Open Access Article Originally Published: July 25, 2009
Now let's throw into the mix our recent addition of a 2008 Tesla Roadster. I and my family already love it dearly: It's a clearly an exceptional vehicle -- great to drive, quick and stunning to watch. Importantly, it also uses our solar PV. Simply plug in the Roadster and it dovetails elegantly with our PV, becoming in essence a solar EV, or what I like to call PV+EV. For people interested in energy security, this is the holy grail of personal transportation --- a gorgeous, fast car that doesn't compromise on performance and is powered by renewable energy.
The green lines are fairly predictable; they're roughly a parabola matching (no surprise!) sunshine. They correspond neatly with the hours that utilities typically charge most money for energy. And, of course, our production of energy from solar power hasn't changed at all since an EV was added to the equation.
But the height and shape of our energy demand, in orange, with an EV is now far different. We consume a good deal more, although it's mostly done at night. The reason is simply that we charge our Roadster late at night through the early morning, when the battery becomes fully recharged and it stops charging.
That's shown in very high 2+ kWh orange bars seen above left and right from a typical day in May, charging at 110 volts and 15 amps. Adding this first EV has suddenly enlarged & shifted our energy-use, something to be mindful of when you're solar powered. To speed charging we've recently upgraded to 240 volts and 30 amps, so the orange bars are now briefer and can get really tall indeed! For live data see http://wildershares.com/solar.php
But before you knock the Roadster for increasing our energy demand, remember: We're not paying a penny for gasoline. And the Roadster has supercar performance and a correspondingly large battery. This battery holds 54 kWh, giving this car great speed and a good range but therefore needing much (solar) 'juice' -- certainly more than a smaller EV that might be used mainly for short trips or inter-city commuting and errands.
Due to cooling and other losses in charging, filling from empty takes about 68 kWh, or 26% more than 54 kWh the battery holds. This 68 kWh is the seminal amount; it quantifies how much truly is needed. We'll reference this number to determine how far we can go from power of the sun alone.
Crucially, we do all EV charging overnight because with Time Of Use (TOU) meter rates, the cost here is 'only' 18 cents/kWh during off-peak hours at night.
By contrast, a peak rate is far higher at 30 cents/kWh from 11 a.m. to 6 p.m., when our PV makes surplus power from the sun and sells it back to the utility, giving us a credit on our bill.
To charge overnight isn't a sacrifice at all; we'd do it anyway. Moreover, this car captures many natural benefits of EVs. It has zippy, always-available torque and doesn't require you to chug up to the peak torque zone like a gasoline car, “gasser.” It feels far more responsive and intuitive than a comparably slow Porsche or BMW. Only the very fastest gassers are in its league or quicker, such as the fastest Ferraris.
Better acceleration than most any gasser and far more fun to drive, with 100% torque -- and it doesn't require the costly, time-consuming maintenance of a gasser. All this, and you're not dependent on vexing oil -- and best of all you possibly can make your own clean fuel such as from renewable sunlight or wind power to boot!
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6 comments so far...
30-Jul-2009
67457
A very well written and informative article. But using more power on a PV system will essentially shorten the pay back period. While the idea was mentioned it did not receive the same analysis as the often mentioned 10 year payback. We would like to have seen some breakdown of how much it would have cost to use a similar powered "gasser" to travel the same distance, not to speak of the maintenance costs that are now helping to pay down the PV system. I am surprised that this vehicle is only using 30% of the power of the PV system. Perhaps it is a big house.
Posted by: Breath on the wind
26-Jul-2009
67424
EV + SOLAR = FREEDOM ! FREEDOM FROM OIL ! A CELAN EATRH FOR THE FUTURE GENERATIONS !
Posted by: Constantin Ungureanu
03-Aug-2009
67514
Thanks for your comments, Bthe Wind. Useful...
A few thoughts:
Good point about car using only 30% of PV system at present. Because most of our driving is in local Encinitas, only about 1/3 of our solar output is going now to car. if we drove more, then that % would definitely go up. I do expect we'll be adding solar PV and maybe small wind power too, since we're getting 2 more EVs ahead. Especially as the new drivers come on board here, that consumption for miles driven can go up. Finally, a 2 HP water pump here creates a fairly large building demand, pushing down the percent of PV going to car; without it, our building footprint would be smaller.
We didn't look in this article at what payback will be with added PV or wind, because we don't yet know what systems we will use, or hence costs (prices coming down fast as poly supply prices drop). Once we make decisions there, will post those $ data. We are hopeful this will shorten time to payback, as you note; will review PV price tag.
Lastly i've seen some good articles out already, comparing costs driving an EV -- as compared to driving a 'regular' gasser - and for that reason i didn't want to go there. But i hadn't seen anyone reviewing how much PV was needed to drive an EV (especially a Tesla), so focused on that aspect. It's a bit hard to compare a Tesla Roadster to a gasser too, since it can't compare to say a typical car; it's a very high-performance sports car... so in my mind it should be compared to say a Porsche Turbo or (fast) Ferrari - then costs per mile driven etc are very different for the gasser to begin with.
Just a few thoughts... thanks again, Bthe Wind... helpful points!
Cheers,
Rob W.
Posted by: Rob Wilder
03-Aug-2009
67518
Thank you, Rob. Your setup is an amazing model for the future.
It's unfortunate there are so few options today for electric vehicles in the cash for clunkers swap. Instead of an 18 to 24 MPG exchange, going electric would truly make progess in getting away from gas and boosting national health and security.
Since seeing your car along with the E Mini in Carlsbad, I've been evangelizing the benefits of electric vehicles to everyone I know. I am amazed by the promise of EV's but public awareness still seems pretty meager. I guess it's a battle for our nation's "sol".
Thanks again for your leadership.
Posted by: Warren Scott
03-Aug-2009
67519
Perhaps some additional calculations might be useful to readers such as:
Miles per kWh achieved by the Tesla Roadster (many conversions get about 4 miles/kWh).
Miles per typical solar panel per day.
Solar panel area per daily mile.
Solar panel cost per daily mile.
These figures could help readers estimate the cost and roof area required to supply their daily commute with solar panels.
Thanks.
Posted by: Mark Seidler
15-Aug-2009
67684
what an interesting guy, and also his article,,,,,,,,,,,, very informative,,,,,,,,,,,,,,, the way forward has to be electric power,,,,,,,,,,,, battery technology is coming on in leaps and bounds,,,,,,,,,, thanks for a great article
