Battery Life Calculator
Estimate how long a battery will last. Enter capacity in mAh, load current (or power + voltage), and an efficiency factor: get runtime in hours, minutes and days.
Last updated: May 2026
Enter capacity and load to estimate battery runtime.
Hours = capacity (mAh) × efficiency ÷ current (mA)
Common scenarios
Battery runtime: what the numbers actually mean
The basic formula is simple: divide capacity (mAh) by current (mA) to get hours. A 2,000 mAh battery at 200 mA = 10 hours. But real devices do not draw constant current; they sleep, spike, and idle. The efficiency factor accounts for voltage regulator losses (a linear LDO wastes voltage as heat; a buck converter at 85% efficiency keeps most of it).
For projects with deep-sleep modes, calculate the average current: (active_mA × active_seconds + sleep_mA × sleep_seconds) ÷ total_cycle_seconds. Enter that average current for a realistic runtime estimate. If you still need to work out that load figure, the power and current tools in our Electronics calculators hub feed straight into this runtime estimate.
Typical current draws
| Device / mode | Typical current |
|---|---|
| ESP32 deep sleep | 10-20 µA |
| ESP32 active (Wi-Fi transmit) | 160-260 mA peak |
| Arduino Uno active | ~50 mA |
| Raspberry Pi Zero 2W (idle) | ~120 mA |
| LoRa module (transmit) | ~100 mA peak |
| LoRa module (receive) | ~12 mA |
| Small OLED display | 10-25 mA |
| GPS module | 20-30 mA |
From load current to how long the pack lasts
This calculator sits at the runtime end of the DC build chain. Every step before it hands you either a current value or a power value that feeds directly into this estimate:
- Start with the base relationships. Voltage, current and resistance are pinned down first. The Ohm's Law calculator is where that begins.
- Tame each LED with a series resistor. The LED resistor calculator gives you the current each branch draws, which you will sum into the total load.
- Split a reference voltage. The voltage divider calculator shows the quiescent current the divider wastes continuously.
- Match values to physical parts. The resistor color code and capacitor code calculator decode what is actually in the bin.
- Size the supply. The watts, volts and amps calculator sums the total power draw and confirms the rail can deliver it.
- Account for wiring losses on long runs. The voltage drop calculator shows how much the cable itself costs you at distance.
- You are here: predict battery runtime. Take the load current from step 1 or 5, enter capacity (mAh) and an efficiency factor, and this calculator gives you hours. Apply an efficiency factor below 100% whenever a regulator sits between the cell and the load: a buck converter at 85% efficiency means the battery sees more current than the load alone implies. For lead-acid packs specifically, also derate the rated capacity at high discharge rates: the Peukert effect means a 100 Ah lead-acid battery delivering 20 A does not last 5 hours.
For circuits driven by AC rather than steady DC, the chain takes a different turn: the impedance calculator and the capacitor reactance and inductor reactance tools handle the reactive side.
Nameplate capacity is optimistic, and lead-acid is the worst offender. A 100 Ah leisure battery will not hand you 100 Ah under a real load: pull more than about 10 A continuously and usable capacity drops away fast. I learned that the hard way and now enter 60 to 70 percent of the rated figure here, with efficiency set to 85 percent for the DC-DC converter, which lands close to the runtime I actually measure.
Frequently Asked Questions
Why does a 2000 mAh battery not last exactly 2000/load hours?
Several factors reduce real runtime below the theoretical figure: (1) Voltage cut-off: batteries are not fully dischargeable; lithium cells cut off at ~3.0 V, losing 5-10% of rated capacity. (2) Temperature: cold reduces effective capacity significantly (Li-ion loses ~20% at 0°C). (3) Regulator losses: a linear LDO dissipates the voltage difference as heat; the efficiency field in this calculator accounts for this. (4) Peukert effect: drawing more current reduces effective capacity; rated mAh is typically at a 0.2C or 1C discharge rate.
What efficiency value should I use for a LiPo with a boost converter?
A good switching boost converter runs at 85-92% efficiency at moderate loads. At very light loads (<10 mA output), efficiency can drop to 70-75% due to switching losses. Use 85% as a conservative starting estimate for projects using a boost regulator from a 3.7 V LiPo to 5 V. If the datasheet shows an efficiency curve, use the value at your expected output current.
How do I extend battery life for a microcontroller project?
The single most effective technique is using deep sleep aggressively. An ESP32 drawing 200 mA active but sleeping 99% of the time averages just ~2 mA, a 100× improvement. After that: (1) reduce clock speed when full performance is not needed; (2) turn off unused peripherals (Wi-Fi, Bluetooth, ADC); (3) switch from a linear LDO to an efficient buck converter; (4) use lower system voltage if the MCU supports it (3.3 V instead of 5 V); (5) replace polling loops with interrupt-driven wakeup from sensors.
How much should I derate lead-acid capacity at high discharge rates?
A 100 Ah lead-acid battery rated at the 20-hour (C/20) discharge rate does not deliver 100 Ah when you draw hard. At a 5-hour rate (20 A) the Peukert exponent for a typical flooded lead-acid cell reduces effective capacity to roughly 70-80 Ah. At a 1-hour rate the usable capacity can fall below 55 Ah. For camper-van or solar storage sizing, enter 60-70% of the nameplate figure into the capacity field when your average load current is above about C/10. Sealed AGM cells tolerate slightly higher discharge rates before the same capacity loss, but the same rule applies: never assume the full nameplate Ah at sustained high current.
Methodology and sources
This tool estimates how long a battery lasts by dividing its usable charge by the load current, then derating for converter and regulator losses. It is straightforward battery physics, not a formal standard.
- Method: runtime in hours = capacity (mAh) × efficiency (%) ÷ load current (mA). In power mode the load current is first derived from power and voltage as current (mA) = power (W) ÷ voltage (V) × 1000, applying Ohm's power law (P = V × I).
- Standards and sources: standard battery and electrical physics (charge ÷ current; P = V × I); no industry standard is implemented. Capacity, cut-off voltage and discharge-rate behaviour follow each cell's chemistry and datasheet (for example the Peukert effect on alkaline capacity).
- Assumptions and limits: the efficiency factor (default 85%) models regulator and converter loss only. It assumes a roughly constant average load, so for sleep-heavy designs you must enter a time-weighted average current yourself. Rated mAh is taken at the manufacturer's reference discharge rate; real usable capacity falls with cold temperatures, voltage cut-off and high discharge current (Peukert effect).
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning aid for estimating battery runtime, not a substitute for a qualified professional or the manufacturer's battery and charging specifications; verify safety-critical battery, wiring and protection decisions against the cell datasheet and local code.
Next step in this workflow
Battery life estimated: explore all electronics calculators.