Power Bank Device Recharge Calculator
Introduction: realistic power bank recharge estimates
Portable power banks promise simple backup power, but the label on the case rarely tells the whole story. A bank sold as 20,000 mAh does not deliver 20,000 mAh directly into a phone, tablet, camera, or laptop. The cells inside the bank sit around 3.7 volts, while USB output is boosted to 5, 9, 12, or 20 volts. That voltage conversion wastes some energy as heat. Cables, charge controllers, and the device's own battery management system take a little more.
This calculator turns the label into a usable estimate. You can enter the power bank in watt-hours or in milliamp-hours, choose a common device preset, adjust charging efficiency, and see both the number of full recharges and the likely time per recharge. The goal is not to make the power bank look better or worse than the box. It is to give you a travel-planning number you can trust.
The recharge formula behind the estimate
The core concept is energy conservation. A power bank stores a certain number of watt-hours. When that energy is transferred to a device, some is lost as heat in the voltage regulation circuitry and cables. The efficiency percentage captures this loss. The usable energy for recharging is the power bank capacity multiplied by the efficiency. Dividing this usable energy by the device's battery capacity gives the number of full recharges. The charging time depends on the power delivered to the device; dividing the device's capacity by the charge power yields the hours required per charge.
Mathematically, the number of full charges N and the per-cycle time T are:
In plain language, multiply the power bank capacity Cpb (in Wh) by the efficiency η expressed as a decimal, then divide by the device battery capacity Cd. The charging time per cycle divides the device capacity by the charging power P in watts. The calculator handles the mAh-to-Wh conversion automatically.
Plain-text formula: bankWh = mAh × nominalVolts ÷ 1000 (or entered directly in Wh); fullCharges = bankWh × (efficiency ÷ 100) ÷ deviceWh; hoursPerCharge = deviceWh ÷ chargeWatts; totalChargingHours = fullCharges × hoursPerCharge.
Source/version metadata: the 3.7 V nominal cell voltage is the lithium-ion industry convention used on power-bank labels, and 80–90% end-to-end conversion efficiency reflects typical USB boost-converter and cable losses; airline rules referenced below follow FAA/IATA lithium-battery limits (100 Wh carry-on without approval). Last reviewed July 2026.
Worked example: a 100 Wh bank and a 12 Wh phone
Consider a worked example. Suppose you have a 100 Wh power bank and want to recharge a 12 Wh smartphone. Assuming 85% efficiency, the usable energy is 85 Wh. Dividing by 12 Wh yields about seven full charges. If the phone charges at 18 W, each charge takes roughly 0.67 hours, or about forty minutes. The calculator will display these numbers and also show a table comparing other power bank capacities.
The table below uses the default device and efficiency inputs and shows how many charges are available from different power bank sizes. This helps illustrate diminishing returns; doubling capacity does not always double usefulness due to weight, cost, and airline restrictions.
| Bank Capacity (Wh) | Full Charges | Total Charging Hours |
|---|
Power bank marketing often lists capacity in milliamp-hours (mAh). To convert mAh at a nominal 3.7 volts to watt-hours, multiply by 3.7 and divide by 1000. For example, a 20,000 mAh bank is roughly 74 Wh before conversion losses. Phone batteries are often close to 3.85 volts, while laptop batteries are usually already labeled in Wh. The calculator lets you use either unit so you do not have to do the conversion by hand.
Assumptions and limitations of the estimate
Limitations of this calculator include assuming constant charging power and ignoring device tapering, where charging slows as the battery fills. It also assumes the power bank can deliver the necessary voltage and current for the device. Some phones require specific fast-charging protocols that may reduce efficiency or limit power if not supported. A laptop may accept 65 W from one USB-C bank and only 30 W from another. A camera may charge slowly even when the bank has plenty of stored energy.
Temperature, age, and cable quality matter too. Cold weather can reduce usable lithium-ion capacity during a trip. Heat can age the cells permanently. A thin or damaged cable can waste power as heat or prevent a fast-charge handshake. For everyday planning, 80 to 90 percent efficiency is a reasonable starting range for phone-sized loads, while laptop charging through a busy USB-C power path may land lower.
Despite these caveats, the calculator is a handy planning tool. It can help you decide whether a compact bank covers a weekend, whether a 100 Wh pack is worth the weight, or whether you should bring a wall charger instead. It also reveals the quiet tradeoff that marketing copy often hides: bigger banks give more energy, but they also add bulk, take longer to refill, and may cross airline battery limits.
For related tools, try the Portable Power Station Solar Recharge Time Calculator and the Portable Projector Battery Life Calculator. Both explore portable energy storage in different contexts.
By quantifying realistic expectations, this calculator helps reduce range anxiety for electronics and encourages efficient energy use on the go.
Power bank questions travelers ask
Why does a 20,000 mAh power bank charge my phone fewer times than the label math suggests?
The 20,000 mAh rating describes the internal 3.7 V cells, which store about 74 Wh. After typical 80โ90% conversion losses you have roughly 60โ66 Wh usable, and a modern phone battery holds 12โ18 Wh, so three and a half to five full charges is normal. Dividing label mAh by phone mAh ignores both the voltage difference and the conversion loss.
Can I take my power bank on a plane?
Under FAA and IATA rules, lithium power banks must travel in carry-on baggage, never checked. Banks up to 100 Wh (about 27,000 mAh at 3.7 V) need no approval; 100 to 160 Wh requires airline approval and is usually limited to two units. Larger banks are prohibited on passenger aircraft, which is why 99 Wh travel banks are so common.
What efficiency percentage should I enter?
Use 85% as a sensible default for phone-class charging over USB. Drop to 80% for older or budget banks, thin cables, or cold conditions; quality USB-C Power Delivery at 15โ20 V into a laptop often reaches about 90% because less voltage boosting is wasted. If you measure real numbers with a USB power meter, enter those instead.
How long does it take to refill the power bank itself?
Divide the bank's watt-hours by the input power of your wall charger and add roughly 20% for charging losses and the slow top-off phase. A 74 Wh bank on an 18 W charger needs about five hours; the same bank on a 65 W USB-C charger can finish in under two, if the bank's input circuitry accepts that power.
How to use the result
If the result says 2.7 full charges, read that as two reliable full recharges plus a partial third charge under similar conditions. If the device is already at 40 percent, the same bank may top it up several more times because each top-up uses only part of the device battery. If you are planning emergency power, round down. If you are planning convenience power for a desk or hotel room, the decimal estimate is usually fine.
This calculator intentionally excludes cost calculations to keep the interface simple, but you can estimate the electricity cost of refilling your power bank by multiplying its capacity by your utility rate. For most small banks, the cost per full recharge is only a few cents. The more important planning questions are weight, allowed carry-on size, charging speed, and whether the power bank supports the device's fast-charge protocol.
Power Relay Arcade
Keep three expedition devices alive by routing charge bursts without emptying your power bank. Every watt-hour you deliver is your score.