A 7,800 mAh power bank usually yields ~23–25 Wh usable—about two phone charges or ~2.5 hours at 10-watt output.
Shoppers look at a power bank number like 7,800 mAh and wonder what that means in real hours. The short answer is: it depends on the device draw, the conversion losses inside the pack, and the cable or wireless method you use. Below you’ll find the plain math, quick estimates, and a few smart tweaks that squeeze more runtime from this capacity class.
Real-World Runtime For A 7,800 mAh Pack
Power banks use lithium-ion cells with a nominal voltage near 3.7–3.87 V at the cell level. Capacity on the box is listed in milliamp-hours at that cell voltage. To turn that into energy, multiply capacity by voltage to get watt-hours. A 7,800 mAh pack holds about 7.8 Ah × 3.7 V ≈ 28.9 Wh of raw energy. Boost circuitry then steps that up to 5 V (or higher with USB-C PD), which costs you some energy. Good units land near 80–90% efficiency on wired output; wireless adds more loss.
That’s why usable energy is lower than the printed figure. With an 82–90% range, the realistic usable window is about 23–26 Wh. That number drives everything else: hours at a given power draw, or charges for a device battery of known watt-hours.
Quick Table: Typical Loads And Hours
Pick the load that matches your device while charging. A phone during fast charge often pulls near 10 W; a handheld console or small router may hover near 7–12 W; tiny gadgets sip 2–5 W.
| Output Load (W) | Hours @ 85% Eff. | Hours @ 70% Eff. (wireless/inefficient) |
|---|---|---|
| 2 W (earbuds/GPS tracker) | ≈ 12.2 h | ≈ 10.1 h |
| 5 W (steady phone top-up) | ≈ 4.9 h | ≈ 4.0 h |
| 7.5 W (handheld console light play) | ≈ 3.3 h | ≈ 2.7 h |
| 10 W (typical fast-charge phase) | ≈ 2.5 h | ≈ 2.1 h |
| 15 W (tablet trickle/PD step) | ≈ 1.6 h | ≈ 1.4 h |
| 20 W (PD burst) | ≈ 1.2 h | ≈ 1.0 h |
How The Math Works (Without The Jargon)
Step 1: Turn mAh into Wh. Use Wh = Ah × V. A 7,800 mAh pack is 7.8 Ah. At 3.7 V, that’s about 28.9 Wh.
Step 2: Account for conversion loss. Boosting to 5 V and dealing with heat costs energy. With an 85% assumption, usable energy is 28.9 × 0.85 ≈ 24.6 Wh.
Step 3: Divide by your device’s draw. If your phone pulls 10 W, runtime ≈ 24.6 ÷ 10 ≈ 2.46 hours—spread across the charge session.
Charges You Can Expect On Phones And Small Tablets
Phone batteries are often in the 12–17 Wh range; compact tablets can sit around 30–39 Wh. A 7,800 mAh pack with roughly 24–25 Wh usable will refill a phone a couple of times, while only topping a tablet partway. Numbers shift with screen-on use during charging, PD voltage steps, and cable losses.
Rough Refill Counts For Common Battery Sizes
The table below uses a conservative 85% efficiency for the pack and assumes wired charging:
| Device Battery (Wh) | Typical Class | Approx. Full Recharges |
|---|---|---|
| 10–12 Wh | Small phones | ~2.0–2.3× |
| 13–15 Wh | Mid/large phones | ~1.6–1.9× |
| 16–18 Wh | Plus-sized phones | ~1.3–1.5× |
| 30–32 Wh | 11-inch tablet | ~0.8× |
| 38–39 Wh | 13-inch tablet | ~0.6× |
What Changes The Result
Wired Vs. Wireless
Wired output wastes less energy. Wireless adds transmitter/receiver losses and heat, which can drop usable energy into the 65–75% band. That’s why the first table includes a 70% column.
Voltage And USB-C PD Behavior
Many devices request higher voltages over USB-C PD—9 V, 15 V, or 20 V—during parts of the charge. The pack must step the 3.7 V cell up to those levels, which trims efficiency a bit. At light loads, efficiency is usually better; at high PD bursts, it can dip.
Screen-On Drain And Background Tasks
If you’re gaming or streaming while plugged in, the device is burning power at the same time it’s charging. That reduces how much of the pack goes into the battery itself. Airplane mode, lower brightness, and closing chatty apps help a lot during a top-up.
Temperature And Age
Cold temps slow chemical reactions; high temps trigger thermal limits. Both shorten sessions. Old packs and old phone batteries also sag sooner than new ones.
Worked Examples You Can Copy
Scenario 1: Fast-Charging A Modern Phone
A recent flagship with a ~13 Wh battery is at 5% and you start a wired fast charge that hovers near 10 W, falling lower later. A 7,800 mAh unit with ~24.6 Wh usable can deliver about two full charges across the day. If you keep using the phone during the session, expect 1.5–1.8 fills instead.
Scenario 2: Topping A Compact Tablet
An 11-inch tablet near 31 Wh won’t reach 100% from empty on this capacity. You’ll get roughly two-thirds of a tank on a wired session, more if the screen stays off and the charge curve spends time at lower power.
Scenario 3: Low-Draw Gadgets
Earbuds in a case, a headlamp, a tiny camera, or a pocket hotspot sip 2–5 W. That turns into long hours from the same bank—anywhere from 5 to 12 hours based on the first table.
How To Estimate Any Device Yourself
Find The Device Battery In Wh
Some makers list watt-hours directly. If you only see mAh, use the same cell-math your pack uses: mAh ÷ 1000 × 3.85–3.87 V ≈ Wh for many phones. For tablets and laptops, spec sheets often show Wh outright.
Compute Usable Energy For The Pack
Take 28.9 Wh as the raw energy for this capacity class. Apply 0.85 for a wired session to get ~24.6 Wh. For wireless, use 0.70–0.75 to be safe.
Convert To Hours Or Charges
Hours: divide usable Wh by load in watts. Charges: divide usable Wh by the device battery Wh, then trim a bit if you plan to use the device while charging.
Small Tweaks That Add Real Runtime
Use A Short, Good Cable
Long, thin cables waste power as heat. A short, certified cable cuts loss and keeps fast-charge modes stable.
Charge In Cool Shade
Heat hurts efficiency. Keep the pack and phone out of direct sun and off a hot dashboard.
Let The Pack Do Wired First
If the bank supports both wired and wireless, start with wired while the phone is low, then switch to wireless for trickle-top-ups later.
Match The Port To The Device
USB-C PD helps larger devices, while USB-A can be fine for small gadgets. Using the right port keeps voltage steps reasonable and trims waste.
Specs And Standards You Can Trust
You’ll see the watt-hour formula written as Wh = Ah × V on aviation safety pages. That same math is echoed in the IATA battery guidance used across air travel. For charging behavior and voltage steps, the USB industry’s own page on USB-C Power Delivery explains the higher voltage modes your devices may request.
Reference Numbers For This Capacity
| Metric | Value/Range | What It Means |
|---|---|---|
| Raw Energy (cell level) | ~28.9 Wh | 7.8 Ah × 3.7 V at the cell inside the bank |
| Usable, Wired | ~23–26 Wh | Assuming ~80–90% conversion efficiency |
| Usable, Wireless | ~19–22 Wh | Extra loss from coils and heat |
| Phone Recharges | ~1.6–2.3× | Across common 12–15 Wh phone batteries |
| Tablet Top-Up | ~0.6–0.8× | Across ~30–39 Wh small tablets |
| 10 W Continuous Output | ~2–3 h | Varies with screen usage and cable loss |
Reality Check Against Real Devices
Modern Phones
Many current phones sit near 12–17 Wh. With ~24–25 Wh usable from this capacity, two full fills is a fair expectation if you keep the screen off during most of the charge. Screen-on use trims the count.
Compact Tablets
Compact models can land near 31–39 Wh. A 7,800 mAh bank nudges these along for commutes and flights but won’t refill them from empty. Pair it with a wall charger at your destination for a full cycle.
Bottom Line Math You Can Rely On
Think in watt-hours. Treat a 7,800 mAh unit as roughly 24–25 Wh when wired. Divide by your device’s watt-hours for charge count, or divide by your average load in watts for hours. Keep the cable short, keep things cool, and wired wins when every minute matters.