^^ x2 Timers are the way to go; make sure it's capable of the electrical load.
I use the block heaters on whatever Jeep will be driven the next day, but doesn't make it into the garage (never my wife's of course) anytime the nighttime temps are below 40 F. A few pennies of electricity (600W heater) is undoubtedly cheaper than burning fuel while idling with prices still where they are. Remote starts or idling gets you 0 mpg. The coolant temp is over 100 F after about 2 hours (when ~30 F ambient). Practically instant heat after you set off. 1/4 mile later, the jacket temps will be upwards of 150. Colder oil from the pan will flow over the hotter cylinder head and in turn, get up to temp faster.
Easy install on the 4.0 while the 4.7 (and 5.7) are not as straightforward. Ask me how I know......
For ~$35, the 4.0 block heater is worth the time to install and then some.
1996 XJ Country 4.0 I won't give it up!