What can I expect using Prius at 25 MPH?

[MountainStone]

In October I will be moving on to McChord Air Force Base in Tacoma, Wa. Living and working on base will allow me to bike to work on nice days, but living in Tacoma means there will be many days where biking will not be enjoyable.

The speed limit on base is 25 MPH, and my base house will be about 2 miles from work. How can I get the most efficiency from a Prius in this situation? I'll add a block heater on a timer to prevent running the ICE, but can I realistically expect to get from house to work on battery power alone (or mostly)? Insignificant elevation changes on this 2-mile road, BTW.

I know an NEV seems suited to this situation, but we will be buying only one vehicle when we get to Tacoma, not two.

I tried to search for the answer but no combination of search terms yielded the answer I was looking for.

[Buckland]

I have driven a mile or so on battery alone, but that's on city streets. If the limit was 25 you could theoretically drive the whole way on battery. But I'm sure the engine will kick in now and then to keep the battery charged, especially if you have AC, heat or lights on.

Buck

[efusco]

The EBH will make all the difference for you in this situation. I think you can expect mid-50s in the summer and probably low-50s to upper 40s MPG in the winter.
That said, I know that a lot of bases have a ton of stop signs and stopping for troops marching and such...those days you're gonna suffer a bit....if you can keep moving the entire 2 miles you'll do much better.

Now...if you add the new Hymotion/A123 5kWh lithium pack and plug that sucker in you'll never need to use any gas!

A NEV (neighborhood EV) like a Zap or something would work well in you situation too...you'd be enclosed and the top speed would be ideal for you.

[Patrick Wong]

If you didn't use an EBH and are only driving 2 miles one-way, my guess is that your mpg will be in the mid-20s.

If you use an EBH, consider the cost of powering that heater for four hours a day or whatever period of time you plan to run the heater. Is it worth it to incur that electricity cost to save a tiny bit of gasoline?

Are you able to plug in the EBH while you are at the office; if not then your EBH savings will only happen from your trip from home to the office and not the reverse trip.

Even if you use an EBH, the engine will still run to bring the operating temp up to 90 degrees C or thereabouts, so I question whether you can expect mpg in the high-40s or better given your very short commute.

Since your commute is so short, mpg does not really seem a major concern since your fuel usage measured in gallons per week is tiny. Over a five-day workweek, you will travel 20 miles to and from work. Therefore at 25 mpg, your fuel consumption traveling round-trip to work for a five-day period is 0.8 gallons.

Suppose you actually got 50 mpg. Your fuel consumption for that week would be 0.4 gallons. A savings of 0.4 gallon x $3.50/gallon = $1.40.

Now think about your kWh electricity cost to run the heater 5 days a week and see how that compares to $1.40. I understand that the heater consumes 600 watts of power or thereabouts.

Good luck with your pending move to Washington.

[efusco]

3 hrs of the heater 5 days of week at 400W ends up being pennies and for the marked improvement in FE, not to mention the CO2 reduction is well worth it.

[Patrick Wong]

Hi Evan,

Since you say "3 hours", I assume you expect the OP will only run the EBH at home. Any mpg savings are applicable only for the home to office commute; while the office to home commute will not benefit from the EBH.

As a result, we are talking only about a one-way commute of 10 miles a week from the OP's home to his office. Suppose I accept your figure of 50 mpg using EBH for the sake of discussion (although I think this is overstated). Lets assume that 25 mpg is reasonable w/o use of EBH. This translates to a savings of 0.2 gallons per week (0.4 gallons needed to travel 10 miles at 25 mpg vs. 0.2 gallons needed at 50 mpg.) Hence the cost savings at $3.50 per gallon is $0.70.

Your figures of 400W EBH power consumption and 15 hours per week translates to an electricity requirement of 6 kWh per week. Southern California Edison is the local electric utility here. The current price for a kWh at the highest price tier (which my household falls into) is $0.38. Therefore at these prices, I would pay $2.28 in electricity to save $0.70 in gas. Use of an EBH clearly is a bad economic decision for many southern Californians to make.

I understand that Midwest electricity rates are much lower, perhaps 8 or 9 cents per kWh. If 8 cents, then you would pay 48 cents in electricity to save 70 cents in gasoline per week. This is nice, but I question whether the savings are enough to write home about especially after you consider the purchase and installation cost of the EBH.

I agree that the mpg indicator on the MFD will read higher than it otherwise might, and I suppose there are some psychic rewards to that.

[MountainStone]

Originally Posted by Patrick Wong

If you use an EBH, consider the cost of powering that heater for four hours a day or whatever period of time you plan to run the heater. Is it worth it to incur that electricity cost to save a tiny bit of gasoline?

Electricity is free in base housing. Wish the Prius was a plug-in...

[Patrick Wong]

Originally Posted by MountainStone

Electricity is free in base housing. Wish the Prius was a plug-in...

Well, in that case why not buy a used golf cart, and use that to commute to the office. Then have a Prius or other gasoline-powered vehicle for your longer trips off-base.

I'm just trying to make the point that the use of EBH is not a "no-brainer"; it depends upon the relative cost of electricity and gasoline in your area.

[efusco]

It may not be a 'no-brainer' from a pure dollars/cents perspective, but the reduction in fuel use is a positive. The access to almost immediate heat from a prewarmed ICE is very nice.

And while you feel the 50mpg is conservative I think it's a very lowball for a 25mph trip...I often get 75mpg in the first 5 minutes with a prewarmed ICE and average 35mph speeds...slower should, in theory, do even better.

[onlynark]

Originally Posted by efusco

It may not be a 'no-brainer' from a pure dollars/cents perspective, but the reduction in fuel use is a positive. The access to almost immediate heat from a prewarmed ICE is very nice.

And while you feel the 50mpg is conservative I think it's a very lowball for a 25mph trip...I often get 75mpg in the first 5 minutes with a prewarmed ICE and average 35mph speeds...slower should, in theory, do even better.

I thought ~25 mp is the most efficient speed for a prius is it not?