Electric Cars and Photovoltaic Solar Cells

Elon Musk, Co-Founder & CEO of Tesla Motors, October 11, 2006

Elon and Martin both recognize the synergy between solar panels and electric cars. As Martin pointed out to Gov. Schwarzenegger, a million solar rooftops in California will not reduce California’s oil dependency by one drop unless we have electric cars also.

You have asked us quite a few questions about photovoltaic cells. Elon and Martin will answer a few of them here:

Question: Doesn’t it take more energy to produce a solar panel than that panel will ever produce in its serviceable life?

Elon: The idea that a photovoltaic (PV) solar panel cannot pay back its energy investment is flat out wrong by a huge margin, but I've heard it repeated by many otherwise intelligent people.

Martin: This reminds me of otherwise intelligent people who think of hydrogen as a fuel – who don’t realize that it takes lots more energy to create hydrogen gas and pressurize it than you could ever get out of it with a fuel cell (or any other way).

Elon: Right. The analysis for photovoltaics is straightforward and has been done by disparate researchers around the world, with the payback results in recent studies varying only by a year or two.

The most common type of solar panel uses single- or multi-crystalline silicon wafers, and is offered by a wide variety of manufacturers, from Sanyo in Japan to Renewable Energy Corporation in Norway to Sunpower in California. Creating the silicon crystal is by far the most energy intensive part of the process, followed by various and sundry manufacturing steps, such as cutting the silicon into wafers, turning the wafers into cells and assembling the cells into modules.

The cumulative energy used can be summed up and accounted for both theoretically (eg. paper by Alsema, Frankl & Kato ) and empirically (eg. paper by Knapp & Jester).

In a modern manufacturing plant, the energy needed to create a frameless PV module from semiconductor scrap material is estimated to be around 600 kWh/m2 for monocrystalline cells and 420 kWh/m2 for multicrystalline cells (source: www.nrel.gov). A big variable is how thin the silicon wafer can be sliced. For ultra-thin cells, like those from Sunpower, the energy to produce a module may be considerably lower.

Taking the monocrystalline example:

Solar incidence (US): 1825 kWh/m2/year
Module efficiency: 18% (Sunpower)
Energy lost in system: 20% (Due to inverter, wires, cell temperature, etc.)
Total energy produced:
263 kWh/m2/year
Energy to create module: 600 kWh/m2 (National Renewable Energy Lab.)
… to build aluminum frame: 80 kWh/m2 (from Alsema et al)
Total energy used:
680 kWh/m2

The above results in a payback period of roughly 2 and a half years. The NREL study similarly calculates the payback period for polycrystalline panels to be 3-5 years, and amorphous silicon panels to be 0.5-2 years. Given that most modules have a 25 year warranty and an expected useful life in excess of 30 years, this indicates about a ten to one advantage for energy generated versus consumed.

Not accounted for is the energy cost of installing the modules, which is quite nebulous and varies depending on the efficiency of the installer. However, also not accounted for is the potential to use reflectors to concentrate solar energy, which can improve the payback period by a factor of two or more, and the ongoing improvements in conversion efficiency and silicon usage efficiency. Additionally, the aluminum frame is completely recyclable, and this recoverable energy is not accounted for.

Some References:J. Mason, "Life Cycle Analysis of a Field, Grid-Connected, Multi-Crystalline PV Plant: A Case Study of Tucson Electric Power's Springerville PV Plant." Final report prepared for Tucson Electric Power, November 2004.

K. Knapp; T.L. Jester, "An Empirical Perspective on the Energy Payback Time for PV Modules." Solar 2000 Conference, Madison, WI, June 16-21, 2000.

E. Alsema, "Energy Requirements and CO2Mitigation Potential of PV Systems," Photovoltaics and the Environment, Keystone, CO. Workshop Proceedings, July 1998.

Question: How many solar panels do I need to power my Tesla Roadster?

Martin: The Tesla Roadster consumes about 200 watt-hours per mile. Suppose you drove 35 miles per day on average (12,775 miles per year). You would need to generate 2.6 MWh/year.

By Elon’s math, monocrystalline solar panels generate about 263 kWh/m2/year in the USA. So you would need about 9.7 square meters of solar panels (a square about 10 feet on a side) to completely offset the energy consumed by your Tesla Roadster.

Elon: Obviously, you can’t fit these on the roof of your car. But you can hire a company like Solar City to install them on your house – where the panels are mounted at the right angle, and are in the shade as little as possible.

Martin: And if you get a time-of-use meter from your power company, you sell solar energy to the power company during the day at a high rate, and buy energy back at night to charge your Tesla Roadster at a lower rate. This gives you about 2:1 leverage, meaning that you need an array only 5’ X 10’ to completely offset the *cost* of energy for your Roadster.

Question: Why don’t you put solar panels on the car to at least partially offset the energy consumed by the car?

Martin: The only practical place to put panels on the Roadster is the roof (about 1 square meter). Ideally, this would then generate 263 kWh/year. However, the Roadster won’t always be in the sun, and it won’t be at its ideal angle. A 60% de-rating would be generous to account for shade and suboptimal angles, so the panel would generate about 150 kWh/year – driving the car an additional 2 miles per day. This is not even a 1% increase in driving range!

Elon: Although the amount of energy that could be recaptured from the top of the Roadster is small, I originally pushed very hard to have this option available. Martin argued that such panels would only be decoration.

Martin: Fins on a ’59 Caddy

Elon: … Thank you Martin …But his real reason was that he needed to keep his engineering resources focused on completing the Tesla Roadster itself.

Martin: We may one day offer a solar-roof option for the Roadster. Perhaps an aftermarket company will beat us to it. In the mean time, we will partner with solar providers like Solar City to offer rooftop-based solar options for Tesla owners. We’d like to see a modular solar carport as well.

Question: Why don’t you put solar panels in front of the headlights to charge the batteries while the lights are on?

Editor: Conservation of Energy.