Battery Life Calculator: How Long Will Your Battery Last?

"How long will this last on battery?" is one of the most common questions in electronics, and the answer is almost never what the spec sheet claims. Manufacturer battery life figures are tested under ideal conditions that don't reflect real-world usage. Knowing the math lets you set realistic expectations — or plan a better power system.

The CalcHub Battery Life Calculator calculates runtime from battery capacity and device power draw, with adjustable efficiency factors.

The Core Formula

Runtime (hours) = Battery Capacity (mAh) ÷ Load Current (mA)

Or in watt-hours:

Runtime (hours) = Battery Capacity (Wh) ÷ Load Power (W)

For a 10,000mAh power bank at 500mA load:
10,000 ÷ 500 = 20 hours theoretical

Real-world will be less due to efficiency losses.

Converting Between Units

• mAh → Wh: Wh = mAh × Voltage ÷ 1,000
• Wh → mAh: mAh = Wh × 1,000 ÷ Voltage

A 10,000mAh battery at 3.7V (typical Li-ion nominal) = 37Wh. When a USB charger converts 3.7V to 5V, you lose ~10–20% in conversion efficiency.

Real-World Efficiency Factors

Batteries don't deliver 100% of their rated capacity:

Factor	Typical Loss
Conversion efficiency (3.7V→5V USB)	10–20%
Battery age/degradation (after 500 cycles)	10–30%
Temperature (cold: -10°C vs 25°C)	20–40%
Depth of discharge limit (Li-ion: stop at 20%)	10–20%
Peukert effect (high current draw)	5–15%

Practical efficiency multiplier: 0.7–0.85 for most real-world conditions.

Revised formula: Runtime = Capacity × Efficiency ÷ Load

10,000mAh power bank at 500mA with 80% efficiency: 10,000 × 0.80 ÷ 500 = 16 hours

Common Device Runtime Reference

Device	Typical Battery	Average Draw	Real Runtime
Smartphone	4,000mAh	200–500mA	8–20 hours mixed use
Laptop	60–80Wh	15–45W	4–8 hours
Wireless earbuds	35–60mAh	10–15mA	4–6 hours/charge
Smartwatch	200–400mAh	10–25mA	1–4 days
Bluetooth speaker	2,000–5,000mAh	500–1,500mA	6–20 hours
Arduino with sensors	2,000mAh	50–100mA	20–40 hours
ESP32 deep sleep	2,000mAh	0.01mA (sleep)	Months (depends on wake frequency)

Off-Grid and Solar System Sizing

For larger battery banks (RV, solar, backup):

Battery bank needed (Wh) = Daily load (Wh) × Days of autonomy ÷ DoD

For a 200Wh/day load, 2 days autonomy, 80% depth of discharge (lead-acid):
200 × 2 ÷ 0.80 = 500Wh minimum bank

At 12V: 500 ÷ 12 = ~42Ah. Use two 6V 50Ah lead-acid batteries in series, or a single 12V 50Ah battery with margin.

Note: Li-ion allows deeper discharge (80–90% DoD vs. 50% for lead-acid), so a 100Ah LiFePO4 at 80% DoD gives 80Ah usable vs. 50Ah usable from a 100Ah lead-acid.

Why does my phone battery not last as long as the specs say?

Manufacturer tests use controlled brightness, no active background apps, minimal radio activity, and optimal temperature. Real usage with a bright screen, streaming, 5G, and apps running in the background might double or triple the power draw.

How do I extend Li-ion battery lifespan?

Avoid full charges to 100% and discharges below 20% for daily cycles — keeping the battery between 20–80% charge can nearly double its cycle life. Heat is the biggest enemy: don't charge a hot phone, don't leave devices in hot cars.

What's the difference between mAh and Wh for comparing batteries?

mAh is meaningful only at a specific voltage. You can't compare a 10,000mAh phone battery (3.7V = 37Wh) to a 10,000mAh car battery (12V = 120Wh) using just mAh. Always compare in watt-hours for a meaningful capacity comparison across different voltage systems.

Related Tools

• Power Consumption Calculator — determine your device's wattage
• LED Resistor Calculator — design efficient low-power LED circuits
• Electrical Load Calculator — for larger system load analysis