Dr. Robert Wilder with his family's Tesla Roadster
Dr. Robert Wilder with his family's Tesla Roadster electric car.

PV + EV = 72 MPS

Dr. Rob Wilder discovers miles per sunlight with his solar-powered Tesla Roadster

By Dr. Rob Wilder

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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