Most power banks store 18–100 Wh (5,000–27,000 mAh), and usable charge is roughly 60–80% of the label due to voltage and conversion losses.
You bought a portable charger, but how much energy does it really give your phone, tablet, or camera? This guide cuts through labels, explains the math behind milliamp-hours and watt-hours, and shows expected real-world recharges by size. You’ll also see airline limits and fast-charging notes so you pick the right pack and avoid headaches at security.
How Much Charge Your Power Bank Really Delivers
Manufacturers print capacity in milliamp-hours (mAh) at the battery’s internal voltage. Most lithium packs inside sit around 3.6–3.7 volts per cell. To compare across devices and understand flight rules, convert to watt-hours (Wh) with a simple formula: Wh = (mAh × V) ÷ 1000. A 10,000 mAh pack at 3.7 V holds about 37 Wh on paper.
That’s the theory. What you can use at the USB port is lower. Energy gets lost when the pack boosts from its 3.x-volt battery to 5–20 V for USB, in the cable, and inside your device while it regulates charging. Quality models waste less; budget models waste more. In practice, plan for roughly 60–80% of the printed mAh to reach your phone or tablet.
Fast Way To Estimate Usable Capacity
Take the labeled mAh and multiply by 0.7 for a quick, realistic number. That “70% rule” works well in day-to-day use. If the brand publishes verified efficiency or Wh at the cell voltage, use that figure instead.
Capacity, Watt-Hours, And Typical Recharges
The table below translates common sizes into watt-hours and rough full recharges for a typical phone (4,000–5,000 mAh battery). It’s a broad guide; your results vary with screen-on time, mobile signal, and cable losses.
| Printed Size | Approx. Wh (3.7 V) | Phone Recharges* |
|---|---|---|
| 5,000 mAh | 18.5 Wh | ~1–2 |
| 10,000 mAh | 37 Wh | ~2–3 |
| 15,000 mAh | 55.5 Wh | ~3–4 |
| 20,000 mAh | 74 Wh | ~3–5 |
| 26,800–27,000 mAh | ~99–100 Wh | ~4–6 |
| 30,000 mAh** | ~111 Wh | ~5–6 |
*Assumes ~70% real-world efficiency and a 4,500 mAh phone. **Check airline rules before flying with >100 Wh packs.
Wh Vs. mAh: Why Watt-Hours Tell The Real Story
mAh alone can mislead because it ignores voltage. Watt-hours capture both voltage and current-hour rating, so they measure energy. That’s the unit airlines use, and it’s the best way to compare a small phone bank and a bigger laptop-grade pack.
Convert mAh To Wh In Seconds
Use this: Wh = (mAh × V) ÷ 1000. Most single-cell lithium packs sit near 3.6–3.7 V; multi-cell designs use series connections that change voltage but not total energy. You’ll often see 37 Wh on a 10,000 mAh label because 10,000 × 3.7 ÷ 1000 ≈ 37.
What Affects How Many Recharges You Get
Plenty of small factors shave output. Here are the big ones you’ll notice.
Conversion And Cable Losses
Boosting the pack’s 3.x-volt battery up to USB levels burns energy as heat. Long or thin cables add drop. Short, good-quality USB-C cables help reduce waste.
Charging Speed And Protocols
USB Power Delivery can push much higher wattage than legacy 5 V ports, which helps tablets and laptops refill faster. PD 3.1 extends headroom up to 240 W with the right charger and cable; large banks that advertise high-watt outputs rely on these rules.
Device Battery Size
Newer phones range from about 4,000 to 6,000 mAh. A gaming phone at 6,000 mAh eats more from the same pack than a compact phone at 4,000 mAh. Tablets and handheld consoles draw more still, while cameras tend to sip.
Age, Heat, And State Of Charge
Lithium cells lose capacity as they age and when stored full in hot places. Keeping a pack out of hot cars and topping it off every month helps it stay ready.
Airline Rules In Plain English
Portable chargers count as spare lithium batteries. Most carriers allow up to 100 Wh in carry-on by default; many also allow two spares from 100–160 Wh with approval. No power banks in checked bags. If your pack lists only mAh, convert to Wh using 3.7 V to check where it falls.
Picking The Right Size For Your Gear
Match the pack to your day. A commuter may only need a slim 5,000 mAh stick for top-ups. A traveler who shoots photos and navigates offline maps may be happier with 10,000–20,000 mAh. If you carry a USB-C laptop, look for higher Wh plus PD output that meets your machine’s wattage.
Quick Fit By Use Case
- Daily carry: 5,000–10,000 mAh for light phone top-ups.
- Weekend getaway: 10,000–20,000 mAh for phones, buds, and an e-reader.
- Creator or gamer: 20,000–27,000 mAh for phones, consoles, action cams.
- USB-C laptop days: High-Wh models that support the wattage your laptop asks for.
Realistic Recharge Math You Can Use
Here’s a simple way to forecast recharges for your exact phone or tablet.
Step 1 — Turn mAh Into Usable mAh
Multiply the printed mAh by 0.7. That’s your rough usable mAh at the port. A good 20,000 mAh bank gives you ~14,000 mAh to share across devices.
Step 2 — Divide By Your Device Battery
Take that usable mAh and divide by your device’s battery mAh. If your phone is 5,000 mAh, the math says ~2.8 full recharges. In practice, screen-on drain during charging and standby use while traveling nudge that number down a bit.
Device Batteries And Expected Top-Ups
Use the cross-reference below to sanity-check your plan. Numbers assume ~70% usable from the bank and full device recharges from 20% to 100% rather than 0% to 100% every time.
| Device Battery | 10,000 mAh Bank | 20,000 mAh Bank |
|---|---|---|
| 4,000 mAh phone | ~1.7–2.0 full charges | ~3.4–3.8 full charges |
| 5,000 mAh phone | ~1.4–1.8 full charges | ~2.8–3.6 full charges |
| 6,000 mAh phone | ~1.2–1.5 full charges | ~2.3–3.0 full charges |
| 9,000 mAh tablet | ~0.9–1.1 full charges | ~1.8–2.2 full charges |
| 10,000 mAh console | ~0.8–1.0 full charges | ~1.6–2.0 full charges |
| 2,000 mAh camera | ~3.5–5.0 battery swaps | ~7.0–10.0 battery swaps |
When A Bigger Bank Makes Sense
Two features separate “phone-only” packs from laptop-friendly models: energy in watt-hours and peak output in watts. If your notebook ships with a 65 W or 100 W charger, pick a bank that can source the same level over USB-C PD and has enough Wh to give you meaningful runtime. A 27,000 mAh unit (~100 Wh) paired with a 65 W PD port can do a short laptop top-up and still refill a phone several times.
About PD Cables
High-watt PD sessions need 5-amp USB-C cables that are electronically marked. They tend to be thicker and are labeled for 240 W or 5 A. Using a 3-amp cable limits available power even if the bank and laptop can do more.
Care Tips So Your Pack Stays Reliable
- Store the bank around half charge when you won’t use it for weeks.
- Top it up every month or two to keep the cells healthy.
- Avoid leaving any lithium pack in hot cars or in direct sun.
- Use short, good USB-C cables to reduce voltage drop and heat.
- If a pack swells, recycle it at an e-waste point and stop using it.
Takeaways You Can Act On Today
- Think in watt-hours for apples-to-apples comparisons and flights.
- Use the 70% rule for quick recharge estimates.
- Match PD wattage and cable rating to your laptop’s needs.
- Keep packs cool and partially charged during storage.