Calculate how long a battery will power your load. Enter battery Ah, voltage, depth of discharge, and load watts to find backup time in hours.
Whether you have a solar battery bank, a deep-cycle battery for off-grid use, or a portable power station, the most important question is: how long will it last under my load? Battery backup time depends on four factors: battery capacity (Ah), voltage, depth of discharge (DoD), and the connected load power.
Depth of discharge is a crucial factor. Lead-acid batteries should only be discharged to 50% (DoD = 0.50) to preserve lifespan. Lithium-ion batteries can safely discharge to 80–90% (DoD = 0.80–0.90). Discharging beyond the recommended DoD dramatically shortens battery life.
This calculator determines backup time from battery specifications and load power. Enter the amp-hour rating, nominal voltage, allowable depth of discharge, and your load in watts. It's essential for off-grid solar system sizing, portable power planning, and emergency backup duration estimation.
This analytical approach supports both immediate cost reduction and long-term sustainability goals, helping organizations balance economic and environmental priorities in their energy management.
Correctly estimating battery runtime prevents unexpected shutdowns and extends battery lifespan. This calculator accounts for depth of discharge limits that many simple calculations ignore. Regular monitoring of this value helps energy teams detect usage anomalies early and address equipment malfunctions or operational issues before they drive utility costs higher. Having accurate metrics readily available streamlines utility bill analysis, budget forecasting, and investment planning for energy efficiency projects and renewable energy installations.
Time (hours) = (Battery Ah × Voltage × DoD) ÷ Load (W)
Result: 4.0 hours
Usable energy: 200 Ah × 12V × 0.50 = 1,200 Wh. Backup time: 1,200 Wh ÷ 300 W = 4.0 hours. A 200 Ah 12V lead-acid battery discharged to 50% powers a 300W load for 4 hours.
Flooded Lead-Acid: Cheapest, requires maintenance (water), 500–1,000 cycles at 50% DoD, 30–50 lbs per kWh. AGM Lead-Acid: Sealed, maintenance-free, 600–1,200 cycles, premium over flooded. LiFePO4: 2,000–5,000 cycles, 80% DoD, lightweight (12–15 lbs/kWh), highest upfront cost but best lifetime value.
Step 1: Determine daily load (Wh). Step 2: Decide on days of autonomy (1–3 days). Step 3: Total Wh = Daily × Days. Step 4: Account for DoD: Ah = Total Wh ÷ (Voltage × DoD). Step 5: Add 15% for losses. Step 6: Select batteries to meet or exceed required Ah.
Battery capacity is rated at 77°F (25°C). At 32°F (0°C), lead-acid loses ~20% capacity. At 0°F (-18°C), it loses ~40%. Lithium batteries lose less (5–15% at 32°F) but most should not be charged below freezing without a heating system.
Depth of discharge is the percentage of battery capacity that is actually used. A 100 Ah battery at 50% DoD provides 50 Ah of usable energy. DoD limits protect battery health — deeper discharges shorten cycle life.
Flooded lead-acid: 50%. AGM/Gel lead-acid: 50–60%. Lithium iron phosphate (LiFePO4): 80%. Lithium-ion (NMC): 80–90%. Using a lower DoD extends battery cycle life but requires more battery capacity.
For parallel batteries: multiply Ah by the number of batteries (voltage stays the same). For series batteries: multiply voltage by the number of batteries (Ah stays the same). Then use the formula with total Ah and total voltage.
Several factors reduce real-world runtime: inverter losses (5–15%), cable losses, temperature effects, battery age (capacity degrades 1–3% per year), and Peukert's effect (lead-acid loses capacity at high discharge rates). Documenting the assumptions behind your calculation makes it easier to update the analysis when input conditions change in the future.
At 50% DoD: flooded lead-acid — 500–1,000 cycles. AGM — 600–1,200 cycles. Gel — 1,000–2,000 cycles. LiFePO4 — 2,000–5,000 cycles. At deeper DoD, cycle life decreases significantly.
Ah = (Load W × Hours) ÷ (Voltage × DoD). For 500W for 8 hours on 12V at 50% DoD: (500 × 8) ÷ (12 × 0.50) = 666 Ah. You'd need two 350 Ah 12V batteries in parallel.