Battery Life (hrs + mins): The total runtime before the battery is completely discharged at constant power draw.
Days at 4 hrs/day: How many days the battery will last if you use the device for 4 hours each day. Adjust this estimate based on your actual daily usage.
Continuous vs. intermittent: These estimates assume steady power draw. Real-world usage with on/off cycles often extends battery life.
Voltage impact: When converting from watts, voltage affects the current draw—higher voltage for the same power reduces current in milliamps.
Multiple devices: Adding more devices multiplies the total power draw, significantly reducing runtime.
How This Calculator Works
This calculator uses the fundamental battery formula: Runtime = Capacity ÷ Current Draw. If your power is in watts, the tool first converts it to milliamps using the battery voltage (Watts ÷ Volts = Amps). When you add multiple devices, it multiplies the per-device current by the device count, then divides the battery capacity by total draw. The daily usage estimate divides total runtime by 4 hours to estimate how many full days the battery will last under that assumption.
Quick Questions
Why is my estimate different from actual runtime? Batteries don't discharge linearly, efficiency varies with temperature, and real-world devices don't draw constant power. This calculator assumes ideal conditions; real results are often longer due to intermittent usage patterns.
What if I don't know the power draw in mA? If you have the spec sheet in watts, switch to the Watts unit and enter your battery voltage (usually printed on the battery). The calculator converts automatically.
How do I find battery capacity and power draw specs? Check the device manual, battery label, or product datasheet. Capacity is in mAh (milliamp-hours) and power draw is either in mA or watts depending on the device.
Does this work for rechargeable batteries? Yes—the formula is the same. Just use the rated capacity (mAh) and the device's power consumption. Older batteries may have lower effective capacity due to degradation.