Device Battery Life

Battery life is calculated in two steps. First, convert the battery capacity from milliamp-hours (mAh) to watt-hours (Wh). Then divide by the average power draw.

Watt-hours (Wh) = (mAh × Voltage) / 1000

Battery Life (hours) = Wh / Average Power Draw (W)

What the variables mean:

• mAh (milliamp-hours): The battery capacity listed on your device specs. Higher means more stored energy.
• Voltage (V): The battery operating voltage. Most smartphones and tablets use 3.7V lithium batteries. Laptops typically use 7.4V or 11.1V.
• Power draw (W): How much power your device consumes on average during use.

Typical battery capacities and power draws:

Practical example: A phone with a 5,000 mAh battery at 3.7V has 18.5 Wh of energy. At an average draw of 3W (light browsing), it lasts about 6 hours and 10 minutes.

Why you will not actually get that. The division above assumes the device drains the cell to nothing and that every stored watt-hour reaches the screen. Neither is true. Regulators and conversion losses eat a slice, and the device cuts off at a set voltage well before the cell is empty, because running a lithium pack flat damages it. Between the two, plan on about 85% of the ideal figure. The calculator shows both so you can see the gap.

Cold weather widens it further. A lithium cell at freezing point delivers roughly a fifth less than the same cell at room temperature, and it comes back once it warms up, which is why a phone that “died” outside will often start again indoors with 20% showing.

One number people mix up: mAh is not energy, and comparing mAh across devices with different voltages is meaningless. A 5,000 mAh phone battery at 3.7V holds 18.5 Wh. A 5,000 mAh laptop battery at 11.1V holds 55.5 Wh, three times as much, from the same headline number. Airlines size their carry-on limits in watt-hours for exactly this reason, and it is why the number on a power bank tells you less than you would think.

Tips: Screen brightness is the single biggest factor in power draw. Reducing brightness from 100% to 50% can extend battery life by 30% or more. WiFi uses less power than cellular data. Background apps and GPS also increase drain significantly.