How long a battery lasts depends on two things: how much energy it really delivers and how fast the load uses it. The label gives a capacity in mAh, Ah or Wh, but you rarely get all of it, and many devices draw power in bursts rather than steadily. This calculator converts any capacity to amp-hours and watt-hours, handles loads given as a current, a power drawn through a converter or inverter, or a sleep/active duty cycle, and applies a usable-capacity allowance so the estimate is realistic.
How to use the battery life calculator
- Enter the battery capacity and choose mAh, Ah, Wh or kWh.
- Enter the battery voltage — nominal values are 1.2 V for NiMH, 1.5 V for alkaline, 3.7 V for lithium-ion, 12 V for lead-acid and 12.8 V for LiFePO4.
- Choose how the load is given: a steady current, a power in watts (with converter or inverter efficiency), or a sleep/active cycle with an active current, the percentage of time active and a sleep current.
- Set the usable capacity. Around 80% suits most lithium batteries; use about 50% for lead-acid to protect its life.
- Read the runtime, the average current and power, and the discharge rate (C-rate).
Battery life formulas
The C-rate is the load current divided by the capacity in Ah. A 0.05C load on a 2 Ah battery is 100 mA and would empty an ideal battery in 20 hours.
Worked examples
2,000 mAh lithium-ion cell, 100 mA load, 80% usable (the default)
The cell stores 2 Ah × 3.7 V = 7.4 Wh. Runtime = 2 Ah × 0.8 ÷ 0.1 A = 16 hours, a gentle 0.05C discharge.
An RV or camping battery. A 12 V, 100 Ah lead-acid battery with 50% usable capacity runs a 60 W load through a 90% efficient inverter. The battery supplies 60 ÷ 0.9 = 66.7 W, or 5.56 A, so the runtime is 100 × 0.5 ÷ 5.56 = 9 hours.
A coin-cell sensor. A 220 mAh, 3 V coin cell powers a sensor that draws 20 mA while transmitting 2% of the time and 15 µA asleep. The average is 20 × 0.02 + 0.015 × 0.98 = 0.4147 mA, and the runtime is 220 × 0.8 ÷ 0.4147 ≈ 424 hours, or 17.7 days. Cutting the active time to 0.5% would stretch that to about two months.
Typical capacities
| Battery | Nominal voltage | Typical capacity | Energy |
|---|---|---|---|
| AA alkaline | 1.5 V | 2,000–2,800 mAh (light loads) | about 3–4 Wh |
| AA NiMH | 1.2 V | 1,900–2,500 mAh | about 2.5–3 Wh |
| CR2032 coin cell | 3 V | 220–240 mAh | about 0.7 Wh |
| 18650 lithium-ion | 3.6–3.7 V | 2,500–3,500 mAh | about 9–13 Wh |
| Phone battery | 3.85 V | 4,000–5,000 mAh | about 15–19 Wh |
| Deep-cycle lead-acid | 12 V | 100 Ah | 1,200 Wh |
Getting a realistic estimate
Alkaline cells lose much of their capacity under heavy loads, so a digital camera may get half the rated mAh. Lithium-ion capacity is quoted at a moderate rate near room temperature; below freezing it can drop by 20% or more. Lead-acid batteries are rated over 20 hours, and drawing the same energy in two hours delivers noticeably less. Repeatedly discharging lead-acid below 50% also shortens its cycle life. For devices that sit unused for months, self-discharge — roughly 2–3% per month for lithium-ion — becomes part of the budget.
To size the solar array that recharges a battery bank, use the solar panel calculator. The amps to watts calculator converts a load’s current into power, and the energy converter translates Wh into joules or BTU.
Estimates assume steady conditions. Charge, store and dispose of batteries according to the manufacturer's instructions; large battery banks and inverters connected to building wiring must follow local electrical code and be installed by a licensed electrician.
Frequently asked questions
How do I calculate battery life?
Divide the usable capacity by the load current: hours = capacity (Ah) × usable fraction ÷ current (A). A 2,000 mAh battery with 80% usable capacity powering a 100 mA load lasts 2 × 0.8 ÷ 0.1 = 16 hours.
How long will a 100Ah battery run a 60 watt load?
A 12 V, 100 Ah battery stores 1,200 Wh. If it is lead-acid, plan on using about half (600 Wh), and through a 90% efficient inverter the 60 W load draws about 66.7 W from the battery, so it runs about 9 hours. A lithium (LiFePO4) battery with 90% usable capacity would run it about twice as long.
What is the difference between mAh and Wh?
mAh measures charge — current over time — and depends on the battery's voltage to mean anything about energy. Wh measures energy directly: Wh = Ah × V. A 3.7 V, 10,000 mAh power bank stores 37 Wh, which is what airlines and chargers actually care about.
Why does my battery run out sooner than calculated?
Usable capacity shrinks with high discharge rates, cold temperatures, age and the device's cutoff voltage, and voltage converters lose some energy. Lead-acid batteries are especially affected by high current (the Peukert effect). The usable-capacity setting lets you allow for these losses.
How do I estimate battery life for a sensor that sleeps most of the time?
Use the sleep/active option. The average current is active current × active fraction + sleep current × sleep fraction. A sensor drawing 20 mA for 2% of the time and 15 µA otherwise averages about 0.41 mA, so a 220 mAh coin cell lasts roughly 17 to 18 days at 80% usable capacity.