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

Nice to see these calculations. We have roughly the same electricity prices (€0.23/kWh) but our gas prices are a lot higher so driving electric makes perfect sense.
We pay around €1.55 per liter (€5.88/gallon which is about $6.90/gallon)

So it as was written before highly dependent on where you live.

 

+1 and not only where you live but also how you drive your PRIME is a very important factor.

As pointed out, Old Bear and I both have a very similar cost of electricity ($0.21/kWh) and gas (~$2/gal), but one big difference is the way we drive our car. For me, even now with COVID-19 restriction and diminished total miles driven, my driving almost always consists of a substantially longer than EV range distance with very low traffic on country roads at 30-50mph without stop and go. For this driving condition, my PRIME gas-only HV mode almost never gets lower than 50mpg except maybe during extremely cold wintertime.

 

As a new 2020 Prius Prime owner I'm learning a lot during my first week. My first Prius was a 2015 Prius III and I assumed that the PRIME would be nearly identical with the exception of the plug-in capability. Boy did I underestimate the Prime. It's so cool that the PRIME's default mode is EV. In regular HV mode my Prime doesn't charge up the battery like it does in my 2015 - I mean it charges it, but doesn't really move the needle so to speak. To move the needle on the SOC (state of charge) I'd have to use Charge Mode.

What I'm saying is, is that I'm really impressed with the Prime. It's a lot more than I thought it would be. It's changing all my preconceived ideas on electric vehicles. "Just drive the dam car" seems to be my default position now. I guess I'm gonna need to figure out the charging schedule so I can take advantage of electric rates at 1am-6:30am. The combined (electric/gas) MPGs are mind-blowing.

 

When you're in the gas-only HV mode, the PRIME operates just like a regular gen 3 Prius, correct? It shuts off the engine at stops and deceleration, and uses electric to get started? I'm finding that using the EV mode (until it runs out) at the beginning of a longer trips, say 100 miles, dramatically increases the overall average MPGs. But I think I'm going to be using HV mode simply to get older gas out of my tank - this thing burns so little fuel.

 

Jeez... With all these graphs and equations you guys are over complicating things.
The EPA says that one US gallon of gas is equivalent to 33.7 kWh of electricity.
So just take your electric cost per kWh and multiply it by 33.7 - thats what you are paying per equivalent gallon of gas.
Doesnt matter what kind of car you drive or how efficiently you drive.
If your gas cost per gallon is less than that number, then its cheaper to use gas.
Oh, and make sure you calculate your electric cost by taking your *total* bill amount divided by the total kWh used for that bill.
In my case at 0.23 cents per kWh I'm paying the equivalent of $7.751 per gallon of gas.
Unless the cost of gas goes over that its cheaper to burn gas than use electricity.

Oh, but you say "It only cost me $1.75 to fully charge my car overnight !"
That may be true, but that $1.75 is equivlent to a cup full of gas that would cost a lot less. Get over it...
(end of rant...)

 

Yes and no. In that it operates so as to use the battery while also maintaining a certain charge level, yes it's the same. But if you're driving a Prime at a steady 50 mph or less, assuming you're not fighting a strong headwind, you'll notice that the engine will shut off by itself for a mile or two at steady speeds and use up quite a bit of charge. Then it will run for a similar distance to bring back the original SOC. Gen 2 & 3 didn't do that. Also, my wife's 2017 Trim two with NiMH battery doesn't do that. Gen 4 with NiMH act like a Gen 3 but more efficient. I haven't driven a Gen 4 with a Li-ion battery, but I suspect it's similar to the Prime only less so. Maybe someone with a Li-ion Gen 4 can fill us in.

 

Maybe not exactly same as Gen3. For example, when I had Gen3, getting mpg above 60 was very difficult for me, but with Prime, I can get 60mpg in HV mode only without an effort. The better comparison is Prius Prime and Gen4 Prius.

For now, the gas is dirt cheap and I am driving only ~50miles/week. I only keep the traction battery partially charged at 30-40% most of the time. I only charge it full if I know for sure I am driving after the charge event. In my driving situation, almost all of my drives are longer than the full EV range on the Prime. I have driven my 2017 Prius PRIME for over two years with default EV mode until it runs out of range and let the HV takes over. No switching EV/HV or using CHG mode. With this method, I was getting ~90mpg overall, 55mpg HV mode and 4.6miles/kWh EV mode while paying $0.045/mile ($0.05/mile on EV mode and $0.04/mile on HV).

When I started experimenting with manually switching EV/HV on my 2017 PRIME before trading to 2020 PRIME, I realized that I can improve the EV efficiency and save some money while keeping the overall mpg high enough. With current rate of $2/gal gas and $0.21/kWh electricity, if I keep driving the way I did on 2017 PRIME that is to use default EV mode until it runs out and let HV mode takes over, I get ~90mpg overall, 55mpg on HV mode and 4.6miles/kWh EV mode while paying $0.05/mile on EV mode and $0.03/mile on HV mode. If I use EV mode on downhill and coasting only to improve the EV efficiency, then I can still get ~80mpg overall but only 20mpg on HV mode and ~16miles/kWh while paying $0.01/mile on EV mode and $0.09/mile on HV. Those are two extreme case scenarios. I am aiming to drive mixed EV mode and HV mode to get ~100mpg overall, 50mpg on HV mode with ~8miles/kWh efficiency on EV mode.

 

Nop, that is not correct. Yeah, you can compare the cost of gas vs and cost of electricity for the same energy content, but that is only a half of equation for PHEV like Prius Prime. Prius Prime has EPA rated 54mpg and 133mpge. For the same energy content of gas or electricity, EV mode is far more efficient. And that is only for the EPA rating. If your driving habit is different, the HV mode mpg and EV mode mpge will change. It is not a simple calculation like you have indicated.

In your case at $0.23/kWh and using EPA rated mpg and mpge, your break even gas price is more like $3/gal not $7.75/gal as you indicated. If your statement is true, then NO ONE is buying EV (PHEV or BEV).

 

Good info....thanks. I'm in the same boat as you on miles. Right now I'm only driving 50-100 miles a week (but I'm gonna take some trips very soon to have some Prime fun). We'll be sitting pretty when/if we ever get to driving at pre-Covid levels or if gas prices ever spiked. Pre-Covid I was doing 600 miles a week or more. I like the idea of keeping the traction battery at 30-40% and only doing a full charge when I know I'm going to be using it.

I guess there's no harm in doing a short plug-in session. Let's say on EV mode I was showing on 8% left. I could plug for a couple hours just to add some charge, without going all the way. In any case, this car seems to know what it's doing. It will probably survive me and maybe out-live me too.

 

Yeah, that is what I do when I deplete the EV range when I come home and don't have a plan to drive again for a while. With the OEM L1 EVSE, 2 hours will give you just about 30-40% SoC in the traction battery.

 

When I bought my Prime I had no idea that Toyota had changed to Li-ion. Shows how much research I did! The NiMH battery in my 2015 still operates like it did when it was new - at least as far as I can tell. It amazes me how few Primes Toyota has sold in the USA, even without the stars having aligned with regards to gas prices, driving demands, and competition .

 

Wait - are you saying that if I drive electric only (*no gas used*) and use 33.7 kWh I will travel more miles than if I drive using gas only (*no electricity added to car*) and use one US gallon of gas?
[Note that regen electricity can be ignored here because it would be a constant in both cases assuming same travel route and style of driving]

 

Distribution was limited to select regions and Toyota was in no rush. Learning from that smaller market and avoiding the tax-credit fallout provided good reason. An undeniable benefit/outcome was RAV4 Prime.

 

I don't know how you drive, so I can't say for sure about your driving. But yes I can drive about 181 miles on EV mode if I keep charging my traction battery from the wall and use total of 33.7kWh. That's 35 miles EV range from a full charge, and a full charge takes ~6.5kWh of electricity.

By definition, EPA rated 133mpge on PRIME means you can drive 133miles with 33.7kWh of electricity. That sure beats 54miles the 1 gal of gas can offer.

 

Well that makes me feel a whole lot better about the economics of the issue, if its true.
Unfortunately I can't test this yet until I get my energy monitor installed so I can measure how much electric energy I'm putting in when I charge.

 

Most people have no problem getting EPA rated 25 miles of EV range out of a full charge. Only reason you do not get that much of EV range would be either it is very cold or very hot or you drive mainly on highway over 70mph. Yeah, at your electric rate of $0.23/kWh which is slightly more than my $0.21/kWh, you will save money by driving HV mode using gasoline until the gas price go above $3. But, at gas price anywhere between $2-$3, the difference is not that much. EV mode MG1 motor has more pleasurable response than gas engine, and it's much quieter. And it's not always money factor. Less CO2 emission is a good thing even if it cost little bit more.

 

What that kWh per gallon ignores is the tremendous energy loss incurred just by the reciprocating mass of the pistons & valves, the energy spent compressing valve springs, the windage of pistons pushing air around (it's basically fighting against the engine braking used in B mode when not in EV), friction from pistons, bearings, etc, and various other built-in inefficiencies in a piston engine. The electric motors don't have to overcome nearly as much resistance. Just try spinning a piston engine by hand and then spin an electric motor by hand and the difference will be apparent. As a result, a most of that 33.7 kWh/gallon in the gas is frittered away in overcoming those internal resistances.

Just look at the m/kWh report on the MID. It won't include the overhead of charging losses, but it's very close.

I average 5.0 miles per kWh, since getting the car) on the car's efficiency report. 33.7 kWh would therefore take me 168.5 miles in EV. In HV mode, I usually get about 65-70 mpg around town. Not even close to 168.5!

Using the meter on my charging outlet, it takes about 6.1 kWh to fully charge a depleted battery. I usually get a little over 30-32 miles on that charge. Those numbers correspond well with the car's numbers.

So, with my electric price of 11 cents/kWh and a current gas price of $2.15/gallon, I can spend $1.32 (66 cents times two) to go 60 miles on electricity, or I can spend $2.15 to go the same distance on gas.

 

You assume that all of the energy contained in a unit of gasoline is converted to the power being transmitted to your car's wheels.

Unfortunately, a good deal of that energy is converted to heat which is disposed of by your car's radiator and exhaust system. (In winter, some of that waste heat is used to heat your car's cabin.) In addition, in most cars, your gasoline engine continues to run while you are stopped at a traffic light. And, when you step on the brakes, the kinetic energy of your moving vehicle is converted to more waste heat by the friction of your brakes.

Electric cars also have energy losses due to electrical resistance, battery efficiency, and plain old friction of moving parts.

But that energy loss pales in comparison to that of an internal combustion engine and conventional friction braking.

 

And besides what I already mentioned and the good stuff added by @Old Bear, there's the compression stroke to keep in mind. You burn a big percentage of the gasoline just compressing the fuel-air mixture. Piston engines are so inefficient, it's amazing they're still so popular. That Toyota has done with hybrids is an absolutely brilliant job of compensating for the inherent shortcomings of traditional engines.

 

The simple formula that I use is based on (1) close to 6Kwh per full charge, (2) around 55mpg in hybrid mode, and (3) around 25-30 miles EV range. That is very close to to a full charge of being equivalent to 1/2 gallon of gasoline. So the break-even point is 12 times the cost per Kwh being the cost of a gallon of gasoline. I pay 9 cents/Kwh for electricity, so if gasoline costs more than $1.08/gallon, I come out ahead with electricity.