RMI Outlet

Plug Into New Ideas

In this first-of-its-kind study for the Colorado Energy Office, RMI performed a comparative analysis of three proposed tariffs that are specifically designed to meet the needs of the unique type of load presented by DC fast chargers (DCFCs) for electric vehicles. The objective of the study was to understand the costs that the chargers might impose on operators of public high-speed EV-charging networks in Colorado, and the utility revenues that will result from them.

The key criterion by which we judged the three tariffs was the cost per mile of range delivered to the vehicle, assuming an energy efficiency of 3.4 miles per kilowatt-hour (kWh).

This project modeled the cost of service for DCFC charging stations over a period of 10 years using the following parameters:

Three tariffs:

  • One proposed by Xcel Energy that features a low fixed monthly charge, lower demand charges than the existing tariff most DCFC stations are currently on in their service area and added charges for energy consumed during “critical peak pricing” (CPP) periods.
  • One from PG&E that eliminates demand charges, offers a three-tier time-of-use (ToU) pricing regime for energy, and requires the customer to select a high fixed monthly “subscription” charge based on their expected consumption.
  • An innovative design developed by RMI that includes Xcel’s fixed charge, offers a two-tier ToU regime for energy pricing, and uses a sliding-scale approach under which volumetric rates for energy decrease and demand charges increase over time as a function of the utilization rate.

Three load profiles:

  • A public DCFC charging depot with two dual-port 50 kW chargers
  • A public DCFC charging depot with two dual-port 150 kW chargers
  • A transit bus depot with twenty-five 100 kW chargers

Three utilization rates on public DCFCs:

  • 5%, which is representative of many DCFCs today
  • 10%, which is representative of the utilization rates that a public DCFC might experience in a maturing EV market within five years or so
  • 30%, which is representative of the utilization rates that a public DCFC might experience in a mature EV market

Our analysis shows that the RMI sliding-scale tariff design results in the most consistent and predictable cost per mile for all utilization rates of both 50 kW and 150 kW public DCFC charging stations. The RMI tariff also results in the lowest cost of energy at the 5% utilization rate, which, of the utilization rates we modeled, is the closest to typical real-world experience. This is vitally important, because while EV adoption is still in its early days and utilization rates on public DCFCs are low, costs must be low enough to encourage charging station operators to continue to deploy more public charging stations. Even while delivering the lowest cost of energy, the RMI tariff is designed to generate the same revenue as Xcel would receive under its own tariff design over the 10-year modeling period.

For the transit bus depot, our analysis shows that the average cost per mile is lowest under Xcel’s S-EV tariff for both charging scenarios we analyzed. Thus, we recommend that tariff for large, stable loads, such as those of a transit bus depot or other similar fleet-charging application.

For public DCFC operators, we believe that the RMI tariff strikes the best balance of the three tariffs we analyzed. By varying charges as a function of utilization, the RMI tariff makes it possible to satisfy three key objectives simultaneously: creating an attractive business opportunity for charging network operators, keeping the cost of charging at or below the equivalent cost of refueling a conventional gasoline or diesel vehicle, and permitting an appropriate level of cost recovery for the host utility.