The power bank last calculator uses usable Wh = mAh×3.7/1000×efficiency to estimate charges and hours for your device.
Here’s a clear way to turn the numbers on your pack into real-world time. You’ll enter your pack’s labeled capacity, a realistic efficiency, and your device’s battery size or average power draw. The tool below converts everything into watt-hours and gives you two answers: the number of full recharges and an estimate of continuous runtime in hours at a chosen load.
Power Bank Runtime Calculator Steps
This quick tool follows industry math. It converts milliamp-hours to watt-hours using a 3.7 V nominal cell value, then applies an efficiency factor to model losses in the boost converter and cables. That usable energy is compared against your device’s battery or average watt draw. You can tweak each field to match what you own.
Try The Runtime & Recharges Calculator
const pbWhRaw = (pbMah/1000) * pbVolt; const pbWhUse = pbWhRaw * eff;
let charges = null, hours = null; if(devMah && devVolt){ const devWh = (devMah/1000) * devVolt; if(devWh>0) charges = pbWhUse / devWh; } if(loadW>0){ hours = pbWhUse / loadW; }
const lines = [ `Usable energy: ${round2(pbWhUse)} Wh (from ${round2(pbWhRaw)} Wh × ${Math.round(eff*100)}%)` ]; if(charges!==null) lines.push(`Estimated full recharges: ${round2(charges)}×`); if(hours!==null) lines.push(`Estimated runtime at ${loadW} W: ${round2(hours)} hours`); if(charges===null && hours===null) lines.push(`Add a device battery or a load to see results.`); document.getElementById('results').innerHTML = lines.map(l=>`
${l}
`).join(""); }
Core Formula And Why The Numbers Make Sense
Capacity labels on packs are printed in mAh at cell level. The energy behind that label is Wh = (mAh ÷ 1000) × cell voltage. Most lithium-ion cells are rated around 3.6–3.7 V nominal, which is a mid-point across the charge curve. Aviation rules publish the same watt-hour math, and many packs also print the Wh value on the case.
From Watt-Hours To Real Use
Once you have watt-hours, apply an efficiency. Energy passes through a boost converter to reach 5 V or a USB-C PD voltage, then through the device’s own charging circuit. Heat and conversion drop total energy available to your phone, tablet, earbuds, or camera. A practical range is 70–90% depending on build quality, cable gauge, temperature, and output voltage. The calculator lets you pick a value that fits your gear and conditions.
Why 10,000 mAh Rarely Equals Two Perfect Phone Charges
Marketing uses cell-level mAh at about 3.7 V, while your phone’s battery sits near 3.8 V nominal and the device draws extra for its screen and radios if you keep using it while charging. These factors explain why real outcomes land below simple “mAh ÷ mAh” arithmetic. Using watt-hours with an efficiency factor gives a closer picture.
Quick Reference: Common Pack Sizes And Realistic Output
This table gives ballpark energy after an 85% setting. The “charges” column shows typical outcomes against 4,500 mAh phone batteries near 3.85 V when the phone is idle during most of the charge.
| Label Capacity | Usable Energy (Wh) | Phone Charges (≈4,500 mAh) |
|---|---|---|
| 5,000 mAh | 15.7 Wh | ≈0.9–1.0× |
| 10,000 mAh | 31.5 Wh | ≈1.8–2.0× |
| 20,000 mAh | 63.0 Wh | ≈3.6–4.0× |
| 27,000 mAh | 85.1 Wh | ≈4.9–5.5× |
| 30,000 mAh | 94.6 Wh | ≈5.5–6.1× |
What Efficiency Setting Should You Use?
Pick a starting point and adjust after a real trip. Many mid-range packs land near 80–85% under 5 V loads. Compact designs with warmer cases or thin cables can dip to the low-70s. Larger packs with quality cells and short USB-C cables can reach the high-80s. USB-C PD at 9–20 V adds conversion on both sides, so sustained high-watt charging can shave a few points off the top. If your phone stays on with maps, hotspot, or camera recording, your “charges” count drops because part of the energy runs the phone rather than filling the battery.
How Output Voltage Changes The Math
Watt-hours stay the same, but higher output voltage often means more converter loss. That is why the same 10,000 mAh pack may deliver more total milliamp-hours at 5 V than at 20 V. When estimating laptop runtime over USB-C PD, use the hours result in the calculator with your average watt load instead of the phone-style “charges.”
Set Up A Realistic Test
For a quick check at home, charge your phone from near 10% to near 100% with the display off and Wi-Fi or data on but idle. Note the percent gained. Repeat until the pack empties, then add the gains together. Compare the total with the calculator’s “charges” number at your chosen efficiency. If your result is far lower, try a shorter, thicker cable, lower the output level where possible, and keep the pack out of hot pockets and dashboards.
Units, Standards, And Safe Limits
Watt-hour ratings are standardized. Aviation guidance defines Wh as nominal voltage multiplied by ampere-hours and asks manufacturers to print Wh on packs above a small threshold. That same math underpins the conversion in the calculator. USB-C and USB Power Delivery define fixed and programmable voltages up to high levels for laptops and monitors. The bigger the draw, the more sensitive the system is to cable quality and heat, which is why short, certified cables are a good pick for high-watt charging.
If you like reading the source rules, see the IATA explanation of Watt-hour calculation and the USB-IF page on USB Power Delivery levels. Both align with the math used here.
Worked Examples You Can Copy
Phone Recharges From A 10,000 mAh Pack
Pack energy: (10,000 ÷ 1000) × 3.7 = 37.0 Wh. At 85%: 31.45 Wh. Phone battery: (4,500 ÷ 1000) × 3.85 ≈ 17.33 Wh. Estimated fills: 31.45 ÷ 17.33 ≈ 1.81×. If you keep the screen on while shooting video, your total will drop.
Camera Runtime In Watts
Pack energy: 20,000 mAh → (20,000 ÷ 1000) × 3.7 = 74.0 Wh. At 80%: 59.2 Wh. If the camera draws 7 W while recording, estimate 59.2 ÷ 7 ≈ 8.46 hours.
Troubleshooting Low Results
Lossy Cable
Thin or long leads drop voltage. That drop forces higher current, which wastes more energy as heat. Try a shorter USB-C cable rated for your target watt level.
Hot Pack Or Hot Phone
High temperature reduces charge acceptance and raises resistance. Shade both devices. Avoid cases that trap heat during high-watt sessions.
Mismatch Between Charger And Device
Some packs lock to 5 V with a basic cable. Some phones expect PD or PPS to request higher voltages. Use a USB-C to USB-C cable that negotiates the right profile for your device.
Phantom Drain
Many packs sip a little power even when idle. If the indicator shows a drop overnight, top off before a trip. Avoid leaving the pack on a hot dash or in a closed tent on sunny days.
Second Reference Table: Output Modes And Expected Efficiency
These ranges help you pick the right efficiency slider for the calculator when you change ports or cables.
| Output Mode | Typical Use | Efficiency Range |
|---|---|---|
| USB-A 5 V | Earbuds, small phones, action cams | 80–90% |
| USB-C PD 9–12 V | Phones/tablets fast charge | 75–88% |
| USB-C PD 15–20 V | Ultrabooks, handheld PCs, Switch | 70–85% |
FAQ-Style Clarifications Without The Fluff
Is mAh Or Wh Better For Estimating?
Use watt-hours for clean comparisons. Milliamp-hours depend on voltage, so they can mislead when you move between different outputs.
Do Higher Voltages Charge Faster?
Higher PD steps allow more watts when the phone or laptop asks for them. That cuts time when the device supports it, but step-up losses can rise a little. Use the hours calculator with your typical watt draw for laptops.
Why Does My Pack Deliver Less Over Time?
Rechargeable cells age with cycles and heat. The management board also uses a little power each time you press the button or leave a cable attached. Expect gradual decline across many trips.
Practical Buying Notes
Look for a printed Wh rating on the case and a spec table that lists PD or PPS profiles that match your devices. A pack near 20,000 mAh balances cabin rules with strong capacity. For phones and compact cameras, 10,000 mAh is a handy size you can pocket. If you often shoot 4K video, edit on a tablet, or game on a handheld PC, step up one size and carry a short, certified cable for best results.
Keep This Page Handy
Bookmark the calculator, save your usual efficiency, and punch in new devices as you upgrade. The watt-hour approach travels well from phones to tablets to cameras and back to laptops. Once you know usable Wh, the rest is simple math.