Perhaps one of you good-looking, brainy, engineer, math wiz, nerd types can help me with this semi-serious question: Approximately how much gasoline (87 octane) is one of the Prius' "50 watt hour" little yellow "PacMan stars" equivalent to?<br><br>Also, it it true that one 50 WH star is approx. equal to running the stock stereo at full blast for say, 5 hours or so ... assuming that the steroe amp is capable of approx. 15 watts RMS total output?<br><br>Along the same lines, another semi useful comparison would be the ratio of 50WH stars to running time of the headlights on low beam.<br><br>Several folks have asked me what the 50 watt hour stars are equivalent to in terms of _mileage_ or functionality of the car, and I didn't have any good answers for them.<br><br>- Peter<br>(Is that the sound of slide rules I hear?)

I figure each little star represents about an ounce of gasoline that will go unburned.<br><br>See my message #369, back in August.<br><br>There are probably lots of ways to approach this question, undoubtedly more accurate than my back-of-the-envelope guess. Any other ideas on how to figure it, and maybe check to see if I got it close?<br><br> --Mike

Re: How much gas is 50Wh equivalent to?<br><br>The US EPA report on the Japanese Prius states (pg 12) that "Toyota claims that the max power capacity of the battery pack is 1800Wh which is equal to to 0.238 gallons of gas". Doing the math, this says that 50Wh = 0.1 oz of gas.<br><br>(I did actually do a little engineering on the Atlas/Centaur rocket early in my career)<br><br>Chuck

&gt; Re: How much gas is 50Wh equivalent to?<br>&gt; The US EPA report on the Japanese Prius states (pg 12) that "Toyota<br>&gt; claims that the max power capacity of the battery pack is <br>&gt; 1800Wh which is equal to to 0.238 gallons of gas". Doing the math, <br>&gt; this says that 50Wh = 0.1 oz of gas.<br><br>Have you got the math right? I figure:<br>50 Wh / 1800 Wh = 0.0277 * 0.238 gal = 0.0066 gal = 0.845 ounce<br><br> --Mike

I wonder that's equivalent to the raw energy of gasoline or the actual effect of using that volume of gas in a car.<br><br>Why? Gasoline engine is only about 11% efficient at best. That means you release 89% of the gasoline energy in the form of heat.... to the environment.<br><br>Therefore, instead of 0.845 ounce, it's about 8 ounces of gasoline in a car.....

&gt; I wonder that's equivalent to the raw energy of gasoline or the actual<br>&gt; effect of using that volume of gas in a car.<br><br>Good question. I think it's the effect of using it in the car. <br><br>Back in message #369 I assumed 10 horsepower output to the wheels to push the car down the road at a steady highway speed, and the 45 mpg EPA highway number for the gasoline it burns to get that output to the wheels, and also got about an ounce. <br><br>That also assumes the regenerated power can be returned to the wheels with 100% efficiency, which is pretty close to accurate. In fact it's the whole point of the car.<br><br> --Mike

&gt;Re: Have you got the math right? I figure:<br>&gt;50 Wh / 1800 Wh = 0.0277 * 0.238 gal = 0.0066 gal = 0.845 ounce<br><br>Mike,<br><br>You're correct (no it wasn't me who caused the last mission to Mars to crash because of a conversion error). <br><br>Amazing how close this comes to your message #369 where you predict 50Wh = 1 oz using a totally different approach.<br><br>Chuck

This is great, folks. Keep it coming! This is exactly the sort of info my captive Prius passengers want to hear (yeah, right)! No, seriously, I very much appreciate the info and education.<br><br>Now, what about the Prius car stereo amplifier and headlight low-beam questions? How do the Prius' 50 watt hour regenerative "stars" approximately equate to the power consumption of the stereo when it's cranked, or to the power consumption of the low-beams? (I don't know offhand the actual max/typical power consumption rates of the stereo or headlights myself, regrets.)<br><br>Is this a straight forward "watts = watts" sort of comparison, or is it more of an apples and oranges kinda thing?<br><br>- Peter<br>(Scary thought: I got "interviewed" by a reporter from the Oakland [CA] Tribune newspaper who may be publishing a story on the Prius soon. Of course I said I absolutely _LOVE_ my Prius! Stay tuned ...)

Chuck said,<br>&gt; Amazing how close this comes to your message #369 where you predict<br>&gt; 50Wh = 1 oz using a totally different approach.<br><br>Especially amazing considering how many wild a** guesses I made. Just lucky. I think we must be pretty close to the real figure.<br><br>So, ee_of_ee, you must have some real figures on all this....<br><br> --Mike

First, I'm not a rocket scientist, just a Prius nut.<br>Second, gasoline contains 33.4 kWh of energy per gallon.<br>Third, if we assume that Prius gets 50.1 mpg (it makes the math easier), that is 750Wh/mile.<br>Fourth, that is about 350 feet of "free" driving per 50Wh star.<br>Now the disclaimer, this is not a precise calculation, just another way to play with numbers.

Hi ee_of_ee,<br>Thanks for the additional hints on meaning of 50Wh stars.<br><br>Let me try to add in some additional complicating factors. Primarily, the inefficiencies of gas engines and batteries. The EPA web site lists a lot of ways that the energy of gasoline becomes heat instead of vehicle motion. Most also apply to running on Electric (drive train losses, accessories, etc.), but idle losses can be removed because Prius ICE doesn't idle often. Braking losses are what the stars represent, so we don't count them either. So that leaves 62.4% loss due to engine inefficiencies. Let's guess with the Prius Atkinson cycle engine that it's more like 50%, so that gallon of gasoline with 33.4 kWh only gives us 16.7 kWh of useful energy.<br><br>Storing electricity in a battery and pulling it back out again is also not really efficient. I think it loses about 30% but I've also seen other numbers. So a 50 Wh star is about 35 Wh of useful energy. That gives us about 480 stars per gallon or about 3.75 oz. of gas per star. Hmm, bigger answer than other methods.<br><br>With the 50% derating for the gas energy, a 50.1 MPG Prius would use 16700/50.1 = 333 Wh/mile. With the 30% derating for battery storage, the 35 Wh star would be about about 550 feet of free driving.<br><br>That poor guy wanted rocket scientists and all he got was us (and a lot more confused :-)<br><br>Robert Snyder<br><br>Your Footage (per Star) May Vary!

I'm sorry, 480 stars per gallon would be about 3.75 stars per oz., not 3.75 oz. per star, so this answer was smaller than other methods. I.e. closer to 1/4 oz. per star.<br><br>Isn't this how rocket scientists make things crash on Mars? Maybe I'm learning how to be one :-)<br><br>Robert Snyder

I'm glad to see others can make math errors, too. <br>As I was waiting to drift off to sleep last night, I realized I'd made a classic in my post yesterday. In my attempt to simplify the math I blew it. A Prius at 50.1 mpg is using 667 Wh/mi not 750. So much for easy math. This also changes the benefit of a 50Wh star to ~396 feet. <br><br>Sorry for the error. I'll be more careful in the future. I, too, may be responsible for the Mars crash.

&lt;In my attempt to simplify the math I blew it&gt;<br><br>Dont feel bad. Certain Persidential Candidate(s) made bigger math errors.<br><br>Cheers