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

The idea of using solar to power electric cars is tremendously appealing in theory, yet critics insist that it's a myth or a pipe dream at least a decade away. But it's here now --- and our Roadster is the proof. Let's examine how we get 72 miles per day from sunlight, or what I affectionately call 72 MPS, in our solar/electric Tesla.

To visualize how sunshine can power very fast cars, start with the solar. Our solar photo voltaic power began with installation in 2003 of a 3.85 kilowatt solar rooftop array on our home in Southern California. Originally powering only a building, the PV has been performing well and should reach payback after roughly 10 years.

The fairly short time to payback is due to two crucial components: 1) *California's solar subsidies, and 2) *Time of Use metering (TOU) by our utility.

This PV is monitored by a web-based real-time monitoring system. In long sunlight in the summer and fall, we generally make on average around 14 kilowatt*hours (kWh) per day from our phase-one panels (below left). In winter, with fewer daylight hours, or on cloudy days with less irradiance (watts/meter2), we generally make much less:

Pleased with phase-one results, we next installed another 2.8 kW of ground-mounted PV (above right) and so total production for both systems is about 6.65 kW overall.

Importantly, since we implemented this solar system in 2003, it has been providing us about 24 kWh per day of electricity. That's roughly enough to meet the needs of many a small business, or a single home. Please remember this 24 kWh per day figure -- it's an amount we think of as “One Sun” and will be relevant with addition of an electric car.

It's also an average. We can make more than 25 kWh on long, sunny, non-foggy days of late summer and fall. Conversely, on shorter winter days, or on any cloudy or foggy days, solar production will be substantially less.

Consider next our billing is by TOU (Time of Use) metering and on a 1-year annual basis -- it's not monthly. So with the grid essentially a battery, and 1-year billing cycle, we can use greater power in late summer and fall to offset winter shortfalls. As the PV in day covers night use over 24 hours, surplus in summer and fall carries winter year in and year out.

Practical Knowledge Gained from Adding an Electric Vehicle

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!

For our EV, 'fuel' comes in essence from PV. And we may soon add a third phase of differing PV, or small wind power for even more renewable fuel. Contrast that with a gasser. It's impossible under virtually any circumstance to make your own gasoline. Yes, it's energy-dense --- but it's finite, dirty, comes primarily from geopolitically instable regions. A gasser can't go 10 feet without it, and even hybrids depend on it.

On the other hand, we're already learning valuable lessons about PV+EV. In a chart above, the Roadster began consuming energy in the evening; by the time it stopped charging, it was only partially charged. Because it draws a maximum of about 15 amps from a common 120V outlet, it needs more time than TOU allows per night to recharge fully.

However we recently changed to a 240V mobile connector, so we're now charging at 240V @30 amps (using a standard NEMA 14-50 4 wire), dramatically shortening the full recharging time from at least 24 hours to less than 8 hours. And we could purchase the high-power connector and charge at 240V @ 70 amps.

A measuring unit to next help explain energy*time is the kilowatt*hour, kWh. Elegantly it can apply equally to energy made by PV-- or energy used in building or car; 500 watts for 2 hours, 1,000 watts for 1 hour, or 2,000 watts for 30 minutes, each = 1 kWh.

Consider now that with TOU, each kWh surplus solar made On-peak, is worth 1.6X each kWh used Off-peak due to a billing ratio of 30:18. So our 25 kWh made On-peak by PV, and leveraged at 30:18 becomes akin to our receiving 41 kWh Off-peak from the grid.

What, next, is our actual range on a 68 kWh fill up? To give an exact range is surprisingly slippery, regardless of solar power or not. Yes, this car impressively is EPA rated at a 244-mile range, or it can go 0-60 in 3.9 seconds. Yet it can't go that far and consistently fast.

The Roadster has several driving modes. We almost always use the default “standard” mode to optimize performance and range. The “range” mode allows more battery charge; it slightly shortens battery life and we sometimes use it if going unusually far -- but it slows the EV considerably so it's more like driving a common gasser. The “performance” mode is typically used on race tracks; it's less efficient so we don't use it at all.

After turning the key in standard mode, you see “ideal” range: it maybe says, 195 miles -- not the EPA rated 244: you don't have access to 100% of the energy in standard mode. You're seeing only 80% of theoretical range. This is partly for battery management; charging to 90% in standard mode prolongs battery life, and 10% more left in reserve also is not shown onscreen.

In our experience, after driving to a half state of charge, we've gone approximately 70-75 miles. Extrapolating and being conservative we normally expect some 140-miles total range; that's without dipping into the 10% reserve and driving in the fast standard mode, which is just too much fun to pass up.

On the other hand, we'll get EPA-rated 244 miles starting off with a full charge and going in range mode.

Now on to a key question: what's real range in this fast EV powered by sunlight? I suggest rephrasing the question: How far does our 6 kW of solar PV make the car go? Recall we make about 24 kWh over an average day; we call this 24 kWh in a day, or 1 “sun.” Broken down as 24 hours, roughly 1 kWh is being made each hour; we call that 1 kWh per hour one “sol”. Two hours is thus 2 kWh, or 2 sol, etc.

As will be shown, we get about 3 miles range from each kWh (sol) in this fast car.

Simply, 24 kWh/Day means roughly this car will drive 72 miles per day from sunlight alone. Thus it has a range of 72 miles per day of sun, or 72 MPS. Translating how far you can go from of sun power alone, and seeing that it's 72 miles per sun (MPS) or 3 miles per sol (3 m/sol), may feel more intuitive and simply more elegant than oily old MPG.

Solar-power is more changeable than a fixed 24 kWh/day, however for simplicity's sake we've kept the 1 sun constant. Yet a second major variable is energy expended in driving. That in turn will be keenly influenced by how and where we drive the EV.

We're estimating our own average demand is 330 Wh/mile overall, after charging losses. This is based on our local situation: we drive local streets 30-60 mph, and the Roadster is very efficient in that zone. We don't do very many freeway miles and are only occasionally in range mode. With our driving mix, we end up with roughly 270 Wh/mile.

We get around 0.270 kWh/mile typically
in 30-60 mph stop & go local traffic.
Given 0.270 kWh/mile from battery as our average, and adding 26% loss charging (going to around 330), means we get roughly 3 miles range for each kWh, or 3 m/sol. (This also is in line with the EPA estimated 0.280 kWh consumption per mile in a combined cycle)

Generating our own PV power makes us more aware of building demand. We are diligent about using regenerative braking to slow down. Why use the brake pedal when you can slow down just as easily by taking your foot off the accelerator -- and make electrons too?

Just lifting off the accelerator slows the car quite sufficiently in most driving situations, particularly from high speeds, when inertia regenerates 30-40 kW back in the battery. For me and for many Tesla owners, the “strong regen” is one of the most interactive and satisfying aspects of driving. Compared to an archaic gasser, which wastefully heats brakes to arrest momentum while putting zero fuel back in the tank, the Roadster can be efficiently controlled with just the slightest movement of your foot on the accelerator.

Now back to the PV connection: We'd estimated its payback in roughly 10 years. Now with 5 ¾ years making solar power under our belt, we see that's about right. Total cost for the first phase was $15,511 (the California subsidies back in 2003 had cut our costs paid in half).

29,000 kWh is generated and measured since a 2nd PV monitoring was installed in 2006 (left).
Or at this sunny On-peak moment (left, top), the Irradiance is a sunny 772 W/M2;
6.5 kW of PV is making 3,698 Watts, the demand is 530 Watts, and 3,167 Watts is being exported.
And our payback might be better yet. We'd looked only at payback for our electricity -- a complex problem but a self-contained one.
Now that we're also avoiding buying gas, a vital second factor is accelerating payback. (Gas costs roughly double cost over electricity alone).

The combination PV+EV works, but there's clearly limits on both sides of the “+”.

So the combination of PV+EV works, though we find limits on both sides of the equation. For instance, this car is thirsty; we're consuming much more PV power than before powering just a building. Rather than PV fully meeting 100% of a smaller demand pie as it did before, now the demand pie is bigger. About 30% of the greater need goes into an EV, and 70% goes into the building. There's no free lunch, even with solar.

How well has the 6 kW PV coped? In cloudy May, demand from both the building and car was 1,160 kWh. In that overcast month, PV made 650 kWh. That sounds a huge shortfall, yet with 30:18 leverage, TOU almost roughly covers it. But during four socked-in, foggy days at the end of the month, PV with TOU would have covered even the bigger combined demand.

650 kWh PV in May becomes 1,070 kWh due to TOU; almost matching 1,160 used.

Clearly it’s not enough watts head to head. But with TOU boost, 650 kWh is like making 1,070 kWh off-peak and just short of running both building and the car together. It’s more complicated, given that some PV power comes off-peak, but of the 1,160 kWh needed, our building alone used 810 kWh or about 70% of total, and car consumed about 350 kWh after charging making up the other roughly 30%.

Over a century ago, people who bought gassers learned that their fuel was toxic and flammable. They knew cars required many gears to compensate for the inefficiencies of internal combustion engines. They knew about the relentless frequency of oil changes, muffler replacements and engine repairs. All the same still holds true for gassers today, but the public simply assumes these things as the price of personal transportation.There are surely limits to EVs -- but they are strikingly different.

We rarely drive this wicked-fast & fun Roadster purely in range mode, so we often don't get an EPA-rated 244-mile range, yet what we get is much more than we need. But that downside pales in comparison to a key point: The Roadster is available today and can travel at any reasonable speed and perform even better than comparable sports cars. It doesn't need futuristic batteries -- what I like to characterize as “unobtainium” (great if it existed, but doesn't). So the biggest barrier to widespread adoption is cost. But the costs are already coming down; the Model S has an anticipated base price of roughly half that of the Roadster.

Now that we've experienced our PV+EV, we are looking forward to adding new methods of clean energy generation to our home -- and new EVs to our fleet. We're through with slow gassers. Our cars going forward are going to be electric!

Lastly we look forward to adding more clean energy that makes this very robustly a 'solar-powered car'. One off-peak electron at night has less value than one in midday and it's that economic reality, that allows 6 kW of solar PV to meet all our demand. But if we say double production of renewable energy, then in physics too we're making enough green electrons to power both building & car(s) in a straight 1:1 swap without TOU boost. Happily, adding PV or small wind isn't technically challenging.

Dr. Rob Wilder is Manager of Encinitas, Calif.-based WilderHill Clean Energy Index (ECO), the first Index on Wall Street for renewable energy, better energy efficiency and zero-carbon solutions. He was previously on faculty at U.C. Santa Barbara, and University of Massachusetts; he has been AAAS/EPA Fellow in Environmental Science & Technology, Fulbright Fellow, and a National Academy of Sciences Young Investigator. For a more extensive look at so-called PV+EV technology, check out http://www.wildershares.com

Times Article Viewed: 20934
Published: 25-Jul-2009


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