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Calculating Power Bank Needs for Laptop Charging: Watt-Hours vs. mAh
Key points
- Watt-hours (Wh) are a better measure of power bank capacity than mAh for laptops.
- Convert mAh to Wh by multiplying by voltage (e.g., 3.7V for lithium-ion cells).
- Laptop battery Wh ratings and estimated power usage help determine power bank needs.
- Charging efficiency losses, typically 20-30%, must be considered.
Understanding Power Bank Capacity
When selecting a power bank for laptop charging, watt-hours (Wh) provide a more accurate measure of capacity than milliamp-hours (mAh). While mAh is commonly listed, it needs to be multiplied by the voltage to determine the true energy stored in Wh, which is crucial for higher-drain devices like laptops. Most power banks use lithium-ion cells at approximately 3.7 volts.
Converting mAh to Wh
To convert mAh to Wh, multiply the mAh value (converted to amp-hours by dividing by 1,000) by the voltage. For example, a 10,000 mAh power bank operating at 3.7V stores roughly 37Wh (3.7V x 10Ah). A 20,000 mAh bank would store approximately 74Wh, assuming consistent voltage.
Estimating Laptop Power Needs
Identify your laptop's battery Wh rating, usually found on the manufacturer's website. To estimate average power usage, divide the battery's Wh capacity by its typical runtime in hours. For instance, a 70Wh battery lasting 5 hours averages 14W of power usage. Instantaneous power usage varies based on tasks, from low for document scrolling to high for video rendering.
Considering Charging Efficiency
The charging process is not 100% efficient. Energy is lost due to conversion and heat, especially with fast charging standards like USB Power Delivery, which require the power bank to increase its voltage. A general rule of thumb for efficiency loss is 20% for normal charging, potentially increasing to 30% or more for faster charging methods.
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Reporting from
This guide explains how to determine the appropriate power bank for charging a laptop by focusing on watt-hours (Wh) for capacity and watts (W) for output, rather than just milliamp-hours (mAh). It details the conversion from mAh to Wh and how to estimate a laptop's power consumption to match it with a suitable power bank.