Battery Runtime & Sizing Calculator
Accurately calculate real-world discharge runtime across any battery chemistry. Automatically accounts for Depth of Discharge (DoD), inverter conversion losses, and load characteristics.
50% (Lead-Acid/AGM)
100% (Direct DC)
50% Usable
How Battery Runtime Is Calculated: The Physics & Formula
Calculating exact battery runtime requires more than simply dividing amp-hours by your device’s current draw. Real-world runtime depends on three critical electrical variables:
1. Nominal Energy vs. Safe Usable Energy (Depth of Discharge)
Batteries should rarely be discharged to 0% state of charge:
- Lead-Acid, AGM & Gel Batteries: Should only be discharged to 50% Depth of Discharge (DoD). Discharging deeper causes severe plate sulfation, rapid capacity loss, and reduces cycle life from 500 cycles down to fewer than 150.
- Lithium Iron Phosphate (LiFePO4): Can safely discharge to 80% to 90% DoD while still delivering 3,000 to 5,000 charge cycles, making them 2x to 3x more usable than lead-acid for the same rated capacity.
- Lithium-Ion (NMC / 18650 / 21700): Safely discharge to 80% DoD for optimal cell longevity.
2. AC Inverter Losses & Idle Draw
When powering standard household AC appliances (like a 120V laptop charger, TV, or CPAP machine) through a 12V or 24V power inverter, the inverter consumes energy during the DC-to-AC conversion process. Standard inverters operate at 85% to 90% efficiency, meaning 10% to 15% of your battery’s stored energy is lost as heat. For maximum runtime when camping or off-grid, always power devices using native 12V DC adapters whenever possible.
3. Converting Between mAh, Ah, and Watt-Hours (Wh)
Because batteries operate at different voltages, comparing capacity using Amp-hours alone can be misleading. A 10Ah 12V battery stores 120 Watt-hours, while a 10Ah 48V battery stores 480 Watt-hours (4x more energy).
- Watt-Hours (Wh) = Amp-Hours (Ah) × Nominal Voltage (V)
- Milliamp-Hours to Watt-Hours:
(mAh × V) / 1000 - For a deep-dive into how capacity ratings work, read our comprehensive guide: What Does ‘mAh’ Mean, And Why Does It Matter?
Frequently Asked Questions (Runtime Calculator)
Why does my battery die faster than the calculator estimates?
Real-world discharge is influenced by Peukert’s Effect (high current draws reduce effective battery capacity in lead-acid batteries), ambient temperature (batteries lose up to 30% of usable energy in sub-freezing weather), and parasitic inverter idle draw.
How do I wire batteries together for more runtime?
To double your runtime while keeping the voltage identical, wire your batteries in parallel (positive to positive, negative to negative). If you need higher voltage for larger inverters, wire in series. See our complete illustrated walkthrough: Wiring Batteries in Series vs Parallel.
What size battery bank do I need for solar?
To size a complete off-grid or RV solar storage bank including days of autonomy and solar panel wattage, use our step-by-step formula: How to Size a Solar Battery Bank (Worksheet).