A 4,000mAh power bank stores about 15Wh; expect roughly one phone charge or 2–3 hours at 5W, depending on efficiency and cable losses.
Shoppers ask this all the time because labels can be confusing. A pack marked 4,000 milliamp-hours holds energy at the cell’s native voltage (about 3.7V for most lithium-ion). Your phone charges at a higher, regulated output (5V via USB, or 9–20V with USB-C PD). Some energy gets lost when the pack boosts voltage and manages heat, so the usable portion is lower than the printed figure. This guide translates that number into hours of run time and likely phone charges, with clear math and practical tips.
Fast Answer With Realistic Math
Here’s the quick way to set expectations for a 4,000mAh pack:
- Energy in watt-hours (Wh) ≈ 4,000 × 3.7 ÷ 1,000 ≈ 14.8Wh.
- Usable energy (wired): assume about 80–85% pack-to-phone efficiency → ~11.8–12.6Wh available at the USB port.
- Run time at a given load: hours ≈ usable Wh ÷ device watts. At 5W that’s ~2.4–2.5 hours; at 10W it’s ~1.2–1.3 hours.
Those ranges align with how power banks step 3.7V cells up to USB output and the typical efficiency losses documented by mainstream power-bank makers and battery references.
Runtime Table For Common Loads
The table below assumes a 4,000mAh pack (~14.8Wh raw) with 85% conversion to the output port (~12.6Wh usable). Wireless adds more loss; see later tips.
| Output Power (W) | Approx Runtime (hh:mm) | What This Covers |
|---|---|---|
| 2.5W | ~5:00 | Low-draw wearables, earbuds cases |
| 5W | ~2:30 | Standard USB-A phone trickle/idle top-ups |
| 7.5W | ~1:40 | Light navigation, social apps |
| 10W | ~1:15 | Screen on, streaming music |
| 15W | ~0:50 | Faster USB-C phone charging |
| 20W | ~0:37 | PD bursts; small tablets |
Why The Label mAh Doesn’t Equal Hours
Milliamp-hours measure charge. Watt-hours measure energy. To compare runtime across devices and outputs, convert to Wh: Wh = mAh × V ÷ 1,000. Most single-cell packs use ~3.6–3.7V cells, so a 4,000mAh unit sits near 14–15Wh before losses. Battery references and industry guides use the same formula and note that energy-based comparisons are more reliable for real-world use.
How Long A 4,000mAh Pack Can Power Your Phone
Phone batteries now cluster around 3,000–5,000mAh at 3.8V class cells. Your pack’s usable 11.8–12.6Wh translates to roughly one full charge for compact phones and a strong top-up for larger models. Fast-charge modes raise voltage on purpose; that shortens the clock time yet doesn’t change the underlying energy math.
If your phone supports USB-C PD, it negotiates output (say 9V at 2A for ~18W). PD standards permit much higher power for bigger gear, up to laptop levels, but a 4,000mAh unit doesn’t hold enough energy to run those long.
Real-World Factors That Shorten Or Extend Runtime
Conversion Efficiency
Inside the bank, a boost converter raises ~3.7V to the target USB voltage. That step and cable resistance waste energy as heat. Mainstream makers explain the drop: your “rated” mAh at cell voltage becomes lower at 5V output, and the pack delivers only a percentage of its raw Wh. Typical figures land in the low-to-mid 80s for decent hardware.
Load And C-Rate
Heavier loads reduce effective capacity a bit because voltage sags more under high current. Battery references describe this with C-rate language: 1C would drain a 1Ah cell in one hour, C/2 in two hours, and so on. The higher the C-rate, the lower the delivered energy.
Wireless Charging Penalty
Qi coils add conversion loss. Expect fewer watt-hours reaching the phone compared with a cable, so the same pack yields fewer charges wirelessly. (Plan on a visible drop even with modern coils.)
Screen-On Use
Using maps, games, or a camera while charging raises the phone’s own draw. The bank must cover both topping up the battery and the live load on the SoC and display, so “hours on cord” can be shorter than the math suggests for a sleeping phone.
Estimate Your Own Runtime In Two Steps
1) Convert To Wh
Energy ≈ mAh × 3.7 ÷ 1,000 → for 4,000mAh, ~14.8Wh. (Battery references illustrate the same math with 3.6–3.7V examples.)
2) Apply Efficiency And Divide By Watts
Usable Wh ≈ 14.8 × 0.85 ≈ 12.6Wh. Run time ≈ 12.6Wh ÷ device watts. A 5W draw → ~2.5 hours; a 10W draw → ~1.25 hours; a 15W draw → ~50 minutes. That’s the cleanest way to turn a printed capacity into time on task.
How Many Phone Charges To Expect
To estimate full charges, compare the pack’s usable Wh to the phone’s battery Wh. Phone battery Wh ≈ phone mAh × 3.85 ÷ 1,000 (many phone cells sit near 3.8–3.85V nominal). Then factor cable and phone intake losses.
| Phone Battery Size | Estimated Full Charges (Wired) | Notes |
|---|---|---|
| 3,000mAh | ~1.1–1.3× | One full charge with a little left |
| 4,000mAh | ~0.8–1.0× | Often lands near a full charge |
| 5,000mAh | ~0.6–0.8× | Strong top-up; not a full refill |
Those ranges reflect the typical 80–85% pack-to-phone efficiency window explained by leading brands. Larger phones with bright displays or high refresh rates tilt to the low end, while smaller models tilt to the high end.
Fast-Charge Notes And What They Mean For Time
USB-C Power Delivery can raise voltage in steps (9V, 15V, 20V) to move energy faster. The energy budget in Wh doesn’t change, but the clock time does. A small pack can finish a top-up sooner at 9V/2A than at 5V/2A, yet the total watt-hours delivered stays bounded by the pack’s usable Wh. USB-IF’s public brief confirms PD power levels up to 240W for bigger equipment; for phones you’ll see common profiles around 18–30W.
Travel Tip: Airline Limits Use Watt-Hours
If you fly with any power bank, airline and security rules measure by Wh, not mAh. Packs up to 100Wh ride in cabin bags without approval in most cases. A 4,000mAh unit is tiny in Wh terms (~15Wh), so it’s well under common limits. Check the official chart before a trip for the latest ranges and approvals.
Ways To Stretch Each Charge From A Small Pack
Choose A Cable That Matches The Port
Poor cables waste energy as heat. Short, quality USB-C cables help the converter hold voltage with less loss. If your phone and bank both support PD, use a certified Type-C to C cable for steadier delivery.
Charge While Idle
Plug in when the phone is dozing. Heavy, screen-on use steals watts that could have gone into the battery.
Stop Wireless When You Need Every Wh
Coils add loss. Switch to wired when you’re low and need the most miles out of a small bank.
Aim For Mid-Level Top-Ups
Keeping a lithium-ion phone near the middle of its range is easy on the cell and avoids waste at the top end. Battery references point out that shallow cycles are kinder than deep ones.
Worked Examples You Can Copy
Example A: Streaming On A Phone (About 5–7W)
Usable energy ≈ 12.6Wh. At 6W average while watching video, time ≈ 12.6 ÷ 6 ≈ 2.1 hours. If the screen is bright and the phone pulls 8W, time drops near 1.6 hours.
Example B: Maps In The Car (About 7–10W)
Navigation, GPS, data, and screen push draw up. Expect ~1.2–1.8 hours from the same pack while routing.
Example C: Wearables And Earbuds (About 1–3W)
Light loads stretch time. Charging a watch or earbuds case at 2W can run for four to five hours before the pack taps out.
What To Look For If You Buy Or Upgrade
Wh Printed On The Label
Some packs print Wh next to mAh. That saves you a conversion step and makes airline checks simple. When comparing two small banks, prefer the higher Wh, not just the higher mAh.
Honest Efficiency And PD Support
Brands that explain 3.7V-to-5V conversion and quote realistic efficiency usually design better power stages. If your phone speaks PD, a bank that supports PD at 18–30W can refill faster during a quick stop. For the PD power ladder straight from the standards body, see the USB-IF overview.
Travel-Friendly Capacity
Even midsize units like 10,000–20,000mAh stay under 100Wh, but always check the airline chart. It’s the clean, official source for what’s allowed in cabin bags.
Key Takeaways
- A 4,000mAh bank holds ~14.8Wh raw; wired usable often lands near 12Wh due to conversion.
- At 5W, plan on about two to three hours; at 10W, a little over an hour.
- For phones, expect about one full charge for small batteries and a strong top-up for larger ones.
- Cables, fast-charge voltage, and screen-on use shift results up or down in the real world.
Method Notes And Sources
All calculations here use the standard energy relation Wh = mAh × V ÷ 1,000 with a 3.6–3.7V nominal cell assumption found in battery references, and PD capability details referenced from the standards body. Airline limits come from the official passenger battery chart. For a plain-English explainer on conversion losses inside power banks, see a mainstream maker’s help article.
Related references you may find helpful: the USB-IF overview of USB-C PD power levels (USB Power Delivery) and the FAA passenger battery guidance (FAA PackSafe chart).