Runtime is usable battery capacity divided by average current draw. A 2000 mAh pack feeding a board that averages 120 mA, at 80% usable capacity, lasts about 13 hours. Change the three inputs below to size your own.
Inputs
Result
13 h 20 m
estimated runtime
13.3 hours at 120 mA average
The formula
runtime (hours) = capacity (mAh) × usable fraction ÷ average draw (mA). Capacity and draw share the same mA unit, so they cancel to hours. Two inputs decide the answer, and both are easy to get wrong: which current you use, and how much of the printed capacity you actually get.
Use the average current, measured over a full duty cycle
The figure that sets runtime is average draw across a complete wake and sleep cycle, in mA. Peaks barely matter. On the OTD L1.01 ESP32-S3 board, the power budget is a 500 mA Wi-Fi transmit peak plus about 50 mA for the rest of the board, roughly 550 mA total against the regulator’s 600 mA ceiling (One Thousand Drones, L1.01 design 2026). That 550 mA peak lasts microseconds during a transmit burst. A sensor node that wakes, samples, sends, then sleeps spends most of its life near its sleep floor, so its average can sit well below 120 mA. Size runtime on the average. Size the regulator and the wiring on the peak.
Usable capacity is never 100%
A LiPo gives your board less than its printed mAh. Some charge sits below the regulator’s dropout cutoff, and a little more goes to converter loss. A usable fraction of 70 to 85% covers most linear-regulator designs; an efficient switch-mode converter can do better. The calculator defaults to 80%.
Worked example
Take a 2000 mAh single-cell LiPo on the L1.01 board, firmware that duty-cycles Wi-Fi down to a 120 mA average, and 80% usable capacity: 2000 × 0.80 ÷ 120 = 13.3 hours. Halve the average draw and the runtime doubles, which is why firmware sleep usually buys more runtime per dollar than a bigger cell.
References
- Espressif Systems. ESP32-S3 datasheet, current consumption characteristics.
- One Thousand Drones. ESP32-S3 USB-C breakout (L1.01), measured power budget. Build the board.