Charging time looks like simple division — battery size divided by charger power — but three things change the answer: the laptop uses part of the charger's power while it runs, conversion is not lossless, and the last part of the charge goes much more slowly. This calculator accounts for all three and gives a realistic estimate.
How to use it
- Enter the battery capacity in Wh and the start and target levels.
- Enter the charger's rated power.
- Enter what the laptop uses while charging (0 if it is off).
- Adjust efficiency if you know it, and optionally a charge-power cap if your laptop limits charging.
How the estimate works
- Power available for the battery = charger power × efficiency − laptop load.
- Lithium-ion charging runs at constant current until roughly 80 %, then at constant voltage while the current tapers. The model counts the part above 80 % at half the power, which matches typical behaviour closely enough for planning.
- Many laptops also limit charging power, or slow down when the battery is warm. Battery-care settings that stop at 80 % shorten the time dramatically.
Worked example: why charging is slow under load
A 56 Wh battery at 20 % on a 65 W charger, with the laptop using 10 W and 88 % efficiency: about 47 W reaches the battery, and a full charge takes a little over an hour. Run a demanding game drawing 45 W and only about 12 W is left for charging — the same charge then takes several hours, and under heavier loads the battery may even drain while plugged in. That is not a fault; the charger is simply too small for the combined demand. Charger sizing is covered by Laptop Charger Compatibility and Wattage Calculator, and USB-C limits by USB-C Power Delivery Calculator.
When the estimate is far off
If a laptop that is switched off takes many times longer than the estimate, look for:
- an underpowered or wrongly identified charger;
- a battery health setting limiting charge;
- a charge current far below normal — measurable across the charger's sense resistor with Current Sense Resistor Calculator;
- a failing battery or charging circuit, as in Laptop Not Charging: Charging Circuit Diagnosis.
FAQ
Why does the last 20 % take so long?
In the constant-voltage phase, the current falls as the battery approaches full. This protects the cells.
Does a bigger charger charge faster?
Only up to the laptop's own charge limit. Beyond that, extra wattage just covers the system load.
Does charging while using the laptop harm the battery?
Not in itself. Heat is the main enemy; heavy use while charging makes the battery warmer.