Gas or Electric: Which one is cheaper for the Prius Prime!

The eternal plug-in question: Is it cheaper to run your Prius Prime on gas or electric?

Its a pretty simple calculation, but much harder to determine for everyone's specific situation. To help people figure that out, I've turned to the tried-and-true engineering textbook solution: lots and lots of graphs!

I've put together a series of graphs to help Primers determine if its cheaper to go gas or electric. Here's an example of how to read the graphs:

• Step 1: On the horizontal axis (x-axis) find the price you pay for electricity
• Step 2: Draw vertically up to the line that best represents your electric driving efficiency (in mi/kWh). This can be found on the Eco Diary screen (figure out either the worst or your average efficiency)
• Step 3: Draw horizontally until you hit the vertical axis (y-axis). This represents your eGallon price (i.e. the 'per gallon'-equivalent price of your electricity rate)

In the above example, the eGallon price of an electric rate of $0.15/kWh for an EV driving efficiency of 4mi/kWh is $2.03/gal. If my local gas prices are below $2.03/gal, then running my Prius on gas will be cheaper. If they are above, then electric/EV will be cheaper.

One slight complication: These graphs will change based on the fuel economy (mpg) of the gas engine too. So I've created additional graphs for those who drive more or less efficient than the EPA-rating on gas.

More efficient:


Less efficient:


And for Canadian readers, here is the same graph (but for the Prime's standard fuel economy) in metric units and CAD!


For those who are interested in how I've created these graphs or the calculations themselves, I've attached the excel file I used. The method is based on the DOE's eGallon website (see here: eGallon | Department of Energy)

tl;dr version: Unless your gas is super cheap and electric rate super expensive, its almost always cheaper to run your Prime in EV mode.

 

While you can find out your specific situation based on the above graphs (or by using the excel file I created), you might be wondering: Are there any places where I should be running on gas instead of electric?

It just so happens there are! But some caveats:

• This is just a snapshot of one moment in time (Feb-April 2019).
• This is based on the average state-wise prices for electricity and gas. If you pay much more or less than the state-wise average for either, the logic may flip on which is cheaper.
• This is based on EPA-rated mpg (54mpg) and 4mi/kWh electric efficiency.

The good news is that pretty much all of the U.S. (46 states + DC), its cheaper to run your Prime on electric than it is gas. The exceptions are states with generally high average electricity rates (e.g. $0.20+/kWh for Connecticut, Massachusetts and Rhode Island and $0.30+/kWh for Hawaii).

If you are charging your Prime at a greater than $0.20/kWh rate, but can get gas for sub $3/gal it may be cheaper to run your Prime on gas or you just need to drive your Prime much more efficiently while in EV mode.

Getting your EV efficiency up to 4.8mi/kWh will make electric charging cheaper for all states but Hawaii.

You will need to hit 5.6mi/kWh or higher to make electric charging cheaper for Hawaii.

 

Ah, good point. Here's the graph shifted to show the common gas price range for Canada:


To read this graph in reverse, if gas cost CAD$1.30/L, your electric rate would have be ~CAD$0.25/kWh or greater for it to be cheaper to run your Prime on gas.

I was going to make a similar to the U.S. map for Canada to show which provinces where it might be cheaper to run on gas, but I couldn't find any that it would apply! Put simply, if you're in Canada it will always be cheaper to run on electric.

I think pretty much most (if not all) of Prius/Prius Plugin-Prime adopters (and especially those on this forum) probably did so for similar reason. The next comparison I want to do is which to compare which is better for Climate Change, running your Prime on gas or electric. I suspect that for environmental reasons, it would probably be better to run your Prime on gas (or pay for renewable energy sourcing for your electric) in the coal heavy-states of the Midwest. However, this type of comparison is a bit more difficult, so it will take me longer to put together than pricing.

 

True, but since your incentivizing more renewables by paying a higher rate, I would consider the electricity you get would be from those sources (i.e. renewable). Its a bit indirect, but it does serve the purpose of getting more renewables on the grid.

I've seen those two, but most of them are reported as in "X vehicle is better than Y". I already have my Prius Prime and I want to know "which areas should I drive on gas vs electric" which I haven't seen done very well. Especially when my EV mi/kWh and HV mpg can fluctuate based on drive or time of year.

 

Oh, they should and some state grids do that (see the ones in the PNW). And honestly, if the fuel actually reduced environmental impacts, particularly climate change, I would pay more at the pump for it. However, although ethanol is renewable, I wouldn't consider it actually better for the environment in the U.S. (where its sourced from corn).

 

I view it as a bit of a hedge since there is great variability in renewable costs based on location. Here's an EIA overview of the levelized cost of energy (LCOE) for electricity in 2015:

There are much bigger ranges on the renewables (particularly the solar thermal, solar PV and offshore wind) than most conventional technologies. However, there are solar PV installations that are actually cheaper than coal.

This graph also shows why onshore wind is rapidly expanding for purely economic reasons (e.g. cheaper than coal, comparable to natural gas)!