You can order the Cisco 1941 with an AC or PoE type of power supply. The Cisco 1941 does not offer a DC version of the power supply, and it does not support an external RPS.
The Cisco 1921 ships with an integrated AC power supply by default, and you do not need to select the AC power supply. In order to power ports on a switch module in a Cisco 1921 or 1941, you must order the router with a PoE power supply. For the Cisco 1921, the PoE power supply is external to the router.