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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

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