Calculating Inverter and Battery Runtime: A Comprehensive Guide

When dealing with renewable energy systems, it's crucial to understand how long your inverter and battery setup can power your loads. This article explores the factors that influence runtime and provides a detailed analysis based on a specific scenario where an individual is trying to determine the operational duration of their system.

Scenario: A 3.5kVA Inverter with 8x200Ah Batteries

Let's consider a scenario similar to the one in the question: A user has an inverter capable of 3.5kVA and an 8-battery bank of 200Ah each. They want to know how long it will take to power a 1250W load. To accurately answer this, we need to delve into several factors, including battery chemistry, inverter efficiency, and voltage degradation.

Understanding the Theoretical Battery Storage

The first step in determining runtime is calculating the theoretical battery storage. For a single 200Ah battery with a typical 12V voltage, the theoretical Wh capacity is 2400 Wh (200Ah x 12V). When we multiply this by the number of batteries (8), we get a total of 19,200 Wh or 19.2kWh.

Impact of Battery Chemistry

However, the actual usable capacity can vary significantly based on the type of battery chemistry:

Flooded, Gel, AGM, Tubular SLA: These types of batteries have a limited depth of discharge (DOD) because they can be damaged if discharged below 50%. Therefore, the usable capacity is effectively halved, resulting in 9.6kWh. Lithium Batteries (LFP or LYP): Lithium batteries are more forgiving and can be discharged deeper, typically to 80% DOD. This increases the usable capacity to about 30% of the labeled Ah, giving us an estimated 15.36kWh.

Load and Inverter Efficiency

In addition to battery capacity, the power consumption and efficiency of the inverter also play a critical role. Assuming an inverter with a likely limit of 2.88kW, it can handle a 1250W load without a startup surge. The load uses 1250Wh per hour, which translates to 1.25kWh per hour at a continuous load.

However, most inverters are not 100% efficient. A typical efficiency range is between 85% and 95%. This means the inverter could use up to 1.3kWh per hour to power 1250W, or up to 1.5kWh per hour at the lower end of efficiency.

Final Runtime Calculation

Considering both battery type and inverter efficiency:

Flooded, Gel, AGM, Tubular SLA Batteries: With a 9.6kWh usable capacity, the runtime at 1.3kWh per hour would be approximately 7.3 hours, and at 1.5kWh per hour, it would be around 6.4 hours. Lithium Batteries: With a 15.36kWh usable capacity, the runtime at 1.3kWh per hour would be roughly 11.8 hours, and at 1.5kWh per hour, it would be about 10.2 hours.

Conclusion

The actual runtime depends on multiple factors, including battery chemistry, inverter efficiency, and load requirements. In this case, the difference between a flooded battery system and a lithium battery system can range from 6.4 to 11.8 hours. This underlines the importance of detailed considerations in renewable energy planning and system design.