How Long Does A 20000mAh Power Bank Last? | Real-World Math

Shoppers ask this all the time because labels list milliamp hours, not hours of run time. Here’s a clear, math-backed way to set expectations before your next trip. We’ll turn that big number on the case into hours, then factor in losses and real charging speeds. You’ll leave with a simple method and quick tables you can trust.

Quick Answer: Runtime Depends On Watts, Not Milliamp Hours

Capacity is stored as energy. Energy shows up here as watt-hours (Wh). A cell inside most banks sits near 3.7 volts. So a 20,000 mAh pack equals about 74 Wh on paper: 20 Ah × 3.7 V. Converters inside take that 3.7 V up to 5–20 V for USB, which sheds some energy as heat. That’s why real output lands lower than the printed figure.

Device Type	Typical Power (W)	Estimated Runtime From 20k mAh
Phone (standard charge)	5–10	6–12 h at 5 W; 3–6 h at 10 W
Phone (fast USB-C PD)	18–27	2–3.5 h
Tablet / iPad class	15–30	2–4 h
Mirrorless camera via USB	5–8	7–11 h at 5 W; 4–7 h at 8 W
Nintendo Switch (handheld)	6–10	5.5–9 h
USB fan / lamp	2–4	14–31 h
Laptop over USB-C PD	30–45	1.2–2 h

Real-World Runtime For A 20,000 mAh Bank

Use this two-step method. First, translate the printed capacity to energy. Second, divide by your device’s draw in watts. That gives hours. Then temper the answer by efficiency.

Step 1: Convert Milliamp Hours To Watt-Hours

Formula: Wh = V × Ah. Cells in these packs run about 3.6–3.7 V. So 20,000 mAh (that’s 20 Ah) × 3.7 V ≈ 74 Wh total.

Step 2: Account For Conversion Efficiency

Electronics inside raise voltage to 5 V, 9 V, 12 V, 15 V, or 20 V. That step isn’t perfect. Quality packs deliver roughly 75–85% of stored energy to your device. That leaves ~55–63 Wh usable from our 74 Wh total.

Step 3: Divide By Device Watts

Runtime ≈ usable Wh ÷ device W. At 10 W (a slow phone charge) you’ll see about 5.5–6.3 hours. At 20 W (a fast USB-C phone charge), plan on ~2.7–3.1 hours. At 30 W (a small laptop sip), you’re in the ~1.8–2.1 hour range.

What “mAh” Leaves Out (And Why Wh Wins)

Milliamp hours don’t include voltage, so two banks with the same mAh can carry different energy if their cells use different nominal voltages. Watt-hours do include voltage, so Wh compares apples to apples and maps directly to time with the simple division above.

Charging Speeds Matter: USB-C PD, PPS, And Old USB

Charging isn’t only about size. Port tech sets the ceiling for speed and efficiency. USB Battery Charging 1.2 ports top out near 1.5 A at 5 V, while USB-C Power Delivery negotiates higher voltages and current, now up to 240 W on compliant cables and devices. Older 5 V only outputs waste time on big tablets and laptops. Modern PD and PPS ports feed phones faster and more efficiently, which shortens the hours a bank can run because your device is drawing more watts in that period.

Typical Draws You’ll See

• Phones on basic 5 V charge: ~5 W.
• Phones on fast charge: 18–27 W depending on model and thermal limits.
• Tablets: 15–30 W while charging during use.
• Compact laptops over USB-C PD: 30–45 W at light work; spikes higher under load.

Real Examples: How Many Full Charges?

Phone and tablet makers list battery size in milliamp hours too, but you can turn those into watt-hours. A phone with a 5,000 mAh pack at 3.85 V stores about 19.25 Wh. With ~55–63 Wh usable from the bank, that’s roughly 2.8–3.3 full cycles. A small tablet near 30 Wh nets ~1.8–2.1 cycles. Leave a little margin for cable losses and hot days.

Device Battery (Wh)	Typical Example	Estimated Full Charges
10–12	Compact phone	4.5–6
15–20	Large phone	2.8–4
25–30	Small tablet / e-reader	1.8–2.5
45–55	Big tablet	1–1.3
6–9	Action cam	6–10
0.8–1.2	Earbuds case	45–75

What Shortens Or Extends Runtime

Cable And Port Quality

Thin, long, or out-of-spec cables drop voltage and waste energy as heat. A short, certified USB-C cable cuts loss and helps the bank hold a higher efficiency band.

Charging While Using The Device

Watching video, hotspot use, or gaming while on the cable raises watts. Since hours equal energy divided by watts, time shrinks when the screen stays lit.

Ambient Temperature

High heat pushes the bank and the phone to throttle. Cold packs sag in voltage. Both conditions can chip away at usable energy and speed.

Bank Design

Two 20k packs can behave differently. Better step-up converters, bigger conductors, and smarter PD firmware waste less. Displays, LED bars, and wireless coils also sip energy even when no phone is attached.

How To Estimate Your Own Runtime (A Handy Template)

Grab your device’s wattage. If you don’t see a number, take the volts × amps on the charger label. Then use 55–63 Wh as the usable energy for a decent 20k pack. Compute hours with the same divide: usable Wh ÷ device W. Adjust down 10–15% if you’ll be scrolling, shooting video, or gaming the whole time.

Why Airlines Talk About Watt-Hours

Air travel rules use Wh, not mAh, since Wh expresses actual energy. A 20k pack at 3.7 V lands near 74 Wh, under the common 100 Wh carry-on threshold many airlines use. Check your carrier for details and keep the printed rating visible on the case.

When A 20k Bank Makes Sense (And When It Doesn’t)

Good Uses

• Weekend trips with two phones.
• All-day shoots with an action camera.
• Light laptop top-ups during meetings.
• Camping lights and fans.

Step Up To A Larger Pack If

• You plan to run a laptop above 45 W for long stretches.
• You want many phone refills without recharging the bank for several days.
• You’ll rely on power-hungry, high-nit tablet screens while streaming.

Specs To Look For On The Label

Clear Energy Rating

Look for both mAh and Wh printed. Wh lets you compare across brands without guessing.

USB-C PD With PPS

PD raises speed for phones and lets small laptops sip at 20 V. PPS fine-tunes voltage and helps phones run cooler during fast charge.

Rated Output And Cable

A bank that claims 45–65 W should ship with a cable rated for 5 A. A weak cable can bottleneck to 3 A and shave hours.

Efficiency Hints

Honest makers publish conversion efficiency. If you can’t find it, use the 75–85% range to estimate.

Worked Scenarios You Can Copy

Streaming On A Tablet At 20 W

Usable energy 55–63 Wh ÷ 20 W ≈ 2.7–3.1 hours of screen-on time.

Phone GPS Navigation At 8 W

Usable energy 55–63 Wh ÷ 8 W ≈ 6.9–7.9 hours while the display stays active.

Light Laptop Notes At 30 W

Usable energy 55–63 Wh ÷ 30 W ≈ 1.8–2.1 hours, enough to bridge a meeting or train ride.

How To Measure Your Device’s Draw

You can grab watts in three ways. First, read the small print on the charger that shipped with the device; multiply volts by amps to get watts. Second, check the maker’s help page for the model’s wired or wireless charging rate. Third, use an inline USB-C meter to watch real-time power while you use the device. That last method shows spikes and dips, which helps you plan for worst-case draw.

Wireless Charging Takes A Cut

Coils add conversion steps and heat. Wired paths waste less. If you stick a magnetic pad on the bank, trim your estimates. A phone that would see 3–3.3 full cycles by cable might land closer to 2.3–2.7 cycles on a pad, especially if the screen stays bright during top-ups.

Helpful Standards You Can Trust

Airlines and shippers use watt-hours, not milliamp hours, to rate energy. The industry formula is simple: Wh = V × Ah. You can see that spelled out in the IATA lithium battery guidance. For charging speeds and port behavior, the USB-IF Power Delivery overview explains how devices and chargers negotiate safe, higher power levels over USB-C.

Care Tips That Preserve Capacity

Keep It Cool During Fast Charges

Heat ages cells. If your phone and bank both feel hot, slow the session: pull the phone case, drop to a lower-wattage port, or give it a few minutes off the cable.

Store Around Mid-Charge

Parking a bank near 40–60% for a week or more is gentler than leaving it empty or topped off for long periods. Top up every couple of months if it sits in a drawer.

Use The Right Cable

A 5 A e-marked USB-C cable enables higher PD rates and trims voltage drop. If the cable is old, kinked, or warm to the touch, replace it.

Reading Labels Without Guesswork

Here’s a quick decoder. If a port says “5V⎓3A,” that’s 15 W. If it reads “9V⎓2A,” that’s 18 W. “20V⎓3A” is 60 W. Add ports? The bank may list a combined cap like “Total 45 W.” The real-world hours still come from usable Wh divided by your active watts at that moment.

Make Your Own Cheat Card

Write these three lines on a note inside your travel pouch:

1. Usable energy for a decent 20k bank: 55–63 Wh.
2. Hours ≈ usable Wh ÷ device W.
3. Trim 10–20% when charging wirelessly or during heavy use.

Bottom Line: How Long Will It Run?

A well-built 20k pack typically feeds phones for 3–6 full cycles, tablets for 1.5–2 cycles, and small laptops for around two hours of light work. Your number lands higher with low-draw tasks and drops when the device pulls hard watts or the cable is poor. Use the simple math here and you’ll predict the window every time.