A 15,000mAh power bank typically yields about 35–50Wh usable, which is 1–2 phone refills or roughly 5–9 hours at a steady 10W.
Shoppers want a straight answer, but real runtime depends on energy, losses, and how fast your gear drinks power. This guide shows clear math, realistic ranges, and quick ways to stretch every watt-hour.
How Long Does A 15,000mAh Charger Last In Real Use?
Start with energy, not just milliamp-hours. Most packs use a single lithium cell around 3.6–3.7V. Converting mAh to watt-hours gives a number you can compare across devices: Wh = (mAh × V) ÷ 1000. For a 15,000mAh cell at 3.7V, stored energy is about 55.5Wh. After conversion to 5V and normal losses, usable energy lands near 35–50Wh.
To turn energy into hours, divide by the power your device draws. For a steady 10W accessory, 40Wh lasts around 4 hours. For a phone that averages 3–6W while charging, the same pack can deliver multiple top-ups spread through the day.
Quick Estimates You Can Trust
Use these ballpark figures as a starting point. The table assumes a realistic 70–90% conversion range and typical device behavior.
| Device | Typical Battery | Estimated From 15,000mAh |
|---|---|---|
| Smartphone (4,000–5,000mAh) | 15–19Wh | ~1.5–2.5 full refills |
| Small Phone (3,000mAh) | 11Wh | ~2–3 refills |
| Tablet (7,000–8,000mAh) | 26–30Wh | ~1–1.5 refills |
| Nintendo Switch (4,310mAh) | ~16Wh | ~1.5–2 refills or 6–10h play |
| Bluetooth Headphones (400–1000mAh) | 1.5–3.7Wh | Many refills; multi-day use |
| USB Desk Fan (5V, 2W) | — | ~18–25 hours |
| Action Cam (5–7W while running) | — | ~6–8 hours |
The Math Behind Those Hours
Step 1: Convert mAh To Watt-Hours
Energy is voltage times charge. A 15,000mAh cell at 3.7V stores roughly 55.5Wh. That one line lets you compare a power bank with phones, tablets, lights, and even small fans that list wattage.
Step 2: Account For Conversion Losses
Power banks step the cell’s ~3.7V up to USB output. Boost circuits and cables waste some energy as heat. Many quality packs deliver around 80–90% efficiency in ideal spots; budget gear can land closer to 60–75% once heat, fast-charge overhead, and cable losses add up.
Step 3: Estimate Your Load
Phone charging draw swings with battery level, screen time, apps, and network. A 20W fast burst may taper to a few watts as the phone nears 100%. Accessories like lights or fans draw steady power, so hours are easy to predict.
Runtime Formula You Can Reuse
Use this quick formula for any device:
Estimated hours = (mAh × 3.7 × efficiency) ÷ (1000 × device watts)
Pick efficiency from 0.6 (rough) to 0.9 (great). If your device reports amps at 5V instead of watts, multiply V × A to get watts.
Worked Scenarios
10W speaker: (15000 × 3.7 × 0.8) ÷ (1000 × 10) ≈ 4.4 hours.
Phone refills: If a phone holds 17Wh (about 4500mAh at 3.8V), usable 40Wh gives around ~2 refills after losses in both the bank and the phone.
Camping light at 2W: 44.4Wh at 0.8 efficiency ≈ 35.5Wh usable; 35.5 ÷ 2 ≈ ~17–18 hours.
What Shrinks Or Stretches Runtime
Charge Protocol And Voltage
USB-A 5V lines are simple but often less efficient at higher currents. USB-C with USB Power Delivery can match device needs with flexible voltage steps, which helps reduce conversion waste and heat at the same wattage.
Cable Quality And Length
Thin or long cables drop voltage under load. The bank compensates with extra current, which eats into stored energy. Use a short, well-made lead, especially for fast-charge sessions.
Fast Charging Behavior
Fast modes front-load current, then taper. The rush phase warms both ends and can lower efficiency. If runtime matters more than speed, a moderate 10–15W setting often goes farther per charge.
Device Battery Size
Big batteries need more energy to reach 100%. Phones around 3,000mAh can see two to three top-ups; 5,000mAh flagships rarely get more than two.
Ambient Temperature
Cold conditions slow chemical reactions and reduce usable capacity; extreme heat forces throttling. Store and use the pack near room temperature for best results.
Realistic Expectations For Popular Use Cases
Here’s how a mid-tier 15,000mAh model behaves in common scenarios, assuming healthy cables and normal temps.
Commuting And Busy Workdays
Think of it as a day-and-a-half buffer. You can top up a modern phone at lunch and again in the evening, with spare energy for earbuds or a smartwatch.
Travel Days
Long flights and maps drain phones faster. Expect one full refill for a big phone plus side charges for a second device. If you carry a tablet, you’ll likely stop at a single solid top-up.
Camping And Festivals
Pair the bank with a low-draw light and you’ll get a few nights of camp lighting plus daily phone boosts. Gamers or action-cam users will burn the tank quicker.
Capacity Labels And Why mAh Can Mislead
The big printed number matches the internal cell at its native voltage. When that energy is boosted to 5V or higher, the figure in mAh must drop to conserve watts. That’s why some brands publish both cell capacity and a lower “rated capacity” at 5V.
Tips To Make 15,000mAh Go Farther
- Charge at moderate speed when time allows.
- Turn off the screen while charging; avoid gaming during top-ups.
- Use short, thick USB-C cables for high current.
- Keep the pack out of hot cars and direct sun.
- Top up more often instead of running every device flat.
How Many Phone Refills Should You Expect?
With 35–50Wh usable, expect around two full refills for a 4,500–5,000mAh phone, or two to three for a compact 3,000–3,500mAh model. If you split energy across two devices, you’ll see fewer complete refills but longer shared uptime.
Charge Speed And Heat
High wattage raises temperature in the bank and the phone. Heat wastes energy and can age batteries faster. When runtime matters, pick a balanced profile over the absolute fastest setting.
Typical Performance Ranges
The ranges below reflect real-world spreads across brands, cables, and loads.
| Factor | Effect On Runtime | Simple Fix |
|---|---|---|
| Converter Efficiency (60–90%) | Lower efficiency cuts hours | Use quality brands; moderate watts |
| Cable Resistance | Voltage drop forces extra current | Short, thick cables |
| Fast-Charge Protocol Mismatch | Extra heat and losses | Use matching USB-C PD chargers |
| Device Draw While In Use | Screen, GPS, and games raise watts | Charge while idle when possible |
| Temperature | Cold reduces capacity; heat throttles | Room-temp storage and use |
Step-By-Step: Calculate Your Own Hours
1) Find The Cell Energy
Multiply 15,000 by 3.7, then divide by 1000. That yields 55.5Wh stored in the cell.
2) Pick A Realistic Efficiency
Use 0.8 for a decent pack and cable, 0.7 if the bank gets warm or the cable looks thin, 0.9 with top-tier gear at modest watts.
3) Get Your Device’s Watts
Many gadgets print volts and amps near the port. Multiply to get watts. If you only know battery size, divide usable Wh by that number to estimate refills.
4) Run The Numbers
Hours ≈ (55.5 × efficiency) ÷ device watts. Refills ≈ usable Wh ÷ device battery Wh.
Troubleshooting Short Runtime
Hot Pack Or Phone
Heat signals wasted energy. Drop the charge rate, swap to a shorter cable, and give the setup airflow.
Saggy Cable
If the phone flickers between charging and not charging, the line drop is high. Replace the cable and retest.
Fast Charge Only Works Once
Some banks limit peak output when the cell is low. Begin the day with the bank topped up so fast modes stay available longer.
Specs Decoder For Smarter Buying
Cell Capacity Vs. Rated Capacity
Cell capacity is the big mAh number at ~3.7V. Rated capacity is the practical figure at 5V after conversion. Both can be honest; they describe different points in the chain.
Output Power
Ports labeled 20W, 30W, or higher fill devices faster but may trim runtime per charge at the extreme. For long days, aim for speed that runs cool.
USB-C Features
Look for PPS or PD support if your phone uses it. Matching the protocol reduces waste, keeps temps in check, and helps the pack deliver more watt-hours to your device.
Method And Sources
This guide converts mAh to Wh with the standard formula and uses public data on USB protocols and typical converter efficiency. For a refresher on the math, see this concise mAh → Wh formula. For charging standards, see the USB-IF page on USB Power Delivery.
Buying Takeaways
If you want a day of confidence for a large phone plus extras, 15,000mAh is a sweet middle ground. If your setup includes a tablet or a gaming handheld, consider stepping up, or carry two mid-size packs so one can charge while the other rides along.