Steganographic capacity is the maximum amount of data that can be communicated over a channel before a passive adversary's ability to detect the communication reaches a given threshold. In this work, a closed-form expression for the steganographic capacity of low-probability of detection (LPD) systems is derived for real-world communication channels, i.e., continuous, memoryless, and band-limited, when the adversary uses an optimal energy detector to identify the communication. Our derivation of the steganographic capacity is based on approximations for the true probability density functions when covert communication is taking place and when it is not, and we show that the maximum absolute percentage error between the true and approximate density functions is small under LPD conditions, i.e., low signal-to-noise ratio. Furthermore, we show the results of a case study where we determine the steganographic capacity of a covert-acoustic LPD system.