FORD 2013 Explorer v.3 - Manual (Page 209)

Using the Engine Block Heater
Make sure the receptacle terminals are clean and dry prior to use. Clean
them with a dry cloth, if necessary.
The heater uses 0.4 to 1.0 kilowatt-hours of energy per hour of use. The
system does not have a thermostat. It achieves maximum temperature
after approximately three hours of operation. Using the heater longer
than three hours does not improve system performance and
unnecessarily uses electricity.
208 Starting and Stopping the Engine
2013 Explorer (exp)
Owners Guide gf, 3rd Printing
USA (fus)