Kevin wants to use less fossil fuel by purchasing an electric vehicle. EV’s are currently about 50% efficient. The grid is about 20% efficient and Kevin’s EV is about 50% efficient. What is the total efficiency of this process?

Question

Kevin wants to use less fossil fuel by purchasing an electric vehicle.  EV’s are currently about 50% efficient. The grid is about 20% efficient and Kevin’s EV is about 50% efficient.  What is the total efficiency of this process?

BlackTom AI · Accepted Answer

Efficiencies in a chain of energy conversions multiply, so the total (overall) efficiency equals the product of each stage's fractional efficiency.

Relevant concepts: Efficiency is the fraction of input energy that becomes useful output energy. When energy passes through sequential stages (here: the grid generation/transmission stage, then the vehicle drivetrain), each stage wastes a portion; the remaining usable fraction from the first stage is the input to the second, so you multiply the fractional efficiencies.

Step-by-step calculation:
1. Convert percentages to decimals:
   - Grid efficiency = 20% = 0.20
   - EV drivetrain efficiency = 50% = 0.50
2. Multiply the fractional efficiencies:
   - Total efficiency (decimal) = 0.20 × 0.50 = 0.10
3. Convert back to percent:
   - 0.10 = 10%

Intermediate arithmetic shown: 20% × 50% → (20/100) × (50/100) = 0.20 × 0.50 = 0.10 → 10%.

Why this is correct: The grid produces or transmits only 20% of the original primary energy as usable electrical energy; of that usable electricity, the EV converts only 50% into motive work. So only half of the 20% survives both stages, i.e., 10% of the original primary energy becomes vehicle propulsion.

Why other approaches would be wrong: Adding the efficiencies (20% + 50% = 70%) incorrectly treats stages as parallel gains rather than sequential conversion; that overestimates usable energy. Dividing or averaging has no physical basis here. The correct physical rule is multiplication for sequential conversion stages.

Conclusion: The combined efficiency of the grid (20%) followed by the EV drivetrain (50%) is 10%, which matches the provided answer.

Kevin wants to use less fossil fuel by purchasing an electric vehicle. EV’s are currently about 50% efficient. The grid is about 20% efficient and Kevin’s EV is about 50% efficient. What is the total efficiency of this process?单项选择题

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