Battery charging time calculator
Enter the battery size, the charger's watts and your starting level. The estimate includes charging losses, the device's own speed limit and the slowdown after 80% that most calculators ignore.
By Zubair Abid. Updated .
Quick answer
Charging time ≈ energy needed (Wh) ÷ (charger watts × efficiency), with the last 20% taking about twice as long. A 5,000 mAh phone (19.25 Wh) on a 25 W charger reaches 80% from 10% in about 38 minutes and 100% in about an hour.
The battery
The charger
How is charging time calculated?
Time (h) = Energy needed (Wh) ÷ (Charging power (W) × Efficiency)
Energy needed is the battery's Wh multiplied by the percentage you want to add. Charging power is the lower of two numbers: what the charger can supply and what the device will accept. A 100 W charger won't charge a phone that is limited to 25 W any faster. Efficiency covers heat and conversion losses, typically 80 to 90%.
Why is the last 20% so slow?
Lithium batteries charge in two stages. First, constant current: the charger pushes in as much as the battery safely takes, and the percentage climbs fast. Around 80%, it switches to constant voltage: the current gradually drops to avoid stressing the cells.
The calculator models this by charging the last 20% at half speed. It's a simplification, but it matches real devices far better than a straight line, and it's why '0 to 50% in 30 minutes' claims are common while '0 to 100%' times are rarely advertised.
Typical charging times
| Device | Battery | Charger | Estimated time |
|---|---|---|---|
| Phone | 5,000 mAh (19.25 Wh) | 25 W | About 1 h |
| Phone | 5,000 mAh (19.25 Wh) | 10 W (old USB-A) | About 2 h 30 min |
| Power bank | 20,000 mAh (74 Wh) | 20 W | About 4 h 45 min |
| Laptop | 60 Wh | 65 W | About 1 h 10 min |
| Power station | 1,024 Wh | 500 W AC input | About 2 h 40 min |
Power stations often advertise much faster AC charging (for example 0 to 80% in under an hour) because they accept 1,000 W or more. Enter the input watts from the spec sheet.
How can you charge faster, and should you?
- Match the charger to the device's maximum. Beyond that, extra watts do nothing.
- Use a cable rated for the power. USB-C above 60 W needs a 5 A e-marked cable. See the laptop charger checker.
- Keep it cool. Phones slow charging when hot. Take off thick cases and avoid charging in direct sun.
- Stop at 80% when you can. Many phones and laptops offer an 80% limit that extends battery life over the years.