Battery Life / Runtime Calculator

How Long Will That Battery Actually Last?

The basic runtime formula — capacity divided by current draw — only tells half the story, because it assumes 100% of a battery's rated capacity is usable and that current draw never changes. Real batteries lose some capacity to internal resistance and voltage sag, and real devices don't draw a constant current the whole time (a sensor that sleeps most of the day and wakes briefly draws almost nothing most of the time). This calculator handles both: a simple mode for a steady current draw, and an advanced mode that averages an active and a sleep/idle current across a day.

How to use it

• Enter your battery's capacity and pick its unit — mAh and Ah are the most common for standalone batteries, Wh shows up more on larger packs and power banks.
• Enter the battery's nominal voltage — this is what lets the calculator convert between capacity units and current/power draw units if they don't match.
• Enter your device's current draw (or power draw in watts), and adjust the efficiency factor down from 100% to account for real-world losses — 80-90% is a reasonable range for most consumer electronics.
• For devices that spend most of their time idle (like a battery-powered sensor or IoT device), switch on advanced mode and set separate active and sleep current draws plus how many hours per day are spent active.

Real-world use cases

Estimating how long a power bank will keep a phone or laptop running, sizing a battery for a remote sensor or IoT project that mostly sleeps and wakes briefly to transmit data, comparing runtime between two battery capacities before buying, or figuring out how many days a set of AA batteries will last in a low-draw device like a remote or flashlight.

Frequently asked questions

Why include an efficiency factor at all?

A battery's printed capacity is measured under ideal lab conditions. In real use, internal resistance, voltage drop under load, temperature, and the device's own power regulation all eat into that number — an 85% efficiency factor is a commonly used rule of thumb that gets you closer to real-world runtime than the raw rated capacity would.

When do I need to enter the battery voltage?

Whenever your capacity unit and draw unit aren't natively compatible — for example, capacity in Wh but draw in mA, or capacity in mAh but draw in W. The voltage is what bridges energy (watt-hours) and charge (amp-hours), so it's needed for any of those cross-unit conversions.

What's a reasonable sleep current for a low-power device?

It varies hugely by design, but a well-optimized microcontroller in deep sleep can draw well under 1mA, sometimes down to single-digit microamps, while a device that's merely "idle" (screen off but still connected to Wi-Fi) might still draw tens of milliamps. Check your device's datasheet for its actual sleep current rather than guessing.

Why does a small sleep current make such a big difference to runtime?

Because sleep time usually dominates the day. A device active for 2 hours and asleep for 22 spends 92% of its time at the sleep current, so even a modest sleep draw ends up carrying most of the weighted average — which is exactly why advanced mode exists instead of just calculating from active current alone.

Does this account for battery self-discharge over long storage periods?

No — this calculates active runtime under continuous use, not shelf life. Batteries also lose a small amount of charge just sitting unused (self-discharge), which matters for infrequently-used devices but is a separate consideration from the runtime-under-load this tool estimates.