A charged power bank lasts from a few hours to multiple days based on capacity (Wh), device draw (W), and conversion losses.
Here’s the short path to a solid estimate: convert the power bank’s milliamp-hours (mAh) to watt-hours (Wh), apply a realistic efficiency figure, then divide by the device’s average watt draw. That single line of math gives you a dependable runtime range without guesswork.
Quick Runtime Formula And A Fast Example
Use this to estimate hours of continuous power for any gadget:
Runtime (hours) ≈ [mAh × 3.7 ÷ 1000 × efficiency] ÷ device watts
- mAh: the labeled capacity on the pack.
- 3.7: nominal voltage of Li-ion cells inside most packs.
- Efficiency: share of stored energy that reaches your device after DC-DC conversion and heat loss (use 0.7–0.9 for most packs).
- Device watts: average power your phone, tablet, or laptop actually pulls while charging or running.
Say your pack is 10,000 mAh. Usable Wh ≈ 10,000 × 3.7 ÷ 1000 × 0.8 = 29.6 Wh. If your phone averages 10 W while topping up and running apps, you’re looking at ~3 hours of sustained 10 W output. Phones don’t pull 10 W nonstop, so this often translates to 2–3 full recharges on typical handsets before the bank is empty.
Broad Ranges At A Glance
This table shows rough real-world outcomes across common sizes. It assumes 80% efficiency and an average 10 W phone draw.
| Bank Size (Label mAh) | Usable Energy (Wh) | Phone Recharges* |
|---|---|---|
| 5,000 | ~14.8 | ~1–1.5 |
| 10,000 | ~29.6 | ~2–3 |
| 20,000 | ~59.2 | ~4–6 |
| 27,000 (≈100 Wh cap) | ~80.0 | ~6–8 |
| 30,000 | ~88.8 | ~7–9 |
*Phone recharge count assumes ~10–12 Wh per full phone charge including overhead. Real results vary by model, battery health, and cable/charger behavior.
Power Bank Full Charge Duration — Real-World Ranges
“How long” can mean two things: total hours of output at a given watt level, or how many device recharges you’ll get before refueling the bank. Here’s how that plays out for phones, tablets, and laptops.
Phones: Typical All-Day Reserve
Most phones hold ~10–15 Wh at 0–100%. A 10,000 mAh pack with ~30 Wh usable often delivers 2–3 phone refills. If you only top up from 20–80% during the day, the same pack stretches across multiple days of light use. Fast charging doesn’t create energy; it just moves it faster. The bank empties sooner when you push higher watt levels.
Tablets: Mid-Range Draw
iPad-class tablets hold ~28–40 Wh. A 20,000 mAh bank with ~59 Wh usable can refill a small tablet once and still cover a phone or two. A large tablet that sips 15–20 W while in use will drain that same bank in about 3–4 hours of steady output.
Laptops: Watch The Watts
Many ultraportables cruise between 10–25 W during light work, while heavier tasks can spike to 40–60 W. A 27,000 mAh (~100 Wh) pack running a 20 W workload lands near 4–5 hours of extra time; at 45 W, it’s closer to ~2 hours. USB-C PD allows the laptop to request what it needs, so higher draw shortens runtime even if the pack size is large.
Why Two 10,000 mAh Packs Don’t Always Perform The Same
Two banks can share a label yet deliver different results. The spread comes from conversion electronics, cell quality, thermal behavior, and the way the load fluctuates.
- Conversion loss: Inside the bank, 3.7 V cell output is stepped up to 5–20 V. That adds heat and waste. Real-world efficiency ranges from about 70% to 90% on common models.
- Cables and sockets: Worn contacts or limp cables add drop. That lost voltage shows up as extra heat, not runtime.
- Protocol overhead: Negotiation under USB-C PD and trickle behavior near full adds tiny losses across each session.
- Temperature: Cold slows chemistry; hot raises resistance. Middle ground helps both the bank and the device.
mAh To Wh: The Only Conversion You Need
Wh is the common yardstick across phones, tablets, and laptops. Get it with one step:
Wh ≈ mAh × 3.7 ÷ 1000
Multiply that by your efficiency pick and divide by the device’s watts to get hours. If your device quotes battery size in Wh already, you can compare directly with the pack’s usable Wh.
What About Fast Charging And “Wattage” Claims?
Output wattage caps how fast the pack can deliver energy, not how much it holds. A 20,000 mAh bank rated for 65 W will drain faster at 65 W than at 18 W, because the same stored energy is leaving in less time. The upside is speed when your laptop or phone needs a quick top-up; the trade-off is a shorter clock time until empty.
Travel Note On Size Limits
Airline rules commonly cap carry-on battery size at 100 Wh, with a pathway up to 160 Wh if your airline approves it first. If your label shows only mAh, convert to Wh to see where you land. Always keep spare banks in cabin bags, not in checked baggage.
Charging Behavior That Extends Lifespan
Runtime on day one is only half the story. As a bank ages, capacity slides. How you charge and store it steers that slope.
- Shallow cycles beat empty-to-full swings: Topping up more often is gentler than deep drains.
- Avoid heat: Don’t leave the pack on a dash or under a pillow while fast charging.
- Storage: If you won’t use it for a while, park it around mid-charge in a cool, dry place.
- Use appropriate chargers: Match the bank’s PD rating; sketchy bricks and cables waste energy and add heat.
Cycle Life And Aging: What To Expect
Lithium-ion loses capacity across charge cycles. Many consumer cells are designed to retain about 80% of original capacity after several hundred full cycles under ideal conditions; some newer packs and phone cells are specified for around a thousand. That means a bank that once pushed 60 Wh might drop near 48 Wh after a long stretch of use, which trims the phone-charge count or hours of laptop time accordingly.
Estimating Your Own Runtime Step By Step
1) Find Or Convert Your Bank’s Wh
Check the label or spec sheet. If you only see mAh, convert with mAh × 3.7 ÷ 1000.
2) Pick An Efficiency Number
Newer, well-built banks with decent electronics often land near 80–90%. Budget models can slip closer to the low 70s, especially at higher outputs or warmer temps.
3) Estimate Device Watts
Phone: 5–10 W while charging in the background. Tablet: 10–20 W. Ultrabook: 15–30 W light work, more under load. If your device shows live watts in a battery app or OS menu, use that number.
4) Do The Math
Hours ≈ (Wh × efficiency) ÷ device W. If you want “number of charges” for a phone, divide usable Wh by the phone’s battery Wh and shave a little for top-off overhead.
Realistic Scenarios You Can Trust
- Camping weekend with a phone: A 20,000 mAh pack often covers a couple of full refills and still has room for headlamp or earbuds.
- Conference day with a laptop: A 27,000 mAh (~100 Wh) bank adds a few hours to a light-draw ultrabook; carry a wall charger for the venue’s outlets and rotate as needed.
- Tablet for reading and notes: A 10,000 mAh bank is a solid cushion; expect a full refill and change.
Second Reference Table: What Shapes Your Result
These are the levers that move runtime up or down.
| Factor | Effect On Runtime | Practical Tip |
|---|---|---|
| Efficiency | Lower efficiency trims usable Wh | Use short, quality cables; avoid heat |
| Output Power | Higher watts drain the bank faster | Only request high-watt modes when needed |
| Device Health | Aged phone batteries waste more energy | Replace tired batteries; limit constant 100% holds |
| Temperature | Hot or cold reduces delivered energy | Charge and use near room temp |
| Cable Loss | High resistance saps runtime | Pick certified USB-C cables rated for your wattage |
| Protocol | PD overhead and top-off trickle shave a bit | Stop charging at ~80–90% when time-boxing |
| Aging | Capacity drops across cycles | Keep cycles shallow and avoid heat |
USB-C PD And Why Watt Caps Matter
USB-C Power Delivery lets a device request the voltage and current it prefers. That’s great for laptops and fast-charge phones, but it also means a bank rated for higher watt levels can empty fast under heavy draw. If your goal is pure runtime, a modest power profile can stretch hours on light tasks. If speed is the goal, enable the higher profile and accept a shorter clock time to empty.
When To Step Up In Size
Pick a capacity that matches your heaviest day. Commuters with a phone and earbuds often land on 10,000 mAh. Travelers juggling a tablet or a Switch do well at 20,000 mAh. Laptop users who want real cushion look near 27,000 mAh (roughly the common airline limit). Bigger only helps if you can carry it and charge it overnight.
Charging The Bank Itself
A large pack can take hours to refill. Look for banks that accept 30–65 W USB-C input and pair them with a matching wall charger. That combo slashes downtime. If your bank supports pass-through, you can power a phone while the bank recharges, but expect more heat and a touch of extra loss.
Helpful References Inside The Text
USB-C PD capabilities are published by the standards body; you’ll see fixed and adjustable voltage modes up to high watt levels on compliant gear. Many airlines and regulators publish battery size limits; the common cap is 100 Wh without prior approval, and up to 160 Wh with approval. Major phone makers publish cycle-retention guidance that signals how capacity changes across heavy use. These references keep your expectations grounded.
Bottom Line
To answer the “how long” question with confidence, stick to math, not marketing. Convert to Wh, pick a fair efficiency number, estimate watts, and run the numbers. With that, you can predict whether a modest pack covers a day out—or whether you need a larger bank for a laptop-heavy stretch.
Learn about USB Power Delivery capabilities and airline 100 Wh battery limits. For cycle-retention guidance on modern phone batteries, see Apple’s notes on battery and performance.