Rob Cockerham has an interesting analysis of the power demands of his CR-10 3D-printer – when only the fans are running, when the nozzle is operating at 185°C, the heating bed is maintained at 45°C, and while the printer is operating on 2 axes.
Analysis of 3D-Printer Power Consumption Using Creality CR-10. TL;DR: 24/7 Operation = 2¢/Hour