Kill-a-Watt owners: PiP kWh usage accurate?

Two and a half weeks and 597 miles after purchase, we finally poured gasoline into the car. I reset the Kill-a-watt and compared the car's kwh report to my own:

Car: 57 kwh - 4.7 gallons saved
Kill-a-watt: 70.31 kwh ($9.84 in my area, roughly 2.5 gallons worth)

This discrepancy is higher than I expected. Is anyone else looking at this?

I'm also wondering at the car's 12 .1 kwh per gallon of gas conversion ratio. How does it figure gas savings?

I wish there was a good Plug in Prius technical reference out there...

The display in the car shows the kWh usage... for propulsion itself, to inform you about vehicle efficiency.

Conversion losses are not included... as the Kill-A-Watt device clearly reveals.

Based on what I know about batteries, it sounds to be in the correct range to me.

Energy losses are part of the charge and the discharge process of the battery, and the amount lost varies depending on the type of battery.

From your data, you got about 81% back of what you put in. That's pretty darn good for a battery.

What efficiencies were you expecting?

Underlying assumption is 33.4 kwh/gallon
Plug in your EV miles and car reported electric kwh use.
You end up with the silly (because it ignores upstream conversion losses) MPG(e)

This ~19% loss is only wall into battery.

How can the car report "Car: 57 kwh - 4.7 gallons saved" if it hasn't actually "saved" those gallons yet?

That's like the U.S. Census Bureau sending me out a census asking the number of people under my roof on a date two weeks in the future, wanting me to return it right away (before the date that I'm reporting about), and threatening me with all kinds of fines if my data for a future event are inaccurate.

I'm sure that you are correct, but it's a funny way for the car to report the battery status...

57 kwh reported divided by 33.4 kwh/gallon would be only 1.7 gallons. The car is reporting 4.7 gallons saved, so it would appear that Toyota is assuming 12 kwh/gallon. Obviously cars (even the Prius) won't convert gasoline to mechanical power at 100% efficiency, so perhaps this 36% efficiency is representative of what the Prius can actually do with gasoline.

This makes sense - I misunderstood the display. In this case, the last squirt of power measured on the Kill-A-Watt isn't reflected in the dash display until I've used it? That would mean I'm actually seeing an 85% efficiency in delivering energy from the outlet to the wheels.

one question - does the car only report electricity used when the car is in EV mode and the ICE is not running? i ask because i have done freeway trips where i have used up the whole battery, and driven, say 15 miles in EV mode, but the display only shows 9 miles of EV, which seems to be closer to the "pure EV" range i actually got.

36% might be a big high for what even a Prius can do with the gasoline engine.

The typical "power to wheels" of a gasoline engine car is 20 to 26%.



Fuel Economy: Where the Energy Goes

Not quite true, MPG(e) as used by the EPA (and other fed agencies) needs to use energy measured at the wall/pump, not as its used in the vehicle.

And the EPA energy conversion is 33.7kWh to a gallon.

One cannot really consider upstream conversion losses unless one knows the upstream source. The upstream sources varies way too much for electricity to meaningfully use an national average.


As to the OPs statement about what it is saying it saved.. I'll leave that to Prius drivers as it makes no sense. Its not using total energy, but its own model of what it saved.
I know John was saying its compared to the Prius milage, but that is not consistent with 57kWh saving 4.7 gallons.

Here is how I see it: 57 kWh is 19 full charges (at 3kw) which is
209 miles (at 11 miles a charge) which is only 4.18 gallons saved. They must use a different number for the power per charge..

Yep, this is correct. Thanks

hmm.... The car knows how many EV miles, so maybe it is just dividing that number by EPA's 50 mpg for CS mode.

If OP averaged about 240 wh/mile, the 57 kwh of energy moved the car 57/(.24) = 237.5 miles.
237.5/50 = 4.75 gallons estimated.

That sounds like the easiest explanation if one knows the EV miles, but since they can also use battery in HV mode, it probably needs to be more sophisticated.

Can anyone answer If one drives exclusively in HV mode (blended).. does a PiP show "gallons saved"?

If they want to capture the electric component of blended miles, again using EPA's combined MPG figure (MPGepa) expressed as GPMepa:

CS is 0.02 gallons a mile

OP's GPM is gallons/total_miles

The difference is gallons/mile saved from the electric use.

How do I know how many kwh I used? I have my car for a week.

+1
In addition, a kill-a-watt provides detail of 120v charging - so obviously he's talking about trickle charging. 120v charging itself is less efficient that 240v charging. Your loss first comes in converting your AC power in to battery storage/DC. Secondly, pushing that DC into the battery itself has loss. And the slower 120v charging has more loss.

Saying, "Not quite true" .... is aptly put. Irony is, EPA energy conversion of 33kWh into "gallons" fails to account for exactly what you mentioned .... namely upstream values. Subtract from the 33kWh value from gas any/all of the following;

We run HUGELY energy intensive oil drills that are miles deeper into the oceon
We run HUGELY energy intensive oil spill clean ups (ah la the recent gulf disaster)
We run a HUGELY energy intensive military machine that in part ensures oil supplies
We run HUGELY energy intensive training / retraining / from deaths of soldiers
We run HUGELY energy intensive health costs via respitory illnes via pollution.

That's just off the cuff.
Then there's the energy to pump, store, pump, refine, pump, deliver.

Most of those uncounted forms of energy are growing all the time. But after all is said and done, we're left with the positive energy ... whether you count it as kWh's ... btu's ... therms ... moles ... etc. It's the return on investment (ROI). Subtract all the loss to get the goo from around the world, into your tank, and THAT's the actual kWh's ... and it aint 33kWh's. It's somewhere between 23kWh's and 27kWh's ... and it fluctuates ... and the ROI continues to get smaller and smaller. It's called, 'the end of cheep fossil fuel'.

.

i don't think the car is going to tell you that number. i believe it only counts how many kwh were used when it was in EV mode and the ICE was not running. the best way is to use something like a kill-a-watt to measure how much input power you have given the charger...

How many amps does the pip draw? My Kill-A-Watt won't plug directly into the outlet because there is an outside cover door on the outlet. I'm using a 6 inch 13 amp grounded cord. ( the kind you use for power adapter extensions on power strips)

The PiP draws up to 12A. I dont think I'd trust one of those 6" extension things. You'd probably be better off/safer getting like a shorter (10-15ft?) 12ga extension cord.

:shudder: