You won't spend more in electricity than you'll save. It's been discussed before (search the archives), but I think we figured it would cost a couple of cents to plug the car in for an hour. At my rates and with my BH wattage (300), it's about 3 cents for the hour.
The timer is a must if you're not around to un/plug manually. More than 2 hrs is wasted energy in normal (cold ambient) circumstances.
I have started to cut my commute into pieces. The first piece is rural hilly roads for 10 miles from my house. At the end of that segment I am usually sitting at about 33mpg. Here is why this is so puzzling. I can coast, drive as slow as I want etc. I see on here entire commutes that are 10 miles and less where people are getting 50mpg+ via SG. The first 2 miles of mine I can only get to about 22mpg. I know everyone's car, drive is different, but I just think I am missing something simple. It takes almost a mile for me to compensate for getting out of my driveway and getting to the first downgrade for coasting.
I know I am asking for critcism, but in reality advice doesn't have to be negative.
Don't feel too bad about dealing with Mother Nature ... and coming up with the short end of the stick!
Look at what you are trying to do...start off from zero MPH and accelerating to "X" MPH, albeit slowly & gently! One, your auto. tranny is gonna slip&slide, trying to move that mass (how many hundreds of pounds?) from zero speed to cruising speed...wasting a lot of gas! Two, and we're talking average MPG here, right? ... at the end of your "trip" (however long) your average MPG is going to (mathematically) include all that "tail" you've been dragging- low speed, high gas consumption. This "tail" of bad numbers will be buried within your last (high speed, low gas consumption) number! Calculate the last, single number (MPG, average) and never ever forget the "tail" in the math! The further you go, the smaller the "tail's" effect on the single number! OK?
And, automatic transmissions are (generally) the curse of the effenciency-seeking driver!
We tried the Scangauge in my friend's auto Yaris. We managed to get over 47 mpg on the 15 mile return trip from Sears, 1/2 freeway, 1/2 city. This was with my coaching and coasting in "N" when possible. On the way to Sears we got only 38 mpg (faster acceleration, less coasting). Elevation change was minimal, little wind.