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Battery Run Time Calculator

Estimate how long a battery bank should run a device using amp-hours, voltage, and either watt draw or current draw. The calculation includes a conservative reserve factor so the result is more useful in the garage, workshop, or field.

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Deep Dive & Real-World Variables

Estimated Runtime Reference Table

Based on a standard 12V 100Ah battery rated at 1200Wh. Lithium estimates use a 8% reserve, while lead-acid estimates use a 50% reserve.

Estimated runtime reference table for common devices on a 12V 100Ah battery
Common DeviceTypical LoadLithium RuntimeLead-Acid Runtime
12V Portable Fridge45W~21.3 Hours~13.3 Hours
CPAP Machine60W~16.0 Hours~10.0 Hours
Laptop Charger90W~10.6 Hours~6.6 Hours
LED Camp Lights15W~64.0 Hours~40.0 Hours
Inverter + Blender600W~1.6 Hours~1.0 Hour

Battery Runtime FAQs

Why is my real-world battery runtime shorter than the calculation?

Battery calculations assume ideal laboratory conditions. In the real world, several factors degrade performance:

  • Inverter inefficiency: Converting 12V DC power to 110V AC power typically wastes 10% to 15% of your total energy as heat.
  • Temperature: Extreme cold slows the chemical reactions inside a battery, temporarily reducing effective capacity.
  • Voltage sag: High-wattage loads draw current so quickly that battery voltage can drop early and trigger low-voltage cutoffs.

What is the difference between Depth of Discharge for Lithium vs. Lead-Acid?

Lead-acid batteries, including AGM, Gel, and Flooded types, should rarely be discharged past 50% of total capacity if you want them to survive more than a few hundred cycles. Lithium LiFePO4 batteries have built-in battery management systems and stable chemistry that usually allow you to use 80% to 90% of capacity without harming battery lifespan.

How do I convert Amps to Watts for the calculator?

Use the formula Watts = Amps x Voltage. For example, a device drawing 5 Amps on a 12V system pulls 60 Watts because 5 x 12 = 60.

Battery compatibility information is based on manufacturer specifications and publicly available data. Always verify battery type before purchase.