A 20,000mAh power bank typically delivers about 3–5 full phone charges, depending on efficiency and your device’s battery size.
Shoppers want to know what a 20,000mAh pack can do on a trip or during a long day. The short answer depends on two things: how much energy the pack can actually deliver to your device and how large your device’s battery is. Below you’ll find quick estimates, the math behind them, and clear tables you can use before your next flight or festival.
How Many Full Charges From A 20,000mAh Pack: Real Math
Portable chargers list capacity in milliamp-hours at the cell’s nominal voltage (lithium cells sit around 3.7V). Your phone, tablet, earbuds, or camera doesn’t charge at 3.7V; the pack’s electronics step that up to 5V (or 9V/12V with USB-C PD). That conversion wastes a bit of energy, so the usable portion is lower than the label.
The Quick Formula You Can Use
Step 1: Convert the pack’s label to watt-hours: Wh = (mAh × 3.7) ÷ 1000. For 20,000mAh, that’s about 74Wh.
Step 2: Apply a fair efficiency range for DC-DC conversion and cable losses. Good packs usually land near 80–90% usable energy.
Step 3: Divide by your device’s battery energy (Wh). If you only know mAh for your device, compare mAh with the same efficiency assumption; it gives a close read.
Typical Outcomes At A Glance (20,000mAh Pack)
Assuming ~85% conversion, you get roughly 62.9Wh of usable energy from a 20k unit. That translates to around 3–5 phone refills, depending on the model.
Estimated Full Charges By Popular Phones
| Device | Battery Size | Est. Full Charges* |
|---|---|---|
| iPhone 15 | ~3,349mAh (12.98Wh) | ~4.8 |
| iPhone 15 Pro | ~3,274mAh (12.70Wh) | ~5.0 |
| Galaxy S24 | ~4,000mAh | ~4.2 |
| Galaxy S24+ | ~4,900mAh | ~3.5 |
| Galaxy S24 Ultra | ~5,000mAh | ~3.4 |
*Assumes a healthy 20k pack, ~85% conversion, and stock cables. Real-world results vary with screen time, background tasks, and ambient temperature.
Why The Label Isn’t The Whole Story
Manufacturers rate portable cells at their internal voltage. Your devices charge at higher output levels (5V base, or 9V/12V/20V with Power Delivery). Moving between voltages costs energy. The pack’s control board, cables, and your device’s charging circuit all sip from the same pie, so the end-to-end number you see is lower than the headline mAh.
Watt-Hours Beat Milliamp-Hours For Comparisons
If you want the cleanest apples-to-apples check, convert both the pack and your device to watt-hours with Wh = (mAh × V) ÷ 1000. That formula is widely used in battery spec sheets and calculators, and it keeps the math honest when voltage differs between sources and loads. You can see the same formula explained by a major battery brand’s guide here: mAh to Wh conversion.
Efficiency: What A Good Range Looks Like
Modern, well-designed USB-C power banks often deliver ~80–90% of their rated cell energy to your device under typical loads. That range lines up with independent tests and brand engineering notes. Quality of the DC-DC converter, cable gauge/length, and load current all influence where you land on that spectrum.
Worked Examples You Can Follow
Example 1: Using Watt-Hours (Best Way)
Pack: 20,000mAh at 3.7V → 74Wh. With 85% conversion, usable energy ≈ 62.9Wh.
Phone A: iPhone 15 battery sits around 12.98Wh from its regulatory listing. 62.9 ÷ 12.98 ≈ ~4.8 full charges.
Phone B: iPhone 15 Pro near 12.7Wh → 62.9 ÷ 12.7 ≈ ~5.0 full charges.
If you want Android numbers, manufacturers publish mAh directly. You can convert to Wh or keep the mAh comparison for quick planning. Samsung lists 4,000mAh, 4,900mAh, and 5,000mAh across S24 models, which lands you in the 3.4–4.2 charge range from a healthy 20k unit.
Example 2: Using Milliamp-Hours (Fast Estimate)
Skip voltage and accept a small margin of error. Take the pack’s mAh × efficiency, then divide by your phone’s mAh.
- Usable mAh ≈ 20,000 × 0.85 = 17,000mAh.
- On a 5,000mAh phone, expect ~3.4 charges.
- On a 4,000mAh phone, expect ~4.2 charges.
This shortcut matches the table above and mirrors what you see on trips: a 20k pack stretches a long weekend for typical users.
What Shrinks Or Boosts Your Charge Count
Cables And Ports
Short, thick cables drop less voltage. That keeps current lower for the same power, which trims loss and heat. If your pack and phone support USB-C PD, stick to PD-rated cables so the devices negotiate cleanly at 9V/12V when needed.
Fast-Charge Profiles
PD and vendor-tuned modes push power harder at higher voltages. That shortens top-ups, but it can nudge losses up at the edges. Over a full discharge, a solid PD design still sits in that 80–90% band with a good cable.
Temperature
Cold cells sag; hot cells age faster. Leaving a pack in a hot car or charging in sub-zero weather chips away at performance. Room-to-mild temps give the best results.
Background Load On Your Phone
Screen brightness, 5G tethering, GPS, and video recording eat energy while you charge. If you plug in during heavy use, a portion of the pack’s power just feeds live usage rather than filling the battery.
Model-Specific Notes And Trusted References
Not all brands share mAh and Wh with the same clarity. Apple lists run-time claims on product pages, while detailed capacities show up in regulatory filings gathered by reputable outlets. Android makers often publish mAh directly in spec sheets. If you’re planning a long trip, you can sanity-check with official listings:
- Samsung’s S24 series shows 4,000mAh, 4,900mAh, and 5,000mAh on its support page: battery capacity for S24 line.
- The iPhone 15 battery energy figures surfaced in public filings and were compiled by tech reporters; these point to about 12.98Wh for iPhone 15 and 12.70Wh for iPhone 15 Pro.
You can also cross-check your own device in settings or on the brand’s regional support pages. For math conversions, see a clear walkthrough of mAh ↔ Wh, which matches how engineers log battery energy.
How I Reached The Estimates
Assumptions And Method
I converted the pack’s 20,000mAh label to energy at 3.7V (74Wh), then multiplied by a realistic conversion band. I used 85% for most examples. That lands at 62.9Wh of usable energy. For devices with known Wh (like iPhone 15 models from regulatory data), I divided usable Wh by device Wh. For devices with only mAh published, I compared mAh with the same efficiency baked in.
Why 80–90% Is A Fair Range
Modern synchronous buck/boost designs are efficient across common USB-C PD voltages when paired with decent cables and loads. Real packs also carry protection circuits and indicator LEDs, so end-to-end numbers won’t hit lab peaks. That’s why the 80–90% window maps well to real life.
What Different Efficiencies Mean In Practice
Here’s how a 20k pack behaves across common efficiency points. Use this if your pack runs warm, you’re fast-charging hard, or you’re working with a budget unit.
| Efficiency | Usable Energy (Wh) | Charges For 5,000mAh Phone |
|---|---|---|
| 70% | ~51.8 | ~2.8 |
| 80% | ~59.2 | ~3.2 |
| 85% | ~62.9 | ~3.4 |
| 90% | ~66.6 | ~3.6 |
These are steady-state estimates with the screen off. High-brightness maps, video shoots, or hotspot duty will trim the count.
How To Get More From The Same Pack
Pick The Right Cable
Use short, certified USB-C cables that match your device’s charge rate. Thin or flaky leads waste energy and can drop the negotiated voltage, which drags charging speed and efficiency down.
Charge At Rest When You Can
Top up while the screen is off and radios are idle. That sends more of the pack’s energy into the battery rather than live use.
Avoid Full Drains And Hot Cars
Keeping the pack between roughly 20% and 80% state of charge and storing it at mild temps helps preserve capacity over time. A healthy pack holds its stated energy better.
Match Output To The Load
Use the USB-C PD port for phones that support it and the regular USB-A for accessories that don’t need PD. Matching the right port can shave waste and reduce heat.
Travel Planning: Can A 20k Pack Cover A Weekend?
Yes, in most cases. A light phone user with two to three hours of screen time per day can stretch ~4 charges across a long weekend. Heavy camera use or hotspot duty eats through energy, but even then, a 20k unit still feels roomy for a single phone. Add a second phone or a tablet, and you’ll lean closer to the 3-charge side of the range unless you ration or recharge the pack at night.
FAQ-Free Takeaways (No Fluff)
- A 20k pack holds ~74Wh; expect ~60–67Wh delivered in the real world.
- Phones near 5,000mAh see about 3–4 charges; smaller batteries get more.
- Good cables, cooler temps, and PD-capable ports improve the outcome.
- Use watt-hours for clean comparisons; link your math to a trusted conversion method.
Citations used for figures and conversions include a clear mAh ↔ Wh method and brand-published battery capacities for the Galaxy S24 family on Samsung’s support site: S24 battery specs. iPhone 15 energy values come from public regulatory listings compiled by reputable outlets.