Overview

Effective charging, system management capabilities, and long-term reliability are often the primary aspects that are considered when deciding on what charge controller model and other system components to use in an off-grid PV System. In off-grid systems, the proper management of the load is not focused on as much as charging, but it is a critical aspect of the system. The life of the battery depends on proper load management just as proper charging does, and reliable operation of the site equipment is the number one goal of the system itself. Even with properly sized systems, ongoing load operation can be put at risk if careful considerations are not made. In this series of technical articles, we will be taking an in-depth look at many aspects of load control solutions as well as the benefits the controller can provide to the load in critical remote power applications. We’ll also be covering many features of just what the controller can provide in terms of reliable load operation for these systems.

Off-grid Site Uptime

Reliable load operation – what does it mean exactly?

Off-grid solar, wind, or hydro systems are designed to provide autonomous power in locations without utility power. Sizing the system properly is the key to getting reliable power. However, dealing with occasional load disconnects due to extended periods without adequate sunlight, wind or hydro energy is often unavoidable. The long-term reliability of the system depends on the load controller’s ability to protect the battery from overdischarge. Allowing deeper discharge of the battery can provide more uptime and less frequent loss of loads but can come at the cost of the health of the battery life and thus reduce long-term reliability. When considering load control, system designers and operators need to weigh out the long-term reliability vs. uptime of the system to best meet their needs and budget.  

Loss of Load Probability (LOLP) – What’s your tolerance and how big is your wallet?

Depending on the location it may be possible to significantly increase the size of the PV array and battery bank so that there would be non-stop continuous load operation without ever experiencing an interruption of the loads for the life of the system. We see this often with systems where load usage is small and the array spends much of the time keeping the battery in float. However, it is costly to do this with a larger system. 

In order to reduce the Loss of Load Probability (LOLP), one must account for the seasonal variability of solar production.  One solution is to have a sizable battery bank that sustains the system through the longest periods of low solar production; but this is expensive. Another option is to add a backup generator as a cost effective way to provide uninterrupted power for critical off-grid loads. However, getting fuel to a remote site, and maintaining a generator can make this an untenable solution. 

The most advanced off-grid solar system sizing calculators are able to estimate the monthly LOLP by taking into account historic weather patterns and variations. This calculation can be most helpful for sizing autonomous systems where the load usage can be accurately estimated.

Even systems that have been designed for 0% LOLP can experience unforeseen issues such as partial shading from leaves or overgrowth of trees or plants over time, loose wires, wire or panel damage, stolen wire or panels, equipment failure or the generator running out of fuel. Therefore, load control is always recommended to prevent the possibility of over-discharging and irreversible harm to the batteries.