Why efficiency matters as much as capacity
A battery's rated capacity in mAh is how much charge it can deliver under ideal test conditions, but a real device rarely gets all of it — power converters lose some as heat, and the battery itself can't sustain its full rating under real loads. The efficiency factor scales the rated capacity down to a more realistic usable amount before dividing by the current draw.
- capacity — battery's rated capacity (mAh)
- load — device's average current draw (mA)
- efficiency — fraction of rated capacity actually usable (0–100%)
More detail
Picking a realistic efficiency number
100% efficiency only applies to a lab test at the battery's rated discharge current. A device powered through a voltage regulator or DC-DC converter typically sees 80–90% efficiency from conversion losses alone; add in a battery that's older, cold, or being discharged faster than its rated current, and usable capacity can drop further. When in doubt, 80–85% is a safer planning number than 100%.
Reading a datasheet's current draw. If a device lists both an active current and a standby/sleep current, this calculator only handles one steady value — run it once with the active current for a worst-case runtime, and once with the average of active/sleep (weighted by duty cycle) for a more realistic estimate.
Frequently asked questions
A 2000mAh battery powers a 500mA load — how long does it last?
At 100% efficiency: 2000 × 1.0 / 500 = 4.0 hours. At a more realistic 85% efficiency: 2000 × 0.85 / 500 = 3.4 hours (3h 24min).
Why does the runtime shown here not match what I measured on a real device?
This calculator assumes a constant current draw and constant usable capacity, but real batteries deliver less capacity at higher discharge currents (the Peukert effect) and their voltage sags as they empty, which can trigger a device's low-battery cutoff before the math says it should run out.
My device draws 150mA on average — how long will a 3000mAh power bank last?
At 85% efficiency: 3000 × 0.85 / 150 = 17.0 hours. Power banks also lose some energy in their own internal conversion, which is exactly what the efficiency factor is meant to approximate — a lower value like 75–80% may be more realistic for a power bank than for a bare cell.
What efficiency should I use for a bare cell with no converter?
Closer to 90–95% is reasonable for a simple resistive or near-ideal load straight off the cell, since there's no conversion stage losing energy — the main loss is the battery's own internal resistance, which is small under light loads.