Just got back from a trip to northwest part of PA. The temperature was in the low teens at night. Since could not find a timer just plugged the block heater in and left it till went out for errands and such. Is there a particular timer that should be used of just the normal household variety as long as it is not outside of course? Is it wasteful/harmful to leave it plugged in for hours at a time? Sure had heat faster this way though and the passengers were thankful of that!!