I recently purchased a block heater. I bought it for the purpose of increasing fuel mileage and also to have heat working sooner when I drive off in the winter.

To me it seemed like basic physics. An engine runs at its peak efficiency when it is at it's optimal operating temperature which is around 195F. I did not initially do any research as what the ROI would be for this purchase. But I am doing some rudimentary analysis now.

I could not find any ROI analysis for a block heater so I figured I might as well create one. Some of the data I used is incorrect since said data does not exist and I do not have the tools to properly collect said data.

I have also found some data (not really 100% credible, but there is no real reason for them to lie) that suggests that the efficiency gains (in terms of reduced fuel consumption) of a block heater remain true in the summer as well as winter.

The unit cost $30 from amazon.

There are 365 days in a year and I estimate that I will use the heater 95% of the time so about 347 days each year I will use the device. In Illinois electricity is about $0.09/kwh. The device consumes 0.2 kw/h and most forums online say it only needs to be plugged in about 2 hours before driving to heat the engine to an optimal temperature (maximization problem with the curve that I am ignoring). So for each day I use it I will probably only have it plugged in for 2 hours. So it will probably take less than $13/year in electricity costs.

Here is where my data is non existent and I used another model which I assume is similar. I am assuming that driving my moms 2002 chrysler 300m takes about five minutes of driving to reach it's optimal temperature (for efficiency). This may be a bit conservative, it probably take a bit longer.

Then I used http://www.metrompg.com/posts/warm-up-idling.htm to get an average of how inefficient the engine would be for the first five minutes (again, no correct data was available but I can see the numbers on that site being reasonable).

So from their data (1.5+1.1+1+.8+.7+.7)/6=.966 so .5(fuel consumption at optimal temp)/.9666(avg of fuel consumption for first 5 minutes of operating)=.517 so 51.7% efficiency.

My mothers car gets about 25mpg when running at optimal temperature, full tires, 10w30, no wind...

so 51.7% of that is 12.93mpg for the first five minutes of driving. I would estimate my average speed for my first 5 mins of driving is about 40mph=.66 miles per minute. so in 5 minutes I travel 3.33 miles where I am only getting about 12.93mpg.

so 347 days*3.33 miles=1156 miles I was driving per year where I was getting 12.93mpg. With gas at $2.55/gallon and assuming that having the heater plugged in for 2 hours will get the engine to it's optimal temperature where it gets 25mpg it would save about $110 in gasoline per year. This is not subtracting the $13/year in electricity costs nor is it taking into consideration the initial $30 purchase price.

I am too lazy right now to do a breakeven analysis to find out how long it would take to break-even and start saving money.

some other info I would like to take into consideration http://priuschat.com/forums/gen-ii-prius-main-forum/23749-some-hard-numbers-block-heater-operation.html#post296616 possibly later (eg, using the heater I think would only provide 19mpg on average for the first 5 mins thus only $73 saved on gas per year)

This is ignoring the obvious benefit of the comfort of having a warm car right away when you drive off on a blisteringly cold day and also ignoring the benefit it would have on the engine (oil is more viscous and it is a lot easier on the engine to start it warm vs cold thus it will probably last longer). It also ignores the heat dispersion/etc and a bunch of mathematical and scientific stuff for now. It also ignores time value of money stuff. investing an extra $5/month for 25 years at a 10% return would result in almost $7k at the end of the 25 years. Again, this ignores the fact that the car will have a longer useful life (hopefully) and yes I know, it's not mine.

Let me know if you find a proper ROI analysis for block heaters.