |
| 30,000 | 72 Wh | ~6–9× |
*Assumes phones with 3,000–5,000 mAh internal batteries. Actual counts vary with screen time, apps, and signal strength.
How The Math Works Step By Step
1) Convert Capacity To Watt-Hours
A 10,000 mAh pack uses cells around 3.7 V, so energy is (10,000 × 3.7)/1000 ≈ 37 Wh. You now have apples-to-apples energy for any device, not just phones.
2) Account For Conversion Losses
Portable batteries use boost converters to raise cell voltage to USB levels, then many devices step it again to charge internal packs. Heat and electronics eat a slice of energy, so plan on ~65% reaching the device. Some high-efficiency designs do better; aging gear can do worse. Anker’s support team summarizes this as “about 60–70% delivered” in normal use (see the explanation).
3) Divide By Your Device’s Watts
Pick a realistic watt number. Small LED lights sip around 2 W. USB fans sit near 5 W. Many tablets pull 8–12 W while charging. Compact laptops can sit at 20–35 W when browsing and spike higher under heavy load. With Hours ≈ (Wh × 0.65) ÷ W, you can plug in anything.
Worked Examples You Can Copy
10,000 mAh Pack → Phone Streaming
Energy: 37 Wh; effective to device: ~24 Wh. If your phone averages 5 W while streaming video with the display on, runtime from the bank is 24 ÷ 5 ≈ 4.8 hours. If the phone’s draw drops to 3 W (audio screen-off), you’re closer to 24 ÷ 3 ≈ 8 hours.
20,000 mAh Pack → Tablet Browsing
Energy: 74 Wh; effective: ~48 Wh. A tablet drawing 9 W gets 48 ÷ 9 ≈ 5.3 hours. Step up to a game at 12 W and you’ll see 48 ÷ 12 = 4 hours.
30,000 mAh Pack → Compact Laptop
Energy: 111 Wh; effective: ~72 Wh. A light laptop at 20 W gets around 72 ÷ 20 ≈ 3.6 hours; a heavier workload at 35 W trims that to about 72 ÷ 35 ≈ 2.1 hours. USB-C Power Delivery can supply these watt levels when both charger and device negotiate the same profile.
USB-C negotiation matters for bigger loads. The official USB Power Delivery spec allows up to 240 W in the newest extended profiles, so runtime then depends on the bank’s stored energy and the watts your gear requests, not the connector type.
Why Two Banks With The Same mAh Can Feel Different
Not every label tells the full story. Two 20,000 mAh packs can deliver different hours because one uses higher-quality converters, thicker cabling, or a smarter charging curve. Temperature and age also change results. Expect fewer hours if the pack is hot, cold, or heavily worn.
Conversion Quality And Cables
Boost and buck stages waste less energy when they run in their sweet spot. Short, good-gauge cables help too by lowering voltage drop. If your device insists on a higher charging voltage via fast-charge standards, the converter works harder and shaves minutes.
Device Behavior While Charging
A phone that’s charging while you scroll pulls energy for the battery and for the system. Background sync, GPS, and bright screens nudge watts up. Airplane mode, battery saver, or dimmer screens nudge watts down. Same bank, different hours.
Age, Cycles, And Storage
Lithium-ion slowly loses capacity across charge cycles and with time. Light packs often ship near nominal specs for the first few months, then drift down. Gentle storage temps and partial charge during long storage slow the slide.
Table: Estimated Hours By Common Loads
The table below uses a realistic 65% efficiency and two popular sizes to give you a fast planning tool.
| Device Load (W) | 10,000 mAh Bank (hrs) | 20,000 mAh Bank (hrs) |
|---|---|---|
| 2 W (LED light) | ~12 | ~24 |
| 5 W (USB fan) | ~4.8 | ~9.6 |
| 10 W (big tablet) | ~2.4 | ~4.8 |
| 20 W (light laptop) | ~1.2 | ~2.4 |
Phone-Only “Extra Hours” Method
Another quick way to set expectations is to compare the bank’s effective Wh to your phone’s internal battery Wh. If your handset carries roughly 12 Wh and your bank delivers ~24 Wh to the device, you can expect around two phone-worths of extra screen-on time at the usage level that drains one internal battery. Power-hungry games or hotspot use will shrink the gain; reading or music playback will stretch it.
Wireless Charging Cuts Runtime
Inductive charging adds extra conversion steps on both sides of the link. That lowers delivered energy and warms the pack. If hours matter, plug in with a cable. If convenience matters more, plan on a smaller number and bring a size up.
Accuracy Tips And Simple Tools
- Grab a tiny USB-C meter to read volts and amps. Multiply those for watts and you’ll see real draw in the moment.
- Sample at idle, during a browse, and during your heaviest task. Average the three and your runtime estimate will be close.
- Re-test once a season. Aging cells and OS updates can shift power draw.
Troubleshooting Short Runtime
Fast-Charge Negotiation Loops
Some devices nudge voltage up and down while they hunt for a profile. Each step wastes a little energy. If hours are falling short, try a normal-speed charger mode in settings, or use a cable and port that stick to a steady profile.
Weak Or Long Cables
High-resistance wires cause voltage sag that the bank tries to compensate for, raising current and heat. Swap in a shorter, certified cable and re-check your meter.
Cold Or Hot Packs
Low temps slow chemistry; high temps raise internal resistance. Keep the pack near room temperature while charging and while powering devices for the most stable results.
Picking The Right Size For Your Day
Pocket Days: 5,000–10,000 mAh
Great for phones and earbuds. Expect a couple of phone recharges or a few hours of steady 5 W power for a fan or camera rig. Slim packs fit in pockets and pass easily through most venue checks.
Work Trips: 20,000–26,800 mAh
A sweet spot for tablets and light laptops. Many models ship with USB-C PD that can hold 20–45 W. You’ll keep a small notebook alive on flights, power a router during an outage, or run a mirrorless camera all afternoon.
Field Use: 30,000 mAh And Up
Handy for drones, projectors, or bigger laptops. Be sure the bank’s Wh falls within transport rules if you plan to fly. Look for higher PD ratings, two-way USB-C, and clear screens that show watts in and out.
Pro Tips To Stretch Runtime
- Charge the bank and the device to healthy levels before heading out; topping off reduces peak watts later.
- Use the shortest cable that reaches, from a brand with low resistance conductors.
- Match the bank’s PD profile to your device; lower, stable watts can beat spiky fast-charge peaks for runtime.
- Keep both bank and device close to room temperature; extremes cut capacity.
- Avoid pass-through charging during mission-critical use; conversions stack and waste energy.
Realistic Expectations And Claims
Marketing blurbs often quote ideal numbers from lab tests with near-idle devices. The hours you see in daily use depend on your apps, radios, and screen brightness. When comparing, rely on watt-hours and the simple formula rather than charge-count slogans.
Final Tally And Handy Formula
If you remember just one thing, make it this: convert to Wh, apply a realistic efficiency, then divide by the watts your gear pulls. That gives you hours you can plan around, no guesswork. Copy the line below into your notes:
Hours ≈ ((mAh × 3.7 ÷ 1000) × 0.65) ÷ device watts
For bigger loads that need more than basic USB, check your device and charger for the PD profiles they speak. USB-C Power Delivery supports high watt levels, which explains why some packs can keep compact laptops going for hours.