Battery Runtime Calculator

What is a battery runtime calculator?

A battery runtime calculator estimates how long a battery will power a device or system based on the battery capacity (in amp-hours or watt-hours) and the load (power consumption). This is essential for planning UPS (uninterruptible power supply) systems, sizing batteries for solar setups, estimating laptop or phone battery life, and designing portable electronics.

The basic formula is simple: runtime = capacity / load. A 100Wh battery powering a 50W device lasts about 2 hours. However, real-world factors reduce this: battery age, temperature, discharge rate efficiency, inverter losses, and the fact that batteries should not be discharged below 50% (for lead-acid) or 20% (for lithium) to preserve longevity.

How to use this tool

Enter the battery capacity (Ah or Wh), voltage, and the load in watts. The tool calculates the estimated runtime accounting for typical efficiency losses. It also shows recommendations for depth of discharge to maximize battery lifespan.

Battery types and characteristics

• Lead-acid (AGM/gel) — affordable, heavy, recommended max 50% discharge depth, 300-500 cycle life.
• Lithium-ion (Li-ion) — lighter, 80% usable capacity, 500-1000+ cycle life, more expensive.
• LiFePO4 (Lithium Iron Phosphate) — safest lithium chemistry, 80% depth of discharge, 2000-5000 cycle life.
• Nickel-metal hydride (NiMH) — common in AA/AAA rechargeable batteries, moderate energy density.

Efficiency factors

Real runtime is always less than the theoretical calculation. Inverter efficiency (converting DC to AC) typically loses 10-15%. Peukert's effect means batteries deliver less total energy at higher discharge rates. Cold temperatures reduce capacity by 10-30%. Battery aging gradually reduces capacity — after 500 cycles, a lithium battery may retain only 80% of its original capacity. This tool factors in typical efficiency losses for more realistic estimates.

Worked example

A 100 Ah, 12 V lead-acid battery stores 1,200 Wh, but you should only draw it to 50% depth of discharge, leaving about 600 usable Wh. Powering a 45 W router and modem, runtime is 600 ÷ 45 ≈ 13.3 hours before inverter losses. Applying an 85% inverter efficiency drops this to about 11.3 hours of real backup.

Common mistakes

The biggest mistake is ignoring depth of discharge: a lead-acid battery repeatedly drained to empty may last only a fraction of its rated cycles. Another is quoting amp-hours without a voltage, since only watt-hours (Ah × V) compare batteries of different voltages. Peukert's effect also means high discharge currents deliver fewer usable amp-hours than the rated capacity suggests.

Frequently asked questions

How do I convert between Ah and Wh?

Watt-hours = Amp-hours times Voltage. A 100Ah 12V battery stores 1,200Wh (100 * 12). A 50Ah 48V battery stores 2,400Wh (50 * 48). Wh is more useful for comparing batteries at different voltages. This tool handles both units automatically.

Why should I not fully discharge my battery?

Deep discharging significantly reduces battery cycle life. A lead-acid battery discharged to 50% may last 500 cycles, but discharged to 80% it might last only 200 cycles. Lithium batteries are more tolerant but still degrade faster with deep discharges. For maximum lifespan, keep lithium batteries between 20% and 80% charge. The tool accounts for recommended depth of discharge in its calculations.

Why do lithium batteries give more usable runtime than lead-acid?

Lithium (LiFePO4) cells tolerate roughly 80–90% depth of discharge versus about 50% for lead-acid, so a lithium pack of the same nominal capacity delivers far more usable energy. They also hold voltage more steadily under load and suffer less capacity loss at high discharge currents.