What this calculator does
Charging time is the energy still needed divided by the power actually going in. A 50 Wh laptop battery at 20% needs 40 Wh to reach full, and a 65 W charger running at 85% efficiency delivers about 55.3 W, which gives roughly 43 minutes.
That estimate is a floor rather than a prediction, because it assumes the charger runs at full power the whole way. Real lithium batteries do not allow that. They accept full power up to somewhere around 80% and then taper sharply to protect the cell, so the last stretch takes far longer than its share of the energy suggests. Treat the figure as accurate in the lower range and optimistic near the top.
The formula
The energy needed is the battery capacity multiplied by the gap between the current and target percentages. The power actually reaching the battery is the charger rating multiplied by the efficiency, since some is lost as heat in the adapter and the charging circuit. Dividing the first by the second gives the time, and the calculation treats the charging rate as constant throughout.
| Term | Meaning |
|---|---|
| Capacity (Wh) | The energy the battery holds when full, in watt-hours. A rating in mAh needs multiplying by the voltage to get there. |
| Charging efficiency | The share of the charger output that reaches the battery, typically 0.80 to 0.90, with the rest lost as heat. |
| Effective charging power | Charger rating multiplied by efficiency, which is what the calculation actually divides by. |
| Taper | The deliberate slowdown in charging above roughly 80%, which this calculation does not model. |
The inputs explained
| Field | What to enter |
|---|---|
| Battery capacity (Wh) | Battery capacity in watt-hours, usually printed on the battery or in the specifications. For a rating in mAh, multiply by the nominal voltage and divide by 1,000. |
| Current charge (%) | The current charge level as a percentage. |
| Target charge (%) | The level you want to reach. Setting this to 80 gives a far more realistic estimate than 100, for the reason described above. |
| Charger power (W) | The charger output in watts. If the charger and the device negotiate a lower rate, use the negotiated figure rather than the rating on the brick. |
| Charging efficiency | Charging efficiency as a decimal. 0.85 is a reasonable default for a modern USB-C charger. |
When to use it
Working out whether a charge will finish in time
The common question before leaving somewhere is whether there is enough time to get usefully charged. Setting a target of 80% rather than 100% gives both a faster answer and a more realistic one.
Comparing chargers
Charger ratings are easy to compare on paper and harder to translate into minutes. Running the same battery against several wattages turns the specification into the figure you actually care about.
Sizing a power bank or a solar setup
The energy needed figure, in watt-hours, is what a power bank has to supply, and it is the number to match against a power bank capacity rather than the charge percentage.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much faster is a higher-wattage charger?
The same charge across a range of charger outputs.
| Charger power | Charge time | Effective charging power | In minutes |
|---|---|---|---|
| 20 W | 2 h 21 min | 17.0 W | 141 min |
| 30 W | 1 h 34 min | 25.5 W | 94 min |
| 45 W | 1 h 2 min | 38.3 W | 63 min |
| 65 W | 0 h 43 min | 55.3 W | 43 min |
| 100 W | 0 h 28 min | 85.0 W | 28 min |
Why stopping at 80% saves more time than it looks like
The same battery and charger, charged to four different target levels.
| Target level | Charge time | Energy needed | In minutes |
|---|---|---|---|
| 50% | 0 h 16 min | 15.0 Wh | 16 min |
| 80% | 0 h 32 min | 30.0 Wh | 33 min |
| 90% | 0 h 38 min | 35.0 Wh | 38 min |
| 100% | 0 h 43 min | 40.0 Wh | 43 min |
Questions
Why does my device charge more slowly than this says?
Most often because of the taper above 80%, which this calculation does not model. Heat is the other common cause: a warm battery, or charging while the device is in use, both reduce the rate the charging circuit will accept. The estimate is closest to right in the lower part of the range.
How do I convert mAh to watt-hours?
Multiply the mAh rating by the nominal voltage and divide by 1,000. A 5,000 mAh phone battery at 3.85 V is 19.25 Wh. Comparing mAh figures across devices at different voltages is meaningless, which is why watt-hours is the useful unit.
What charging efficiency should I use?
Between 0.80 and 0.90 covers most modern equipment, and 0.85 is a reasonable default. Wireless charging is considerably worse, often nearer 0.60 to 0.70, because the energy crosses an air gap. The heat a charger gives off is the losses you are accounting for.
Does a bigger charger damage the battery?
No. The device negotiates how much power it will draw, so a higher-rated charger simply has headroom it does not use. What it cannot do is push more power than the device asks for. Heat and sustained time at 100% are what age a lithium battery, not charger capacity.
For converting between watt-hours, amp-hours and milliamp-hours, see the watt-hours calculator. For the cost and time of charging an electric car, see the EV charging cost calculator.