Charge counts from a power bank depend on watt-hours, device battery size, and 60–90% conversion losses.
If you want a plain answer you can use today, here it is: divide your power bank’s usable watt-hours by your device’s battery watt-hours. That ratio is your ballpark number of full recharges. Usable energy is always below the printed capacity because energy is lost in voltage conversion, cables, heat, and your device running while it tops up. The first sections below show the math, a quick table, and the knobs that change the final count.
The Formula That Predicts Charge Counts
The math is simple and reliable across brands. First turn everything into watt-hours (Wh), then account for efficiency, then divide:
- Convert to Wh: Wh = (mAh × V) ÷ 1000. Most single-cell Li-ion packs inside banks and phones use a nominal 3.7 V.
- Estimate efficiency: choose η between 0.60 and 0.90. Quality banks land closer to 0.8–0.9 at moderate loads; small or bargain units run lower.
- Charges ≈ (Bank Wh × η) ÷ Device Wh.
This approach reflects how energy moves from a 3.7 V cell up to 5 V (or 9–20 V for USB-C PD), then down into the device battery. Each step wastes a little energy as heat in boost/buck converters and cables. Industry papers show boost/buck stages often reach around the low-to-mid-90s percent on paper, yet real-world use slides lower once you include cable drops, phone draw during charging, and thermal limits.
Fast Lookup: Common Bank Sizes To Phone Recharges
The table below assumes a phone with a 4,000 mAh battery at 3.7 V (≈14.8 Wh) and a realistic 80% overall delivery efficiency from bank to phone. If your phone’s battery is larger or smaller, your count scales up or down by the same ratio.
| Bank Label (mAh @3.7 V) | Usable Energy (Wh @80%) | Full Phone Recharges (≈14.8 Wh) |
|---|---|---|
| 5,000 | 14.8 × 0.80 = 11.8 | ~0.8× |
| 10,000 | 37.0 × 0.80 = 29.6 | ~2.0× |
| 15,000 | 55.5 × 0.80 = 44.4 | ~3.0× |
| 20,000 | 74.0 × 0.80 = 59.2 | ~4.0× |
| 26,800 (≈100 Wh airline limit) | 99.2 × 0.80 = 79.4 | ~5.4× |
| 30,000 | 111.0 × 0.80 = 88.8 | ~6.0× |
Why does a 10,000 mAh bank look like “about two full phone charges” in practice? A 10,000 mAh pack at 3.7 V stores ~37 Wh. Your phone at ~14.8 Wh would get 37 Wh only in a perfect world. After conversion and overhead, roughly 30 Wh reach the battery, which divides out to near two full recharges.
How To Convert mAh And Volts To Watt-Hours
You only need one formula. Grab the capacity (mAh) and cell voltage from the label or spec sheet:
- Wh = (mAh × 3.7) ÷ 1000 for single-cell packs. If a label lists Wh directly, use that instead.
- Pick an efficiency: use 0.8 for a mid-grade estimate; bump to 0.9 for a premium bank with short, thick cables; drop toward 0.7 for tiny or stressed setups.
- Compute charges: (Bank Wh × η) ÷ Device Wh.
Need a refresher on USB power levels and PD voltages? Texas Instruments’ overview shows how Type-C and USB Power Delivery step through 5 V, 9 V, 15 V, and 20 V profiles, which is why voltage conversion losses matter in the field. Read the brief under USB Power Delivery basics.
How Many Recharges From A Power Bank: The Variables
The math gives you a good baseline. Real life adds a few dials that swing the count up or down. Here’s what moves the needle most:
Conversion Efficiency And Cable Loss
A boost converter in the bank raises 3.7 V to 5 V (or higher for PD). Your device then bucks it back down for its own pack. Each hop burns off energy. Hardware notes from chip makers put ideal converter stages near the low-90s percent at sweet-spot loads, but current spikes, long leads, thin cables, and heat bring the end-to-end number closer to 0.75–0.85 for many users. If your screen stays on while charging, that draw steals a slice too. A short, low-resistance cable and a moderate charge rate preserve more of the stored energy.
Battery Health And Temperature
Worn cells store less than the label suggests. Cold weather lowers available capacity; hot weather wastes energy as heat while the bank throttles to stay safe. Both banks and phones prefer mild temps around room level.
Charging Speed And Voltage Profile
Faster charging isn’t free. Higher currents raise losses in cells, cables, and converters. PD at 9–20 V can be efficient when designed well, especially if the device bucks down near its sweet spot. That’s why two banks with the same label can deliver different totals on the same phone.
What Your Device Is Doing While It Charges
Navigation, gaming, or a bright display pull watts that never reach the battery. If you want the largest number of full recharges, put the device in airplane mode, dim the screen, and leave it alone while it tops up.
Worked Examples You Can Copy
10,000 mAh Bank To A 5,000 mAh Phone
Bank Wh ≈ 10,000 × 3.7 ÷ 1000 = 37 Wh. Phone Wh ≈ 5,000 × 3.7 ÷ 1000 = 18.5 Wh. Using 80% efficiency, usable energy ≈ 29.6 Wh. Charges ≈ 29.6 ÷ 18.5 = ~1.6×. Expect a full top-off and a healthy partial.
20,000 mAh Bank To A 4,000 mAh Phone
Bank Wh ≈ 74 Wh. Phone Wh ≈ 14.8 Wh. With 80% efficiency, usable ≈ 59.2 Wh. Charges ≈ 59.2 ÷ 14.8 = ~4.0×. With screen-on use while charging, that may land near 3.5×.
26,800 mAh Bank To A Small Tablet (7,500 mAh)
Bank Wh ≈ 99.2 Wh. Tablet Wh ≈ 27.8 Wh. Usable at 80% ≈ 79.4 Wh. Charges ≈ 79.4 ÷ 27.8 = ~2.9×. That’s enough for a weekend of maps and movies if you top up at idle.
Phones, Tablets, Earbuds, And Laptops
Charge counts scale with battery size. Many modern phones sit around 3,000–5,000 mAh; small tablets run 6,000–8,000 mAh; ultralight laptops vary widely and often expect higher PD voltages. If you plan to run a laptop, check its input wattage and the bank’s PD rating. A 45–65 W laptop needs a bank and cable that hold that wattage for the whole session.
Why Wh Beats mAh For Planning
mAh alone misleads because it ignores voltage. Wh captures actual energy. Brands and agencies use this unit to describe limits and capacity because it compares apples to apples across devices. If a spec sheet lists both, use Wh for the math.
USB Power Levels At A Glance
Classic USB tops at 5 V. Type-C with PD negotiates 5/9/15/20 V, and higher current where allowed. That profile affects heat, cable losses, and charge time. The TI brief linked earlier maps those steps clearly. For a reader-friendly take on USB charging limits, Battery University’s overview of charging from a USB port explains current caps and why big batteries can stall on weak sources.
Second Lookup Table: How Many Full Charges From A 10,000 mAh Bank?
This table assumes a 10,000 mAh bank at 3.7 V (≈37 Wh) delivering 80% of its energy to the device (≈29.6 Wh usable). Match your device battery size to get a quick count.
| Device Battery (mAh) | Approx. Wh (@3.7 V) | Full Charges From 10,000 mAh Bank |
|---|---|---|
| 2,000 | 7.4 | ~4.0× |
| 3,000 | 11.1 | ~2.7× |
| 4,000 | 14.8 | ~2.0× |
| 5,000 | 18.5 | ~1.6× |
| 6,000 | 22.2 | ~1.3× |
| 7,500 | 27.8 | ~1.1× |
| 10,000 | 37.0 | ~0.8× |
How To Raise Your Real-World Charge Count
Use Short, Low-Loss Cables
Thicker conductors lower voltage drop, which keeps converters in their efficient zone. Keep cables short when you can.
Charge At Moderate Speeds
Extreme fast charging heats up cells and increases conversion loss. A steady 18–30 W PD session often wastes less energy than a bursty high-rate sprint.
Let The Device Rest While Topping Up
Screen time, GPS, and gaming turn some of your bank’s energy into on-the-spot power instead of stored charge. Idle devices make the math line up with the tables.
Mind Temperature And Ventilation
Banks and phones prefer cool, shaded spots. Don’t wrap them in blankets or stick them under a pillow while charging. Warm air helps converters run closer to their sweet spot without throttling.
What About Laptops?
Laptops don’t follow the simple “mAh at 3.7 V” pattern because many packs use series cells and publish Wh on the label. If your ultrabook carries a 50 Wh battery and your bank can deliver 65 W PD with ~79 Wh usable energy (a high-capacity unit near the airline limit), you’ll see roughly 1.5× when the laptop is sleeping, and less while you work. Match the bank’s PD wattage to the laptop’s input requirement so the session doesn’t step down and waste time.
Assumptions, Sources, And Method
This guide uses Wh-based math because it tracks energy, not just charge at a given voltage. The 3.7 V nominal value for single-cell lithium-ion is a standard convention in spec sheets. Efficiency ranges reflect real behavior of boost/buck converters and cable drops in common USB and USB-C PD setups. For a primer on USB charging limits and practical behavior on a port, see Battery University’s page on charging from a USB port. For PD voltage steps and system design notes that explain why conversion losses show up in the field, see TI’s short deck on USB Power Delivery.
DIY Calculator You Can Run Anytime
Here’s a quick routine you can use on any bank and any device:
- Look for the bank’s Wh. If only mAh is listed, multiply by 3.7 and divide by 1000.
- Multiply by 0.8 for an everyday estimate, or 0.9 for a best-case premium setup.
- Find your device Wh the same way. Many phones: 3,000–5,000 mAh → 11.1–18.5 Wh.
- Divide usable bank Wh by device Wh. That’s your expected full-charge count.
If you want to sanity-check the Wh↔mAh relationship with another reference, brands that publish calculators use the same formula: Wh = (mAh × V) ÷ 1000 and mAh = (Wh × 1000) ÷ V. The relationship is simple and universal across lithium-ion packs.
Quick Answers To Common What-Ifs
“Why Does My 10,000 mAh Bank Feel Like Two Phone Charges?”
Because the label reports storage at 3.7 V, not the 5–9–20 V you see on the cable. After conversion and overhead, only a slice reaches the phone battery. Two full charges from 10,000 mAh lines up with the math and the first table.
“My Count Dropped Over Time—Did The Bank Shrink?”
Cells age, and efficiency dips if the bank runs hotter or cables are worn. Your phone’s battery health also matters; older packs waste more energy as heat during charging.
“Does Fast Charging Change The Count?”
It can. Higher current raises losses. You might save time but lose a portion of stored energy to heat, which trims the number of full recharges.
Bring It All Together
You don’t need a brand-specific chart to plan your day. Translate capacity to Wh, apply a realistic efficiency, divide by your device’s Wh, and you’ve got a number you can trust. Use the two tables for a quick ballpark, then fine-tune with your own battery size. Keep cables short, avoid heat, and let the device rest during charging to stretch every watt-hour.