you don't need to plug you engine unless it's -20°C or colder.  0°F or colder for the US guys.
Gilles
2.3 Ho
 
As long as you know it takes a long while to heat your engine, then i guess it should be fine.
As for the rest of your question, i dont understand.
 
 
 Cost of operation varies greatly depending on cost of electricity, wattage of heater, how long it is on.
  As an example say  electricity costs 10 cents kilowatt/hour (made up number). 
  Basically this means you can consume 1000 watts (= 1 kilowatt) for  1 hour at a cost of 10 cents.
  If you use a 500 watt electrical whatever, 10 cents will last you 2 hours, a 60 watt light bulb would run for a little over 16 hours on a dime. 
  I hope this makes sense.  It is 0440h local.