Imagine night has fallen, and your solar panels can no longer replenish your off-grid power system. How long will your batteries sustain your essential loads? This practical question was recently raised by a user operating a system with two 12V batteries (200Ah and 300Ah) connected in parallel, creating a total capacity of 500Ah. The system powers a 500W load through a 4000W inverter.

To estimate battery runtime, we must first calculate the system's total energy storage. The 500Ah battery bank at 12V stores 6,000 watt-hours of energy (500Ah × 12V = 6,000Wh). However, real-world usage requires accounting for battery chemistry limitations. For lead-acid batteries—the most common type in off-grid systems—manufacturers typically recommend not exceeding a 50% depth of discharge (DoD) to maximize battery lifespan. This reduces the usable capacity to approximately 3,000 watt-hours.

The theoretical runtime can be calculated by dividing usable capacity by load power: 3,000Wh ÷ 500W = 6 hours . But this doesn't account for system inefficiencies. Inverters typically operate at about 85% efficiency, meaning the actual power available to the load would be roughly 425W (500W × 0.85). This adjustment extends the runtime calculation to approximately 5.1 hours (3,000Wh ÷ 588W).

These calculations represent best-case estimates. Real-world performance varies significantly based on factors including battery age, ambient temperature, and load fluctuations. Regular battery maintenance and adaptive power management strategies are recommended for optimal off-grid system performance.