I happen to have a JustPlug power supply (JP5770). It's output is 24VDC and its max load is 1000 mA. To use it, you must connect it to a Lights & Hub Set (JP5700), Expansion Hub, or a Sequencing Hub. Originally, I bought it for some HO houses and street lights for the Steubenville portion of the layout. While the JustPlug is simple and convenient, it's pretty pricey.
I connected a JustPlug power supply to the Lights & Hub Set and then tested its output with a multimeter. Using the dimmer controls, you can get anywhere from 0 to 22.9 VDC.
I have a Woodland Scenics Rustic Water tower, which is labeled as drawing 30 mA. Unless I want to run 40' of wire to it from the Lights & Hub Set, I'm going to have to figure out something else (or buy another set up). Ouch.
George