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

[Emerson, Tom (*IC)]

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...