A 1,200-mAh power bank stores about 4.4 Wh and, in practice, delivers ~3–4 Wh—enough for 1–3 small-device charges or a 20–35% phone top-up.
A tiny 1,200-mAh pack looks handy, but what does that mean in hours or extra percentage on a phone? The answer hinges on energy (watt-hours), conversion losses, and the device you’re charging. Below you’ll find quick estimates, clear formulas, and realistic scenarios so you can predict runtime without guessing.
Quick Estimates For A 1,200-mAh Pack
Most single-cell lithium-ion packs inside pocket chargers run at about 3.7 V. Energy in watt-hours (Wh) equals capacity times voltage, divided by 1,000. After accounting for conversion to 5 V and heat, only part of that energy reaches your gadget. Use the table to set expectations fast.
| Use Case | What To Expect | Why |
|---|---|---|
| Phone Boost (mid-range battery ~11–13 Wh) | ~20–35% extra | ~3–4 Wh delivered vs ~11–13 Wh phone battery |
| Small Phone (older compact ~7–8 Wh) | ~35–50% extra | Smaller battery needs less energy for the same % |
| Smartwatch (200–400 mAh at ~3.8 V) | ~2–3 full charges | Each charge is about 0.8–1.5 Wh |
| Earbuds Case (300–600 mAh at ~3.7 V) | ~1–2 full refills | One case refill is ~1.1–2.2 Wh |
| Action Cam Pack (~1,220 mAh at ~3.85 V) | ~60–80% refill | Needs ~4.7 Wh; the bank delivers ~3–4 Wh |
| USB Fan (5 V, 1 W) | ~3–4 hours | Draw matches the energy the bank can output |
| USB Light (5 V, 0.5 W) | ~6–8 hours | Lower load stretches runtime |
How The Math Works (So You Can Check Any Gadget)
Step 1: Convert mAh to Wh. Multiply capacity by cell voltage, then divide by 1,000. With a 1,200-mAh, 3.7 V cell: 1,200 × 3.7 ÷ 1,000 ≈ 4.44 Wh. You can confirm the formula on engineering-focused references such as the mAh → Wh rule.
Step 2: Account for conversion losses. A power bank boosts 3.7 V up to 5 V. Quality boost converters often reach the low-to-mid 90s at the sweet spot; real-world use sits lower due to heat, cable loss, and light or very heavy loads. Texas Instruments datasheets show peak figures above 90% for modern synchronous designs (TI example). Assume ~85–90% for a small pack.
Step 3: Estimate delivered energy. 4.44 Wh × 0.85–0.90 ≈ 3.8–4.0 Wh at the USB port. The device then steps 5 V down to its battery voltage, losing a little more, so what ends up stored is often ~3.2–3.6 Wh.
Step 4: Compare to the device battery. Divide delivered Wh by the device’s Wh to get the refill fraction. A phone with a ~12 Wh pack will see ~0.3 of a charge. A smartwatch at ~1.2 Wh might see nearly three charges.
Close Variant: How Long Will A 1,200-mAh Power Bank Run? Practical Scenarios
Runtime depends on load. If you power something that draws a steady wattage, you can estimate hours by dividing available Wh by the load in watts. For intermittent loads like phones, think in percentages rather than hours, since charging slows near full and the device draws extra overhead for screens and radios.
Phones: Percentage Gains You Can Expect
Most current phones carry 10–15 Wh inside. Older compacts sit lower. A small pack provides a welcome top-up on a travel day, best used to lift the phone from low to midrange rather than aiming for a full charge. For best results, start charging around 20–40% and stop near 80–90% to avoid long, inefficient tail charging.
Earbuds And Smartwatches: Pocket Bank Sweet Spot
Wearables and earbuds sip power. That makes a tiny bank shine. Earbud cases often land around 300–600 mAh at ~3.7 V, while many watch cells sit near 200–400 mAh at ~3.8 V. With ~3–4 Wh available, expect a couple of watch refills or one to two case refills with room to spare for a quick phone bump.
Action Cameras And Compact Cameras
Plenty of action cams use batteries in the 1,100–1,300 mAh range at ~3.85 V. A well-known example is a 1,220 mAh pack used across several models. Topping one of those from a small bank lands short of a full refill, so plan for a strong partial and budget recording time accordingly.
Assumptions, Limits, And Why Your Result Might Differ
Cell voltage and labeling. Power bank packaging highlights mAh at the internal cell voltage. The USB output delivers at 5 V, so watt-hours are the equalizer. Think in Wh whenever you compare gadgets.
Boost converter efficiency. Peak numbers look great on charts; real life includes heat, cable drop, and light-load overhead. Small enclosures shed heat poorly, and thin cables waste energy. Expect a few percentage points lost before power reaches your device.
Charging curve behavior. Li-ion charging slows near full. If you chase the last 10%, time stretches and efficiency falls. That’s why a partial refill feels “fast,” while going all the way can drain a small bank sooner than the math suggests.
Device overhead. Phones continue to run radios, screens, and apps while charging. If you’re navigating, streaming, or recording video, part of the bank’s energy feeds the workload instead of the battery.
Temperature. Cold cells resist charging and discharging. Hot cells throttle to stay safe. Mid-room temps deliver the best results.
Make Better Predictions With Watt-Hours
Watt-hours let you compare apples to apples. If your device lists mAh and voltage, convert to Wh. If only mAh appears, most small gadgets use cells around 3.6–3.85 V. Multiply and divide by 1,000 to get close.
Rule Of Thumb For Tiny Banks
- Energy on label: 1,200 mAh × 3.7 V ≈ 4.4 Wh.
- At the USB port (after boosting): ~3.8–4.0 Wh.
- Stored in the device battery (after step-down + heat): ~3.2–3.6 Wh.
- What that means: a couple of wearable refills or a modest phone bump.
Worked Examples You Can Copy
Example 1: Phone With An 11.5 Wh Battery
Delivered energy from the bank: ~3.4 Wh. 3.4 ÷ 11.5 ≈ 0.30. Expect about a 30% boost in ideal conditions. With screen on and navigation active, shave a bit off.
Example 2: Smartwatch With A 300 mAh Cell At 3.8 V
Battery energy ≈ 300 × 3.8 ÷ 1,000 = 1.14 Wh. The bank can store roughly three of those. Realistically, plan for two to three quick refills.
Example 3: Earbuds Case At 500 mAh, 3.7 V
Battery energy ≈ 500 × 3.7 ÷ 1,000 = 1.85 Wh. Expect one full refill plus a little extra, or two smaller refills if you stop shy of 100% each time.
Charging Time And Output Current
Small banks usually cap output at 1 A. Some reach 2 A, but sustained high current in a tiny shell builds heat and trims efficiency. Since energy is limited, pushing current faster doesn’t increase total delivered Wh, it just ends the session sooner. Use the table to gauge session length by port rating.
| USB Output | Approx. Session Length* | Best Use |
|---|---|---|
| 5 V / 0.5 A | ~6–8 hours for 0.5 W loads; ~1.5–2 hours when charging phones | LED lights, wearables, slow phone top-ups |
| 5 V / 1 A | ~3–4 hours for 1 W loads; ~45–60 minutes near max phone draw | Fans, small routers, quicker phone bumps |
| 5 V / 2 A | Shorter sessions; heat rises, efficiency dips | Only if the bank and cable are rated for it |
*Session length reflects available ~3–4 Wh at the port. Phones don’t draw full current all the time; charging slows near full.
Tips To Stretch Runtime From A Tiny Pack
Charge Smart, Not Long
Top up when your device sits between 20–50%. Stop around 80–90% if you only need to reach evening. That sweet spot wastes less energy on the slow tail.
Use Short, Thick Cables
Thin, bargain cables drop voltage. The bank then works harder, gets warmer, and delivers fewer watt-hours. A short, quality cable pays for itself in saved energy.
Keep It Cool
High heat robs efficiency. Don’t bundle the charger and phone inside a tight pocket while charging. Give the pair some airflow.
Silence The Energy Hogs
During a quick top-up, dim the screen, pause GPS-heavy apps, and turn off hotspots. Less load means more of the bank’s energy reaches the battery.
When A 1,200-mAh Power Bank Makes Sense
This size shines for commuters and travelers who carry wearables, earbuds, and a compact phone. It’s feather-light, fits any pocket, and can save a day when you need a pickup late afternoon. If your main target is a big-battery phone or a tablet, step up to a larger pack measured in tens of watt-hours.
Spec Sheet Clues That Matter
Labeled Capacity And Cell Voltage
Look for figures that state capacity at 3.6–3.8 V. Some marketing copy quotes “equivalent at 5 V,” which can confuse comparisons. Converting everything to Wh keeps it straight.
Output Ratings And Efficiency
Peak efficiency often appears on converter datasheets and can cross 90% under ideal lab loads (see the TI boost-converter spec). Real life sits lower, especially with heat and small enclosures. Treat 85–90% as a sensible range for planning.
Cycle Life And Care
Lithium-ion cells hold up best when kept out of extreme heat and stored near mid-charge. Shallow cycles stress them less than frequent 0→100% swings. That keeps small banks useful longer.
DIY Check: Run Your Own Estimate
- Find your device battery in Wh, or convert from mAh × V ÷ 1,000.
- Use 4.44 Wh as the internal energy for a 1,200-mAh bank at 3.7 V.
- Multiply by 0.85–0.90 to estimate energy at the USB port.
- Divide by your device’s Wh to get an expected percentage or number of refills.
If you’d like a second opinion on the conversion itself, check an engineering-grade explanation of the Wh formula. You can also browse a manufacturer datasheet to see how high-efficiency boost stages reach into the 90% range under ideal test conditions (technical reference).
Bottom Line For Everyday Use
A 1,200-mAh pocket charger is best for wearables and earbuds, handy for a modest phone bump, and not the right tool for tablets or power-hungry cameras. Think in watt-hours, aim for partial refills, and mind heat and cables. With those habits, this tiny pack punches above its size when you need it.