Metrics

There are several metrics by which we quantify the benefits of a proposed energy technology.

Economics

• Economics is often the deciding factor for many energy decisions.

Central questions

• How much does something cost per unit of use?

• What does your car cost per mile?

• What does your phone cost per minute of use?

• Which is cheaper, buying a more efficient product or paying the extra energy?

• How does this answer change if we add a carbon tax?

Life-cycle assessment

The analysis of all costs, benefits, and impacts over the entire lifetime of a product.

Cost of conserved energy

• How much does it cost to save energy?

• You can purchase energy

• You can purchase a device that "buys" you that energy

Conserved Cost of Energy

• Has dimensions of cost per unit of energy

Simple definition

$CCE = \frac{\textrm{Investment}}{\textrm{Energy Savings}}$

Conserved Cost of Energy

• For long term investments, we do this for a year at a time

$$ CCE = \frac{\textrm{Annual Investment}}{\textrm{Annual Energy Savings}}

Lifetime Energy

• Most energy devices (lights, cars) have both recurring energy costs

  and initial energy costs

• Recurring energy: electricity to emit light or gasoline to drive

• Initial energy: energy to manufacture the bulb or the car

Levelized energy use

• We can estimate the energy per unit of light or mile driven by

• Adding energy over the life of the device

• Dividing the energy by the amount of use over the device

• Dimensions of energy per unit of use (hours light, miles driven)

Energy Return on Investment (EROI)

• $EROI = \frac{E_o}{E_i}$

• This is the ratio of the energy out for a product divided by the energy in to get that energy.

• It should be greater than one to be useful.

Payback Time

• How long will it take for an investment to pay back the initial cost in energy savings?

Further Reading

• Energy For Sustainability