Estimate Battery Backup Time from Capacity and Load
Battery Capacity (Ah)
Battery Voltage (V)
Load Power (W)
Efficiency (%)
Quick Load Presets
Result
Runtime
Wh Capacity
Load Current
Step-by-Step Calculation
Battery Runtime Formulas
Wh = Ah × V (watt-hours)
Runtime(h) = Ah × V × η / P_load
Runtime(h) = Wh × η / P_load
I_load = P_load / V (DC current draw)
Battery runtime depends on total stored energy (Ah × V), the load power, and system efficiency. Inverters, voltage converters, and battery internal resistance all reduce usable energy. Real-world runtime is usually 70-90% of theoretical. For critical applications, test under actual load conditions.
⚠Battery capacity ratings assume a specific discharge rate (typically C/20 for lead-acid, C/5 for lithium). Drawing more current reduces usable capacity (Peukert effect). Runtime estimates should include a 20% safety margin for real-world conditions.
Understanding Battery Runtime
Battery runtime is the time a battery can power a load before needing recharge. It depends on: battery capacity (Ah), system voltage (V), load power (W), and system efficiency (η). Higher capacity, higher voltage, lower load, and higher efficiency all increase runtime. The calculation assumes constant load and ideal conditions.
Lead-Acid Battery
Capacity: 7-200Ah. Voltage: 2V (cell), 6V, 12V. Efficiency: 80-85%. Deep cycle: 50% DoD recommended. 100Ah usable: 50Ah.
Lithium LiFePO4
Capacity: 10-300Ah. Voltage: 3.2V (cell), 12.8V, 25.6V. Efficiency: 90-95%. 80-100% DoD allowed. Lighter, longer life, more expensive.
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