Final answer:
The major weakness of locally supervised alarm controls is their dependence on external power, leading to potential failures during grid outages and requiring costly battery systems for stand-alone operation.
Step-by-step explanation:
The major weakness of locally supervised alarm controls is their dependence on external power. Without an external power source, these systems can fail to operate, which is a significant limitation considering that these types of security systems are meant to be reliable at all times. In situations where the power grid is disrupted, this dependency becomes a major weakness, as seen in the example of solar-powered homes losing power even during daylight due to grid failures. Additional concerns include the cost and maintenance demands associated with battery backups, which are necessary for stand-alone operation but can double the initial investment and add ongoing maintenance and replacement costs.