My math is as follows (though I don't have a REX):
The EPA says 33.7 kilowatt hours of electricity is equivalent to one gallon of gasoline. That means your cost for the same amount of energy as a gallon of gas is:
Home: 0.09$/kWh * 33.7kWh/"gal" = $ 3.03 $/"gal"
Away: 0.43$/kWh * 33.7kWh/"gal" = $14.49 $/"gal" Yikes.
But EVs don't waste most of their energy like gas cars do, so we can take efficiency into account to get a cost per mile:
The next part depends a little on which i3 REX you have, but I'll assume the 60ah battery.
With efficiency #s from here: https://www.fueleconomy.gov/feg/Find.do?action=sbs&id=35279&id=38229&id=43745
It has an MPGe value of 117 so your cost per mile on electric is:
Home: 3.03$/gal / 117mi/gal = 0.0259 $/mi
Away: 14.49$/gal / 117mi/gal = 0.1239 $/mi
Multiplying by the rating of 39MPG while running on the REX and we get the break-even point of a gallon of gas running the petrol engine:
Home: 0.0259 $/mi * 39 mi/gal =
1.01 $/gal
Away: 0.1239 $/mi * 39 mi/gal =
4.83 $/gal
So, yes, if your gas is cheaper than $4.83/gallon, it'd be cheaper to run the REX than pay 43c/kWh.
If my logic is right, the 120ah i3 is less efficient with electricity (100 MPGe which is 85% as efficient as the 60ah) but even LESS efficient with gas (31mpg which is 79% as efficient as the 60ah), so this tips the scale in favor of public charging a bit
Home: ( 3.03$/gal / 100mi/gal) * 31mi/gal =
0.94 $/gal
Away: (14.49$/gal / 100mi/gal) * 31mi/gal =
4.49 $/gal
And because I did the other two, I don't want the 94ah to feel left out:
Home: ( 3.03$/gal / 111mi/gal) * 35mi/gal =
0.96 $/gal
Away: (14.49$/gal / 111mi/gal) * 35mi/gal =
4.57 $/gal
Now I just hope my math is right
