Battery Life Calculator

The Battery Life Calculator estimates how long a rechargeable or disposable battery will power an electronic device before requiring replacement or recharging. You provide four inputs: battery capacity in milliampere-hours (mAh), nominal voltage in volts (V), active power consumption in milliwatts (mW), and standby drain in milliwatts. The calculator converts the electrical charge capacity into actual energy storage (watt-hours), then divides that energy by your device's power draw to produce runtime estimates for both active use and idle standby. This gives hardware designers, hobbyists, and everyday consumers a quick way to compare batteries, plan device usage, or diagnose unexpectedly short runtimes.

The calculation rests on a two-step conversion. First, because milliampere-hours measure charge rather than energy, we convert to watt-hours using the battery voltage:

Wh = (mAh x V) / 1000

For example, a smartphone battery rated at 5000 mAh with a nominal voltage of 3.7 volts stores (5000 x 3.7) / 1000 = 18.5 watt-hours of energy. This is the total work the battery can perform before depletion.

Second, we divide the stored energy by the device's power consumption, converted from milliwatts to watts:

Runtime (hours) = Wh / (mW / 1000)

If that same smartphone draws 2000 mW (2 watts) during active use, the theoretical runtime is 18.5 / 2 = 9.25 hours. Standby runtime uses the same formula but substitutes the lower standby drain — often 20 to 100 mW for modern phones — yielding standby times measured in days or weeks. These figures represent ideal laboratory conditions; real-world efficiency losses from voltage converters, temperature variation, and battery aging typically reduce actual runtime by 10-20 percent.

Suppose you have a compact flashlight powered by a single 18650 lithium-ion cell and want to know how long it will last on a camping trip. You look up the cell specifications and find 3400 mAh capacity at 3.7V. The manufacturer rates the LED at 3000 mW (3 watts) on high mode, with a parasitic drain of 10 mW when the switch is off.

Step 1 — Convert capacity to watt-hours: (3400 x 3.7) / 1000 = 12.58 Wh.

Step 2 — Calculate active runtime on high: 12.58 Wh / 3 W = 4.19 hours, or roughly 4 hours and 11 minutes of continuous bright light.

Step 3 — Calculate standby time if left switched off in a backpack: 12.58 Wh / 0.01 W = 1,258 hours, about 52 days before the cell self-discharges and parasitic drain deplete it.

In practice, you would get slightly less because LED drivers are not 100% efficient and cold temperatures reduce lithium-ion performance. Still, the estimate tells you that one fully charged 18650 cell is sufficient for several nights of intermittent use.

Product design and prototyping: Hardware engineers use battery life estimates early in development to select appropriately sized cells. A wearable that must last 7 days between charges needs a fundamentally different battery chemistry and capacity than one recharged nightly.

IoT and sensor deployment: Remote sensors running on solar-charged batteries or primary cells in inaccessible locations require precise runtime calculations to determine maintenance intervals. Underestimating drain leads to failed data collection; overestimating means unnecessarily large and expensive batteries.

Consumer purchasing decisions: Comparing two power banks both rated at 10000 mAh is misleading if one operates at 3.7V and the other at 5V output. Converting both to watt-hours reveals which stores more usable energy.

Troubleshooting unexpected drain: If your device dies faster than specifications claim, measuring actual power draw with a USB meter and plugging values into this calculator identifies whether the battery is degraded or the device is consuming more power than designed. Need to calculate percentages for efficiency? Try our Percentage Calculator.

A 9-volt battery is one of the most commonly searched battery types because it powers smoke detectors, guitar pedals, wireless microphones, and small electronics. A typical alkaline 9V battery stores about 500–600 mAh at 9 volts, which equals roughly 4.5–5.4 watt-hours of energy. A high-quality lithium 9V can store up to 1,200 mAh (10.8 Wh), more than doubling runtime.

To calculate 9V battery life, enter 500–600 in the capacity field, 9 in the voltage field, and your device draw in milliwatts. A smoke detector drawing 0.5 mW will last over 10,000 hours (more than a year), which matches real-world replacement recommendations. A guitar pedal drawing 50 mW will last about 90–100 hours on alkaline, or 200+ hours on lithium. Always check your device manual for actual current draw — many guitar pedals draw 10–100 mA at 9V (90–900 mW), which significantly shortens battery life.

Many users search for battery life in watts because watt-hours provide a universal energy measurement independent of voltage. Two batteries with the same mAh rating can deliver completely different runtimes if their voltages differ. For example, a 2,000 mAh AA alkaline cell at 1.5V stores 3 Wh, while a 2,000 mAh 18650 lithium-ion cell at 3.7V stores 7.4 Wh — more than double the usable energy.

The calculator above automatically converts your mAh and voltage inputs into watt-hours, displayed in the first result card. This lets you compare batteries across chemistries and form factors. When shopping for power banks, look for the Wh rating — airlines regulate lithium batteries by watt-hours (typically 100 Wh per battery for carry-on). A 10,000 mAh power bank at 3.7V is 37 Wh, well within limits. A 30,000 mAh power bank at 3.7V is 111 Wh and may require airline approval.