[SGVLUG] Off-topic - for Tom - plug-in car that can power yourhouse

[Jeremy Leader]

Tom, my impression was that the "100mpg" figure was an average (assuming some 
unspecified use profile).  In reality, the plugin hybrid gets infinite mpg for a 
short distance (I think the article claimed 40 mile range on battery alone), and 
then reverts to roughly the same mileage as a non-plugin (a little worse because 
of the extra battery weight, I'd assume).

So for example, I commute 20 miles to work, and 20 miles back.  So I could just 
about do my daily commute for only the cost to recharge the battery each night; 
if my employer provided free outlets in the parking garage, it could cost me 
even less.  If I also use the car for short errands on the weekend, I could be 
saving roughly $2 to $3/day * 365 days/year or up to $1000/year.  At that rate, 
it would take 10 years for the add-on to pay for itself (ignoring interest).  I 
wonder if the batteries could be expected to last >3600 charge/discharge cycles?

Then again, who says gas will still be $3/gallon in 5-10 years?

-- 
Jeremy Leader
jleader at alumni.caltech.edu
leaderj at yahoo-inc.com (work)

on 04/12/2007 06:35 PM Emerson, Tom (*IC)  wrote:
> I think this calls for some back-of-the-napkin calculations...
> 
> 
>> -----Original Message----- Of Dustin Laurence
>> On Tue, Apr 10, 2007 at 01:50:41PM -0400, Robert Leyva wrote:
>>> Hello Tom (and everyone)
>>>
>>> Saw this article about having a plug-in car also be set up with a 
>>> grid-intertie inverter so it can be used to power your house.
>>>
>>> Kinda cool.
>> It would be even cooler if Tom's Prius were new enough to be 
>> a plug-in candidate, so he'd be motivated to do the 
>> conversion and then show us. What's $10,000 when the planet's 
>> at stake?  :-)
> 
> Hmmm... As much as I'd like to agree with you and your "smiley",
> something doesn't add up here...
> 
> The buyback for a $10k add-on would (naturally) be $10k of gasoline NOT
> purchased over the lifetime of the vehicle.  The third generation prius
> (which is what we're talking about here) nominally gets 50-60mpg, this
> add-on boosts that to 100+mpg.  Let's use 50 as that makes the figuring
> easier -- you effectively have to buy half as much gasoline with this
> addition to drive the same distance without it.  Though the price
> fluctuates, in the long run it has been hanging around $3/gallon.  At
> that price, $10k buys 3,333.3333 gallons (plus a drop or two...), and at
> 50mpg that pushes your car around the 1-mile oval 166,666.665 times.
> 
> In order to SAVE $10,000, you would have to drive the car that much IN
> ADDITION TO the miles you "pay" for with direct gasoline purchases, or
> 333,333.333 miles overall.  In other words, for $20,000, you could drive
> a prius this far.  For $10,000 in gasoline PLUS $10,000 for the add-on,
> you could drive a prius exactly the same distance (though you've done
> the planet a "favor" by burning half the fuel).
> 
> For comparison, I've **just** recently rolled past the 80k mile mark in
> my 2001 prius -- at that rate, I'll have to drive it another 15 years to
> have spent $20,000 in fuel (provided fuel really does stay at $3/gallon
> for that long -- all this goes out the window when the price of gasoline
> say TRIPLES from what it is now...)  That is without the "benefit" of
> this add-on
> 
> Now, for the discussion at hand: the site talks about "selling" the
> excess energy in your vehicle as a "profit center" for joe average.
> Unfortunately, this doesn't take into account that if you sell off this
> "excess" energy, your MPG rating drops back down to 50mpg since you stop
> getting the benefit of using it to push your car around.  (actually,
> your mpg will probably drop BELOW what you would get without this add-on
> for two reasons: it's heavier, and with the battery "depleted", the
> on-board computer actually runs your car at idle longer to re-charge the
> battery)
> 
> But that is a red herring :)
> 
> You see, what the story is REALLY telling people to do is to "sell" the
> excess energy in your car DURING PEAK HOURS, then "buy" cheaper
> electricity at night to recharge the battery for your morning commute.
> You still get (better than) 50 mpg since (at least in the morning) you
> are running with a charged battery.  Presumably, "at work" your car will
> be used as a source during the peak period, so you'll either have a
> partial-charge or a depleted battery, so your evening commute will be
> back at 50mpg or less.
> 
> BUT... Why bother going to this much trouble?  If the "deal" is to buy
> low/sell high by storing electricity in a battery for about 12 hours,
> why not just skip the part about having wheels on it?.  Or to put it as
> is often seen on slashdot:
> 
>    1) buy "cheap" electricity during off-peak hours
>    2) charge a battery with it
>    3) ??? [or in this case, "wait 12 hours"]
>    4) sell back "expensive" stored electricity
>    5) profit!
> 
> Unfortunately, this will only work if nobody else does this.  If
> "everybody" were to put a car-sized battery in their garage, "buy" cheap
> electricity off-peak, and "sell" it during peak, eventually the
> "off-peak" demand will reach or exceed the "peak" demand...
> 
> Note: I was going to leave it is as an exercise for the reader to
> determine how long you would have to do this to make any measureable
> dent in your own electricity costs, but figuring out these things is so
> much damn fun :)
> 
> Per this website, for our area, so cal edison charges about $130 for a
> "megawatt/hour"
> [http://www.jea.com/services/electric/rates_quarterly.asp]  but this is
> kind of lump-sum (i.e., doesn't take into account peak vs. off-peak)  It
> does, though, work out as an "average" of 13 cents/kwh which is about
> right -- A little more digging finds this site,
> http://www.riversideca.gov/utilities/resi-urate.asp, which shows "peak"
> rates at 20 cents and off-peak at 5 cents. (gratuitisly rounded)
> Averaging (20+5)/2 gets 12.5 cents/kwh, so we're in the same ballpark.
> 
> HOWEVER, the real gem here is noting that there is a 15 cents "profit
> margin" per unit (well, kilo-unit ;) )  If your "normal" usage of
> electricity is evenly balanced between peak/off-peak periods, you'll pay
> the same regardless of whether you use the "average" rate or per-period
> rate.  If, in addition to that, you "buy" 1kw per hour off-peak and
> "sell" it all back, you'll be making 12*$0.15/day, or $1.80/day, or
> $54/month.  If your electric bill is less than that, your making a
> profit.  Of course, you do need to figure in the initial cost and
> depreciation of a battery capable of storing 12kwh -- if your "bill" is
> $50/month, it may take longer than you'd like to "pay off" this purchase
> with only $4/month to show for it...
> 
> OK, it's time for me to head for the meeting -- expect this discussion
> to continue at BC tonight...
>