Run time depends on watt-hours, output draw, and efficiency; a 10,000 mAh pack delivers about 6–7 hours at a steady 5-watt load.
A 5-volt power bank can keep a device going for hours, but the exact number hinges on a few clear variables: the pack’s stored energy in watt-hours, the load your gadget actually pulls, conversion losses inside the boost converter, and any cable or device overhead. This guide shows the math step-by-step, offers quick reference tables, and flags the caveats that shorten (or lengthen) real-world results.
5-Volt Power Bank Runtime—Real-World Method
Most packs list capacity in milliamp-hours at a cell’s nominal voltage (usually 3.7 V for a Li-ion pouch or 18650 cell). To get practical run time at USB output, convert that figure to watt-hours, account for losses, then divide by your device’s average watt draw.
Core math: Wh = (mAh × 3.7) / 1000 → apply efficiency (typical 80–90%) → Hours ≈ (Wh × Efficiency) / Load_Watts.
Example with a steady 5 W draw: a labeled 10,000 mAh pack has about (10,000 × 3.7)/1000 = 37 Wh. At 85% overall efficiency, usable energy is 31.5 Wh. Run time is 31.5 ÷ 5 ≈ 6.3 hours.
Quick Reference: Labeled Size Vs. Usable Energy
Use this broad table to sanity-check what a pack can supply after conversion losses. The “5 V mAh Equivalent” helps when a device lists needs in mA at 5 V.
| Labeled Capacity (mAh @3.7 V) | Usable Energy (Wh @85% eff.) | 5 V mAh Equivalent* |
|---|---|---|
| 5,000 | 15.7 | 3,140 |
| 10,000 | 31.5 | 6,300 |
| 15,000 | 47.2 | 9,440 |
| 20,000 | 63.0 | 12,600 |
| 26,800 | 84.5 | 16,900 |
*5 V mAh Equivalent ≈ Usable Wh × 1000 ÷ 5
Why The Label Rarely Matches Hours You See
Boost Conversion And Heat
Cells sit around 3.7 V but your port supplies 5 V or higher with USB-C PD. The step-up circuit wastes a slice of energy as heat. Quality packs come in near 90% under moderate loads; cheaper units drop closer to 80%. That gap alone can swing an hour or more on common sizes.
Cable Drop And Negotiation Overhead
Thin or long cables add voltage drop. Devices also sip power while negotiating charge rates, lighting screens, and running radios. Those small extras add up during long sessions.
Temperature And Age
Cold slows chemical reactions and trims available capacity. Hot storage accelerates aging. Over a year or two, you can lose a chunk of usable energy even if the pack still works.
Step-By-Step: From Box Specs To Hours
- Read the label. Find mAh and assume 3.7 V nominal unless the maker lists watt-hours directly.
- Convert to Wh. Multiply mAh by 3.7 and divide by 1000.
- Pick a fair efficiency. Start at 85% for a decent pack; adjust up or down based on build quality and load.
- Estimate your device draw. Many phones average 4–7 W while charging; small tablets land near 8–12 W; gaming handhelds climb higher during play.
- Divide. Usable Wh ÷ load watts = expected hours at that draw.
If your pack supports USB-C PD at higher voltages, energy in Wh doesn’t change, but current at the cable does. That can cut cable losses and improve stability at bigger loads. The USB-IF outlines the range PD can offer, up to 240 W on capable gear; phone-class packs sit far lower but still gain from smart negotiation. See the official USB Charger (USB PD) page for the envelope and fixed voltage steps.
Worked Examples With Common Loads
10,000 mAh Pack, Steady 5 W Gadget
Wh = 37.0 → 85% → 31.5 Wh usable → Hours ≈ 31.5 ÷ 5 = 6.3 hours.
20,000 mAh Pack, Small Tablet Averaging 9 W
Wh = 74.0 → 85% → 62.9 Wh usable → Hours ≈ 62.9 ÷ 9 ≈ 7.0 hours.
26,800 mAh Pack, Handheld Console At 12 W
Wh = 99.2 → 85% → 84.3 Wh usable → Hours ≈ 84.3 ÷ 12 ≈ 7.0 hours.
Picking A Fair Efficiency Number
Why 85%? Bench tests across brand-name packs land near that mark under 5–12 W loads. Lighter draws nudge higher; heavy loads dip. Aging cells, warm weather, and bargain electronics shave more. If your first charge session ends short, drop your efficiency figure by a few points and re-run the math.
Does Capacity In mAh Tell The Whole Story?
mAh looks large on the box, but watt-hours capture the real stash of energy. Two packs with the same mAh can carry different Wh if cell voltage differs. When a product lists Wh, you can skip conversions entirely during planning.
Charge Speed Vs. Run Time
Fast output doesn’t create energy; it just moves it faster. If a console drinks 15 W from a small pack, hours fall. The trade-off is convenience. USB-C PD can hold a stable higher voltage that keeps draws smooth under load, which helps bigger devices stay powered while running.
How Many Phone Charges Is That?
Instead of hours at a steady watt draw, you might care about full phone cycles. Take the phone’s battery in Wh and divide into the pack’s usable Wh. A modern handset sits roughly between 12–20 Wh. So a fresh 20,000 mAh unit with ~63 Wh usable lands near three full cycles on a 18–20 Wh device, assuming screen-on charging and overhead that trims a slice each fill.
Real-World Estimates By Load And Pack Size
Use this table as a quick planner once you know your device’s average draw during use. Numbers assume 85% pack efficiency and steady-state charging.
| Pack Size (mAh @3.7 V) | Average Load (W) | Estimated Hours |
|---|---|---|
| 10,000 | 5 | ≈ 6.3 |
| 10,000 | 8 | ≈ 3.9 |
| 10,000 | 12 | ≈ 2.6 |
| 20,000 | 5 | ≈ 12.6 |
| 20,000 | 9 | ≈ 7.0 |
| 26,800 | 12 | ≈ 7.0 |
Ways To Stretch Run Time
Match The Cable And Port
Use a short, well-made cable rated for your charge rate. A PD-capable cable on a PD-capable port holds voltage steady, which trims waste and heat at higher draws.
Charge While Idle
Set the device down during charging. Screen, camera, GPS, and gaming all inflate watt draw, stealing hours from the pack.
Stay In The Comfort Zone
Room-temperature storage and operation protect capacity. Avoid leaving the pack in a hot car, and don’t fast-charge immediately after a heavy discharge session.
Avoid Deep Sleep Loss
Packs lose a little charge each month. Topping up every few months keeps the cells healthy and the gauge honest. Battery care guides point to cycle life dropping with heat and deep discharges, so a gentle routine pays off over time. For a broader primer on care and aging, see the well-known Li-ion longevity guide.
Troubleshooting Shorter-Than-Expected Hours
The Pack Feels Warm
Warmth means conversion losses. Step down the load if you can, swap to a thicker cable, or move to a port that negotiates a higher voltage for the same power so current falls.
The Device Pulls In Bursts
Some gadgets jump between low and high draws. Average wattage can exceed your guess. A USB power meter reveals the real profile across a session.
The Label Lists Only mAh
Convert to Wh and use the method above. If the maker lists Wh directly, rely on that figure instead of back-calculating from mAh.
FAQ-Style Clarifications Without The Fluff
Does A Higher Output Port Make It Last Longer?
No. Output modes change voltage and current delivery, not total energy. Pick the mode that keeps your device stable; the energy pie stays the same.
Will A Heavier Load Hurt The Pack?
Staying within the rated port power is fine. Pushing a tiny pack near its ceiling for hours warms the electronics and trims efficiency, so total hours fall compared with a lighter draw.
Can Two Devices Charge At Once?
Sure, but both eat from the same pool. Add the watt draws together and divide into usable Wh to estimate shared run time.
Planning Cheats You Can Trust
- 5 W phone use: 10,000 mAh ≈ 6 hours; 20,000 mAh ≈ 12–13 hours.
- 8–10 W small tablet: 10,000 mAh ≈ ~4 hours; 20,000 mAh ≈ ~7–8 hours.
- 12–15 W handheld gaming: 20,000 mAh ≈ ~5–7 hours depending on brightness and Wi-Fi.
Once you test your own device and see a repeatable average, you can forecast like a pro with one quick division.
Method And Constraints
All estimates assume healthy cells near room temperature, steady draws, and a fair 85% conversion figure. Real sessions drift as radios wake, screens brighten, and heat rises or falls. Treat the tables as planning tools. If your pack lists watt-hours, plug that value in place of the converted figure for tighter results. For charge profiles and PD voltage steps, the USB-IF PD reference gives the official range. For care and cycle-life patterns, the Li-ion longevity guide offers a broad overview rooted in lab work and field use.
Bottom Line Math You Can Reuse
If you remember one set of steps, let it be this: convert labeled mAh to Wh at 3.7 V, multiply by a realistic efficiency, then divide by the watts your device actually draws during use. Pick a better cable, charge while idle, and store the pack cool to make those hours hold.