A power bank’s full-charge count equals usable watt-hours divided by your device’s battery watt-hours.
A quick estimate saves hassle on trips and long days out. You only need three numbers: the bank’s labeled milliamp-hours, the cell voltage (usually 3.6–3.7V), and your device’s battery size. With those, you can convert to watt-hours, factor in conversion losses, and get a practical answer. The sections below show the math, fast rules, and edge cases so you can pick the right capacity, not a guess.
Quick Formula And A Fast Example
Use this two-step path. First, convert the power bank’s label to energy: Wh = (mAh × 3.7) ÷ 1000. Second, apply real-world losses: multiply by an efficiency range, commonly 0.6–0.8. The final number is usable watt-hours. To estimate full recharges, divide that by your device’s battery watt-hours.
Example: a 10,000 mAh unit contains about 37 Wh. With 75% efficiency, usable energy is roughly 27.8 Wh. If your phone battery is near 15.4 Wh, expect about 1.8 full recharges. Cable quality, speed, and heat can nudge that up or down.
Broad Estimates By Capacity (Table Within 30%)
These ballpark figures assume 3.7V cells and 75% efficiency. Phone battery used for the count is ~15.4 Wh (a mid-size model).
| Power Bank Label (mAh) | Usable Energy (Wh) | Approx Phone Recharges |
|---|---|---|
| 5,000 | 13.9 | ~0.9× |
| 10,000 | 27.8 | ~1.8× |
| 20,000 | 55.5 | ~3.6× |
| 26,800 | 74.4 | ~4.8× |
| 40,000 | 111.0 | ~7.2× |
How Many Recharges From One Power Bank: Clear Steps
Step 1: Convert Label To Watt-Hours
Bank labels use milliamp-hours, but devices consume watt-hours. Convert with this standard relation: Wh = (mAh × V) ÷ 1000. Most packs use 3.6–3.7V cells. If the maker prints watt-hours on the case, use that directly.
Step 2: Apply Efficiency
Energy is lost in voltage boost, cable resistance, and charge management. A fair planning range is 60–80%. Pick 70–75% for typical use, lower in cold or when fast charging near max output.
Step 3: Divide By Your Device’s Battery
Find your device’s watt-hours. Many phones sit between 12–20 Wh. Tablets spread wider, from ~20 to 40+ Wh. Laptops vary a lot; 40–60 Wh is common on thin-and-light models. The final step is simple: usable Wh ÷ device Wh = full charges.
What Changes The Count In Daily Use
Charge Speed And Heat
High wattage routes waste more energy as heat. If your pack and phone push near the top of USB-C PD, expect fewer full cycles than the paper math suggests. Slower 5–10 W sessions waste less but take longer.
Cable And Adapter Losses
Thin or long leads add resistance. A low-quality cable can drop voltage and trigger inefficiency. Short, certified leads keep losses modest and maintain stable current.
Temperature
Cold lowers performance and warm days add loss. Mid-range temperatures give the best results. Stash the pack out of direct sun and keep it off hot dashboards.
Device Behavior
Phones sip power while charging due to radios and screens. Starting at a low state of charge, then letting the phone rest, returns better results than topping up while streaming video or using GPS.
Pick The Right Size For Your Day
Match your pack to your routine. Commuters who charge once in the afternoon can use a 5,000–10,000 mAh unit. Travelers running maps, photos, and hotspots benefit from 20,000 mAh or higher. Creators and laptop users should track watt-hours, not just mAh, and confirm the pack’s output meets their device’s draw.
Method, Assumptions, And A Handy Link
The math here follows the common conversion between charge and energy. You can double-check with a trusted calculator by searching for the standard mAh to Wh formula. Aviation rules also tie capacity limits to watt-hours; see the FAA’s PackSafe lithium battery guide for the thresholds that apply when you fly with a spare pack. Those pages confirm the conversions and the limits you’ll see on retail labels.
Device Battery Sizes And What To Expect (Table After 60%)
This table uses a 20,000 mAh pack as a reference. That’s about 74 Wh at the cell level and ~55 Wh usable at 75% efficiency. Counts are full cycles from near-empty, not short top-ups.
| Device Type | Typical Battery (Wh) | Charges From ~55 Wh Usable |
|---|---|---|
| Compact Phone | 12–13 | ~4× |
| Mid-Size Phone | 14–16 | ~3–3.5× |
| Large Phone | 17–20 | ~2.7–3.2× |
| Nintendo Switch | ~16 | ~3.3× |
| Small Tablet | 20–30 | ~1.8–2.7× |
| Big Tablet | 30–45 | ~1.2–1.8× |
| Wireless Earbuds Case | ~1–3 | ~18–55× |
| Ultraportable Laptop | 40–60 | ~0.9–1.4× |
USB-C PD Output And Why It Matters
Energy capacity is only half the story. Your device also needs the right voltage and wattage profile. A phone charge at 18–30 W is routine. Many thin laptops sip 45–65 W. Some gaming rigs want 100 W or more. Check the pack’s PD rating and the cable’s rating before you bet on laptop top-ups.
Realistic Planning Tips That Save You Time
Match Chemistry And Labeling
Most packs use lithium-ion cells near 3.7V. If a case lists both mAh and Wh, trust the Wh figure. It removes the guesswork.
Budget For Losses
Plan on 25–40% loss from boost, cable, and charge overhead. Fast sessions tend to sit near the high end of that range.
Set A Floor For Trips
For a weekend with maps and photos, aim near 20,000 mAh. That size leaves room for friends’ phones, cameras, and a console.
Mind Temperature And Storage
Room temperature is ideal. Avoid full discharges. Top up to around 40–60% for storage between outings. Keep packs dry and away from direct heat.
Troubleshooting Short Counts
The Pack Feels Hot
Heat suggests high loss. Drop the charge rate if your gear allows it. Swap to a shorter cable.
The Phone Won’t Hit 100%
Phones throttle near full. The last few percent can take a long time and burn overhead. Count full cycles from low to about 90% for planning math.
The Laptop Barely Gains
Your device may need a PD profile the pack can’t serve. Check the label for 20V support and the wattage tier your laptop expects.
Worked Examples You Can Copy
Example A: City Day With A Mid-Size Phone
Gear: 10,000 mAh pack, 3.7V cells, 75% efficiency. Usable energy ~27.8 Wh. Phone battery ~15.4 Wh. Expect 1–2 full recharges, or one full cycle plus top-ups for maps and photos.
Example B: Travel Day With Two Phones
Gear: 20,000 mAh pack, 3.7V cells, 70% efficiency due to fast charging. Usable energy ~51.8 Wh. Two phones near 15–16 Wh each. Expect about three full cycles split between the pair.
Example C: Thin-And-Light Laptop On A Train
Gear: 26,800 mAh pack with 60 W PD. Usable energy ~74.4 Wh × 0.75 ≈ 55.8 Wh. Laptop pack ~50 Wh. Expect roughly one full cycle if the laptop is idle or sleeping during charge.
Safety, Rules, And Packing Notes
Airlines set watt-hour limits for spare batteries. Under 100 Wh is widely accepted in carry-on bags. Packs between 101–160 Wh usually need airline approval, and big bricks beyond that are banned. Always carry banks in the cabin, protect the ports, and use a sleeve to prevent shorts.
Key Takeaways
Estimate charges by converting the label to watt-hours, applying an efficiency range, and dividing by your device’s battery. Size your pack to your day, bring a short certified cable, and watch temperature and charge speed. With that, your count in real life will match the math on paper. Keep spares dry, labeled.