Tesla Roadster - Energy Efficiency - Petroleum-equivalent Efficiency

Petroleum-equivalent Efficiency

See also: electric car#Comparison with internal combustion engine vehicles (ICEVs)

The Roadster does not actually use gasoline; therefore, petroleum efficiency (MPG, L/100 km) cannot be measured directly but instead is calculated using one of several equivalent methods:

A number comparable to the typical Monroney sticker's "pump-to-wheel" fuel efficiency can be calculated based on regulations from the DOE and its energy content for a U.S. gallon of gasoline of 33,705 W·h⁄gal (also called the Lower Heating Value (LHV) of gasoline): frac{33705,frac{mathrm{W cdot h}}{mathrm{gal_{ge}}}} {135,frac{mathrm{W cdot h}}{mathrm{km}} times frac{1.6, mathrm{km}}{mathrm{mi}}} times 77.6 % {mathrm{_{charging eff.}}}= 120 ,mathrm{mpg_{ge}} = 1.95 frac{mathrm{L_{ge}}}{100, mathrm{km}}

For CAFE regulatory purposes, the DOE's full petroleum-equivalency equation combines the primary energy efficiencies of the USA electric grid and the well-to-pump path with a "fuel content factor" that quantifies the value of conservation, scarcity of fuels, and energy security in the USA. This combination yields a factor of 82,049 W·h⁄gal in the above equation and a regulatory fuel efficiency of 293 mpggeCAFE.

Recharging with electricity from the average USA grid, the factor changes to 12,307 W·h⁄galUS to remove the "fuel content factor" = 1⁄0.15 and the above equation yields a full-cycle energy-equivalency of 44.0 mpgge full-cycle. For full-cycle comparisons, the sticker or "pump-to-wheel" value from a gasoline-fueled vehicle must be multiplied by the fuel's "well-to-pump" efficiency; the DOE regulation specifies a "well-to-pump" efficiency of 83% for gasoline. The Prius' sticker 46 miles per US gallon (5.1 L/100 km; 55 mpg), for example, converts to a full-cycle energy-equivalent of 38.2 mpgfull-cycle.

Recharging with electricity generated by newer, 58% efficiency CCGT power plants, changes the factor to 21,763 W·h⁄gal in the above equation and yields a fuel efficiency of 77.7 mpgge.

Recharging with non-fossil fuel electricity sources such as hydroelectric, solar power, wind or nuclear, the petroleum equivalent efficiency can be even higher as fossil fuel is not directly used in refueling.

Monetary cost offers another way to find an equivalent fuel efficiency. Tesla Motors reports an energy cost of approximately US 1.4¢ per mile when using PG&E's E-9A rate plan (off-peak night-time incentive charging) cost of 5.294¢ per kWh which is available in the two U.S. states covered by PG&E. Comparison with a gasoline price of US$4.00/ U.S. gallon, for instance, results in an equivalent of 270 mpgge using the E-9 rate or 123 mpgge using the USA average residential electricity price of 11.6¢ per kWh. Including the battery replacement cost at its warranty limit, the cost per mile increases and the equivalent miles per gallon are reduced. The time value of money, improving battery technology, and a 6-8% annual reduction in battery cost all lower the net present costs of battery replacement. These factors allow Tesla Motors to sell customers a replacement battery pack for US$12,000, to be installed after the 7-year/70,000 miles (110,000 km) lifespan of the original pack, resulting in a net battery cost of 17.1¢/mile. Adding this to the electricity costs of 1.4¢/mile above results in an overall cost equivalence of $4/gal ÷ 18.5¢/mile = 21.5 mpgge, which can be compared to conventional car ratings after transmission, engine, and other drivetrain replacement costs are factored into the mpg ratings for those cars.

Read more about this topic:  Tesla Roadster, Energy Efficiency

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