Battery Runtime
Calculator
Accurately estimate discharge time for off-grid, RV, marine, and solar battery storage banks using capacity (Ah), voltage (V), and load power (W).
Enter Battery & Load Details
Runtime (hours) ≈ [Capacity (Ah) × Voltage (V) × (DoD% ÷ 100)] ÷ Load Power (W).Note: Standard nominal formula assumes ideal conversion. Real-world inverters and line resistance typically introduce a 5–15% efficiency penalty.
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How to Estimate Battery Runtime from Capacity, Voltage, and Load Power
The battery runtime calculator by SolarMathDesk enables electrical engineers, off-grid homeowners, and solar installers to accurately predict how long a battery bank can energize connected loads. Sizing storage correctly protects your system from blackouts, prevents destructive deep-cycle events, and saves unnecessary battery replacement costs.
Understanding the Core Battery Runtime Formula
The mathematical relationship linking electrical charge, system potential, and power dissipation is expressed through basic Joule-Watt relationships:
Runtime (Hours) = [Capacity (Ah) × System Voltage (V) × Depth of Discharge (% / 100)] ÷ Load Power (W)To visualize this, consider a typical residential scenario: a 12V 100Ah LiFePO4 battery running a continuous 100W load at an 80% Depth of Discharge (DoD):
Gross Watt-Hours: 100Ah × 12V = 1,200 Wh
Usable Watt-Hours: 1,200 Wh × 0.80 = 960 Wh
Expected Duration: 960 Wh ÷ 100W = 9.6 Hours
Step-by-Step Instructions
- Enter Battery Capacity: Look at the nominal amp-hour (Ah) rating printed on the battery's specification tag (e.g., 50Ah, 100Ah, 200Ah).
- Select System Voltage: Specify your battery configuration (12V for compact RV setups, 24V or 48V for whole-home solar storage banks).
- Enter Total Connected Load: Sum up the wattage of all simultaneous AC and DC devices. Remember to account for phantom loads from idling inverters.
- Specify Depth of Discharge (DoD): Restrict lead-acid (FLA/AGM) to 50% to prevent sulfation. For lithium (LiFePO4), set between 80% and 90%.
- Press Calculate: Review the precise discharge runtime in hours and fractional days.
Key Physical Variables That Affect Real-World Discharge
- Peukert's Law: Flooded and sealed lead-acid batteries suffer heavy capacity de-rating when discharged at high C-rates (e.g., C/5 vs C/20).
- Inverter Efficiency: Pure sine wave inverters lose 7% to 15% of total throughput as heat during DC-to-AC power transformation.
- Thermal De-Rating: Operating batteries at sub-freezing temperatures slows chemical ion transit, reducing available capacity by 20% to 35%.
- Cable Voltage Drop: Sub-optimal cable cross-sections cause resistive I²R heat losses along the DC busbars.
Calculation Methodology & Engineering Review
This calculator is developed according to standard energy balance principles defined in IEEE 1013 (Recommended Practice for Sizing Lead-Acid Batteries) and IEC 62619 (Safety requirements for secondary lithium cells). Sizing algorithms are authored and maintained by the technical editorial team at SolarMathDesk.
Frequently Asked Questions (FAQ)
How long will a 12V 100Ah battery run a 100W appliance?
Why does my battery drain faster than what the calculator indicates?
1. Inverter Idle Consumption: Inverters consume 15W to 40W simply remaining powered on.
2. Peukert's Effect: Heavy current draws dramatically decrease usable capacity in lead-acid chemistries.
3. Cold Weather: Temperatures below 10°C (50°F) temporarily degrade chemical reaction kinetics.
What Depth of Discharge (DoD) should I set for my battery type?
• Lead-Acid (AGM, Gel, Flooded): Keep DoD at 50%. Discharging below 50% causes premature plate degradation and sulfation.
• Lithium Iron Phosphate (LiFePO4): Set DoD between 80% and 90%. LiFePO4 cells can easily sustain 3,000 to 5,000 cycles at an 80% discharge depth.