With 70–85% efficiency, a 4,000mAh pack delivers ~10–12Wh—about 6–20 hours for small loads or roughly one phone charge.
A 4,000mAh portable pack sounds straightforward: bigger number, more runtime. In practice, the answer depends on voltage conversion, efficiency, the device you’re powering, cable quality, and how you use the pack while it’s charging something. This guide gives you the quick math, sample runtimes, and honest expectations so you can plan a commute, a flight, or a day trip without guesswork.
Quick Formula You Can Use
Power banks store energy in watt-hours (Wh), even when the label shows milliamp-hours (mAh). To estimate runtime, convert to Wh, account for efficiency, then divide by your device’s power draw.
Step-By-Step
- Convert capacity to Wh: Wh ≈
capacity_mAh × 3.7 ÷ 1000(most packs use 3.7V lithium cells). - Account for losses: usable_Wh ≈
Wh × efficiency(typical 0.70–0.85 from cell to USB output). - Estimate runtime: hours ≈
usable_Wh ÷ device_watts.
Why 3.7V? That’s the common nominal voltage for lithium-ion cells; energy accounting in Wh avoids confusion with 5V USB output. For clarity on units, see Samsung SDI’s short explainer on mAh vs Wh, which matches this approach (mAh and Wh basics).
Plug-In Example
4,000mAh → Wh ≈ 4,000 × 3.7 ÷ 1000 ≈ 14.8Wh. With 75% overall efficiency, usable ≈ 11.1Wh. If a device draws 2W, runtime ≈ 11.1 ÷ 2 ≈ 5.5 hours. A lighter 0.5W draw can stretch beyond 20 hours.
4000mAh Power Bank Runtime Estimates With Real-World Losses
The table below shows broad scenarios using a 4,000mAh pack and ~75% efficiency. Your exact number shifts with efficiency, temperature, cable quality, and whether you use the device while charging.
Estimated Runtime By Use Case
| Use Case | Typical Draw (W) | Estimated Hours |
|---|---|---|
| Fitness Band / Earbuds Case | 0.2–0.5 | 22–55 |
| Small USB Fan (Low) | 1.0 | 10–12 |
| E-reader / Low-Power Light | 1.5 | 7–8 |
| Phone Idle Top-Up | 2.0 | 5–6 |
| Phone In Use (Screen On) | 3.0–4.0 | 3–4 |
| Compact Action Cam | 4.0–5.0 | 2–3 |
These ranges start from usable energy of ~10–12Wh (70–85% efficiency). For a hands-on way to sanity-check load vs. runtime, Digi-Key’s battery life calculator uses the same capacity-and-current logic many engineers rely on (battery life calculator).
How Many Phone Charges To Expect
Phone batteries vary from ~3,000mAh up to ~6,000mAh. Charging isn’t 1:1 against label numbers because energy moves from a 3.7V cell to 5V USB, then to your phone’s battery, with conversion losses in each step. Quality packs land near 70–85% overall efficiency end-to-end, which aligns with public vendor guidance across the industry.
Ballpark Charge Counts
To convert quickly, think in Wh: a 4,000mAh bank stores ~14.8Wh; usable is ~10–12Wh. Many modern phones sit roughly between 12–20Wh. That means a small phone can get close to one full charge, while a large model may see a solid top-up but not a full cycle.
Approximate Full Recharges By Phone Size
| Phone Battery Size | Approx Recharges | Notes |
|---|---|---|
| 3,000–3,500mAh | 0.9–1.1× | Near full from low single digits |
| 4,000–4,500mAh | 0.6–0.9× | Full if phone off; less if screen stays on |
| 5,000–6,000mAh | 0.5–0.7× | Good top-up, not a full cycle |
Why A 4,000mAh Label Doesn’t Equal 4,000mAh At USB
Voltage Mismatch
The internal cell sits at ~3.7V nominal, while USB delivers 5V or higher with fast-charge protocols. Energy stays in Wh; current (mAh) resettles once voltage changes. This is why energy math in Wh stays clean and predictable.
Conversion Losses
DC-DC conversion, heat, cable resistance, and the phone’s charging circuitry eat into the headline number. Reputable brands publicly place end-to-end efficiency in the 70–85% range, which lines up with user measurements and lab tests.
Temperature And Age
Lithium cells deliver less at low temps and degrade with cycles and time. A new pack on a warm day looks better than an older pack on a cold platform. Expect a gentle slide across months of use.
Worked Examples You Can Adapt
Case A: Wireless Earbuds Case
An earbuds case that sips ~0.3W: usable ~11Wh ÷ 0.3 ≈ 36 hours of case runtime. That might cover many top-ups across a week.
Case B: Phone Navigation In The Car
Maps with screen on can pull ~3–4W. With ~11Wh available, expect 3–4 hours of sustained offset for battery drain. If the phone was at 30%, you’ll crawl toward full, but screen time trims the count.
Case C: Compact Camera Or Action Cam
Many action cams hover near 4–5W while recording. Expect 2–3 hours of extra record time, less with high frame rates or stabilization cranked up.
Picking The Right Pack Size
A 4,000mAh unit is a lightweight backup for a day out. If you need true multi-day coverage or several phone refills, bump to 10,000–20,000mAh. Brands often state realistic charge counts for common phones; match those listings to your model’s battery size before buying.
What Efficiency Number Should You Assume?
Use 0.75 as a safe middle ground for cable-to-phone. Top-tier models can land closer to 0.85 in gentle conditions; budget models can dip nearer to 0.70. That single assumption swings your estimate more than any other variable. If you want a vendor perspective on ranges, Anker’s guidance routinely quotes figures in this band for everyday use (capacity vs delivered mAh).
USB Modes That Change The Math
Standard 5V USB
Most small loads sit here. Assumptions above fit this case well.
Fast-Charge Protocols
Quick Charge, USB PD, and vendor modes bump voltage (9V, 12V, 20V). Higher voltage can boost transfer at lower current, but cell-level energy and end-to-end losses still rule. In other words, fast doesn’t add energy; it moves the same energy sooner, with similar or slightly higher losses under heavy load.
Wireless Charging
Qi introduces extra loss. Expect fewer watt-hours to reach the battery than over a cable. If your goal is runtime, prefer a short, decent cable over a pad when you can.
How To Measure Your Own Draw
Want confidence beyond estimates? A small USB power meter shows live voltage and current. Plug the meter in-line with your device and watch watts change with screen brightness, camera use, or gaming. Once you see an average number in watts, divide usable Wh by that average, and you’ll have a tight forecast for your gear.
Practical Tips To Stretch Runtime
Use A Short, Quality Cable
Thin, long cables waste energy as heat. A short, thicker cable trims loss and keeps both pack and phone cooler.
Charge While The Device Sleeps
Screen-off charging lands closer to those upper estimates. Heavy use while charging drags efficiency down.
Avoid Full-To-Empty Cycles
Top-ups between 20–80% put less stress on cells and often deliver a better feel of “always enough” during a day out.
Watch Ambient Temperature
Cold slows chemistry. Keep the pack in a bag or pocket when you’re outdoors in winter.
Troubleshooting Short Runtimes
If You’re Seeing Way Less Than Expected
- The pack is aging: capacity drops as cycles add up.
- Draw is higher than assumed: screen brightness, GPS, or a camera can pull several watts.
- Cable loss: swap to a short, known-good cable.
- Wireless loss: switch to wired charging when you need every watt-hour.
FAQ-Style Clarifications (No FAQ Markup)
Does A 4,000mAh Pack Charge A 5,000mAh Phone Fully?
Not from near empty. With ~10–12Wh usable, it delivers a strong top-up, but large phones carry ~18–20Wh batteries. Expect roughly half to two-thirds of a full cycle, more if the phone is off during charging.
Why Does “mAh At 5V” Look Lower Than The Label?
Because current depends on voltage. The label reflects 3.7V cell capacity. After boost to 5V, the same energy translates to a smaller mAh number. Accounting in Wh keeps the math consistent with how batteries store energy.
What About Tablets Or Handheld Consoles?
Those loads often sit between 6–12W when the screen is on. With ~11Wh usable, you’re looking at about an hour to a couple of hours of extra play, depending on brightness and game load.
Copy-Ready Calculation Template
Paste this into a notes app and swap numbers for your gear:
Bank capacity: 4,000mAh
Cell voltage: 3.7V
Estimated efficiency: 0.75
1) Wh = 4000 × 3.7 ÷ 1000 = 14.8Wh
2) Usable Wh = 14.8 × 0.75 = 11.1Wh
3) My device draws: ____ W
4) Runtime ≈ 11.1 ÷ ____ W = ____ hours
Bottom Line
A 4,000mAh power bank is a light, pocketable safety net. Expect ~10–12Wh of usable energy in real use. That covers many hours for tiny loads, a solid top-up for large phones, and close to one full cycle for compact phones. If you need multiple full phone charges or you plan to run a bright screen for long stretches, step up to 10,000–20,000mAh. If you want to validate your own numbers, lean on a simple engineering calculator and think in watt-hours, not just mAh—this keeps estimates honest and repeatable.