Question: How many solar panels do I need to power my Tesla Roadster?I didn't realize that you only needed 10 square meters of solar cells to allow for 12,000 miles of driving a year. That seems very doable. If we were to power cars with cellulosic ethanol instead it would require 32 times more land.
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.
Question: Doesn’t it take more energy to produce a solar panel than that panel will ever produce in its serviceable life?Another way to think about this is that about 10% of the energy generated by the solar cells over their lifetime is needed to produce them. So, you need an extra 10% of solar cells to take care of the production energy. Instead of 10 square meters to power your car, you really need 11 to take into account the energy needed to produce the solar cells.
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.
via Tesla Motors Blog