A 4000 mAh power bank holds about 14.8 Wh and typically delivers 8–12 Wh to devices, good for roughly one phone charge.
Shoppers ask this all the time. The short answer depends on three things: how much energy the bank stores, how much of that energy reaches your gadget after conversion losses, and what your gadget actually draws while charging or running. This guide gives you quick math, realistic ranges, and a simple table so you can estimate runtime without guesswork.
What 4000 mAh Means In Watt-Hours
Milliamp-hours describe charge. To get energy, convert to watt-hours with the standard formula: Wh = (mAh × V) ÷ 1000. Most single-cell packs inside portable chargers use a nominal 3.7 V lithium-ion cell, so 4000 mAh × 3.7 V ÷ 1000 ≈ 14.8 Wh. That figure represents the energy stored in the cell itself, not what appears at the USB port.
| Step | Value | Why It Matters |
|---|---|---|
| Rated capacity | 4000 mAh | Label on the bank |
| Nominal cell voltage | 3.7 V | Typical single-cell lithium-ion |
| Stored energy | 14.8 Wh | 4000 × 3.7 ÷ 1000 |
| Conversion efficiency | ~60–85% | Losses in boost electronics and heat |
| Deliverable energy | ~8.9–12.6 Wh | 14.8 Wh × efficiency range |
Why the drop from 14.8 Wh to a smaller usable number? Power banks boost the cell’s ~3.7 V up to 5–20 V at the port. That step burns some energy, the cable wastes a little more, and the phone’s charging circuit takes a cut. In real life you see something like 60–85% of the stored energy at the device, with top brands landing near the higher end.
4000 mAh Power Bank Runtime — Real-World Factors
Runtime shifts with output voltage, device battery size, screen-on use, cable quality, and ambient heat. A light, cool load over a short cable wastes less. A warm phone running a GPS app while charging wastes more.
Fast Charging And USB-C PD
Modern chargers can raise port voltage above 5 V to move the same energy with less current, which helps cables and heat. USB Power Delivery supports fixed steps beyond 5 V, and even higher levels with EPR. You still pay a little in conversion overhead, but the bigger uptick is speed, not capacity.
Typical Outcomes You Can Expect
With a mid-range phone whose internal battery sits near 10–13 Wh, a 4 Ah pack usually gives about one full recharge. Small earbuds cases might recharge several times. A compact tablet can sip a partial top-up. The table below gives ballpark counts you can adapt to your device.
Estimated Full-Charge Counts By Device Type
Use your device’s battery spec in Wh if you have it. If you only see mAh, multiply by your device’s nominal voltage (phones often list 3.7–3.85 V) and divide by 1000 to get Wh. Then divide the bank’s deliverable Wh by that number.
| Device Type | Typical Battery (Wh) | Estimated Full Charges* |
|---|---|---|
| Earbuds case | 2–3 Wh | 3–5× |
| Smartwatch | 1–2 Wh | 5–8× |
| Phone (compact) | 10–11 Wh | 0.8–1.1× |
| Phone (large) | 12–15 Wh | 0.6–1.0× |
| Handheld console | 16–18 Wh | 0.5–0.7× |
| Small tablet | 18–25 Wh | 0.4–0.6× |
*Based on 8.9–12.6 Wh delivered to the device from a 4 Ah bank.
Quick Math: Turn Any Device Spec Into An Estimate
Step 1 — Convert The Power Bank Rating
Stored energy: 4000 mAh × 3.7 V ÷ 1000 ≈ 14.8 Wh.
Step 2 — Apply A Realistic Efficiency
Pick a middle value near 75–80% if you have a decent brand and a short cable. Usable energy: 14.8 Wh × 0.8 ≈ 11.8 Wh.
Step 3 — Compare To Your Device
If your phone has a 12.5 Wh pack, 11.8 ÷ 12.5 ≈ 0.94. That means close to one full charge from empty. If you plug in at 30% instead of zero, you’ll reach full once with a little buffer left.
Why Manufacturers Quote mAh And Not Wh
mAh looks friendly on a box, but energy is what matters. Two packs with the same mAh can hold different energy if their cell voltages differ. Using Wh avoids that trap and makes airline limits and safety rules easier to read.
Does Fast Charging Change The Total?
Higher voltage profiles don’t give you more energy; they mainly shorten charging time. They can trim cable losses a bit. Net delivered Wh stays set by the stored energy and the efficiency of both ends.
How Long In Hours? Convert Wh To Runtime
If you power a small USB light at 1 W from a 4 Ah pack that delivers 11.8 Wh, the light runs near 11.8 hours. A 2 W fan would run near 5–6 hours. A 5 W camera would get about 2 hours. The table below lists common loads.
| Load (W) | Estimated Runtime (h) | Notes |
|---|---|---|
| 0.5 W | 18–25 | Very light draw |
| 1 W | 9–13 | USB light, tracker, sensor |
| 2 W | 4–6 | Mini fan, action cam trickle |
| 5 W | 1.7–2.5 | Slow phone charge |
| 10 W | 0.8–1.2 | Fast phone charge burst |
Ways To Get Closer To The High End Of The Range
- Use a short, thick cable. Less resistance means less heat loss.
- Charge with the screen off. Background draw eats capacity.
- Keep both battery and phone cool. Heat raises losses.
- Avoid pass-through charging chains. Each hop wastes energy.
- Stop at 80–90% when you only need a top-up. The last few percent run slower and waste more.
How To Read Your Device’s Battery Spec
Phone listings may show mAh at 3.7–3.85 V. Multiply and divide to get Wh. Many spec sheets also give video playback hours; those vary with brightness and signal quality and don’t translate cleanly to charging math. Trust Wh for capacity comparisons.
Airline And USB Standards You Should Know
Airlines regulate packs by watt-hours, not mAh. Quick math: 4000 mAh at 3.7 V equals about 14.8 Wh, which sits well under the common 100 Wh carry-on limit. USB-C PD defines fixed voltage steps and higher ceilings for power flow, which affects speed, not the energy inside your pack.
Simple Calculator You Can Reuse
Formula
Deliverable Wh ≈ (mAh × 3.7 ÷ 1000) × efficiency. Estimated full charges ≈ deliverable Wh ÷ device Wh.
Worked Example With A Large Phone
Pack: 4000 mAh → 14.8 Wh. Assume 80%: 11.8 Wh. Phone: 12.9 Wh. Charge count: 11.8 ÷ 12.9 ≈ 0.91. If you start at 20%, you’ll likely reach full once.
Troubleshooting If Your Results Look Low
- Old cells sag under load. Capacity falls with age and cycles.
- Poor cables or loose ports waste energy. Try another lead.
- Apps running in the background keep the phone warm and hungry. Flip on airplane mode while topping up.
- Hot weather compounds losses. Shade helps.
- Some displays show “percent charged” that rises fast early and slows near full. That pattern is normal and skews a seat-of-pants read.
When A 4 Ah Pack Makes Sense
It slips into a pocket and covers a day of heavy messaging with one full phone refill. Travellers who shoot long video, play games, or run tethering may want 10K or 20K models; those sizes add weight, but they give headroom for tablets and consoles.
Key Takeaways You Can Act On
- A 4 Ah bank stores ~14.8 Wh and commonly delivers ~9–12.6 Wh to devices.
- That equals about one full phone charge, several earbud case fills, or a small top-up for a tablet.
- Use the Wh math to compare models and to check airline rules.
- Short cables, cool temps, and sensible charging habits stretch each pack.
Sources and further reading: Anker’s Wh↔mAh conversion guide and the USB-IF overview of USB Power Delivery.