I used the medium heater for a mid-sized (and moderately insulated) engine compartment, and it successfully kept the engine and components from freezing, even during -10 degree F cold snaps. Incredible! I did not want to winterize the engine and fresh water system and not be able to use the boat all winter, so this was a perfect solution. Very well built and reliable. Buy the mount too -- allows for easy removal for spring, or easier installation overall if you're leaving in in permanently.
Pro tip -- if your dock power isn't entirely reliable and you're using the unit as described (to protect an unwinterized engine), I would encourage the installation of a power or temperature monitor. For us, the combo of Victron Cerbo (with temp sensor), BMV 712 battery monitor and 4G LTE Modem (cheap IOT sim card) give us remote alarms when the dock power is lost. I haven't seen temps in the engine room below the mid 40's with the heater installed -- usually hovering at about 48 degrees F.