A 20,000mAh power bank can deliver roughly 3–5 full phone charges, depending on battery size and charging efficiency.
A quick, reliable estimate comes from working in watt-hours, not just milliamp-hours. A typical 20k pack holds about 20,000 mAh at 3.7 V inside its cells, which equals roughly 74 Wh. After conversion losses, only part of that reaches your phone. Good units land around 70–85% overall efficiency in day-to-day use, so you get about 52–63 Wh available to refill your handset.
How Many Phones From A 20,000mAh Power Bank: Tested Math
Below is a practical table that converts a 20k unit’s energy into the number of 0–100% refills for common phone battery sizes. The math assumes a 3.7 V internal pack (≈74 Wh) and three realistic efficiency tiers: 70%, 80%, and 85%. If your phone has a 5,000 mAh battery, look at the 5,000 row. If you own a smaller device, use the 3,000 or 4,000 rows.
| Phone Battery Size | Usable Energy Tier | Estimated Full Charges |
|---|---|---|
| 3,000 mAh (≈11.6 Wh) | 70% / 80% / 85% | 4.5 / 5.2 / 5.5 |
| 4,000 mAh (≈15.4 Wh) | 70% / 80% / 85% | 3.4 / 3.9 / 4.1 |
| 5,000 mAh (≈19.3 Wh) | 70% / 80% / 85% | 2.7 / 3.1 / 3.3 |
| 6,000 mAh (≈23.1 Wh) | 70% / 80% / 85% | 2.2 / 2.6 / 2.7 |
Why Watt-Hours Tell The Truth
Milliamp-hours alone can be misleading because a phone battery and a portable charger use different voltages. Cells in a pack are rated around 3.6–3.7 V nominal, while USB output is 5 V or higher under fast-charge modes. Converting between those levels wastes energy as heat. That’s why two packs with the same mAh can deliver different real-world results.
To compare apples to apples, convert capacity to watt-hours: Wh = (mAh ÷ 1000) × V. A 20,000 mAh pack at 3.7 V holds about 74 Wh inside its cells. From there, multiply by an efficiency factor to estimate the energy that actually reaches your phone during a session.
What Counts As A Good Efficiency?
Modern, reputable packs often land in the 70–85% range across cables and electronics. Temperature, cable resistance, and fast-charge profiles all move the needle. Lower-quality units may perform below that range.
Step-By-Step: Estimate Your Own Charges
- Find your phone’s battery in watt-hours. If the spec is in mAh, multiply by the battery’s nominal voltage (many phones are near 3.85 V). A 5,000 mAh cell is about 19.3 Wh.
- Convert the power bank to watt-hours. For a 20k pack: 20 Ah × 3.7 V ≈ 74 Wh.
- Apply efficiency. Choose 70% (tough conditions), 80% (typical), or 85% (great cable and moderate speed). Usable energy becomes ~52 / 59 / 63 Wh.
- Divide. Usable Wh ÷ phone Wh = expected 0–100% refills.
Want the most accurate figure? Charge the phone from a known level to full and read how many percent the pack lost. Repeat a few times and average.
Real-World Factors That Change The Count
Fast-Charge Overhead
High-wattage modes shorten time on the cable, but they push more current through conversion hardware and cables. That increases heat and trims effective efficiency, which means fewer refills per cycle than the math above might suggest.
Cables And Connectors
Old or thin leads add resistance. Long runs waste more energy than short ones. If you use USB-C to USB-C with quality e-marked cable, losses tend to be lower. The difference can easily shift your result by a few percentage points.
Charging While Using The Phone
Playing games or recording video while plugged in draws power at the same time the phone is trying to refill. Some energy never reaches the battery at all, which reduces the number of complete refills you’ll see from the pack.
Temperature
Cold cells slow chemical reactions and warm cells raise resistance. Both effects sap efficiency. If you’re topping up on a hot dashboard or on a ski lift, expect fewer round-trip watt-hours.
Battery Health
A phone with aged cells may accept charge less efficiently. Packs lose capacity with cycles too, so an older 20k unit won’t match its day-one figures.
About USB-C And Power Delivery Speeds
USB Power Delivery overview shows how USB-C can negotiate higher voltages and currents, raising total charging power well beyond legacy 5 V ports. That improves speed, not the total energy stored. A quick session—say, fifteen minutes at lunch—moved more watt-hours in less time, but it didn’t change the number of full refills available from the same pack.
Worked Examples With Popular Battery Sizes
These walk-throughs show how the numbers play out using typical phone capacities. Pick the line closest to your handset and compare the efficiency tiers to match your gear and habits.
| Phone Battery Size | Usable Wh From 20k Pack | Estimated Full Refills |
|---|---|---|
| ~3,300 mAh (≈12.7 Wh) | 52 / 59 / 63 | 4.1 / 4.6 / 5.0 |
| ~4,500 mAh (≈17.3 Wh) | 52 / 59 / 63 | 3.0 / 3.4 / 3.6 |
| ~5,500 mAh (≈21.2 Wh) | 52 / 59 / 63 | 2.5 / 2.8 / 3.0 |
How To Squeeze More Refills From The Same Pack
Use A Short, Quality USB-C Cable
Keep runs under one meter and avoid worn connectors. Good copper and clean contacts cut resistive loss.
Skip Max-Watt Fast Modes When You Can
Medium speed often wastes less energy. If time allows, charge at a gentler profile for more total watt-hours delivered over the day.
Top Up Earlier
Lithium cells dislike deep discharge. Small top-ups while the phone sits near room temperature can be kinder to both devices and keep efficiency steady.
Travel And Safety Notes
Around airports and trains, you’ll see limits expressed in watt-hours. A 20k pack at ~74 Wh sits within common 100 Wh carry-on rules, which is why that size is popular. Always check the transport operator’s page before a long trip.
Sources And Method At A Glance
This guide treats a 20k pack as ≈74 Wh based on a 3.6–3.7 V cell rating and then applies reasonable efficiency bands drawn from reputable charging manufacturers. USB-C PD specs explain why speed and direction of power flow don’t change total energy, just time to fill.
External references used for core facts: lithium-ion nominal voltage and watt-hour basics; and the official USB Power Delivery overview for power ranges and negotiation behavior.
Bottom-Line Takeaways
- A 20k pack nets ~52–63 Wh to share after losses.
- That equals about 3–5 full refills for common phone sizes.
- Cable quality, fast-charge levels, temperature, and device use swing the result.