California has unique authority under Section 209 of the Clean Air Act  to issue vehicle emission standards that are stricter than the federal vehicle standards . Other states can adopt California’s standards at their discretion, but cannot adopt their own standards under Section 177 of the Clean Air Act . (Here’s a brief history of vehicle emission standards  in California and at the federal level.)
Nine states are following California’s lead in requiring automakers to produce zero emission vehicles (ZEVs) to improve local air quality and reduce the emissions contributing to climate change. As part of its larger Advanced Clean Cars Program , California’s ZEV program requires major manufacturers of passenger cars and light trucks (up to 8,500 pounds) to attain a certain number of ZEV credits depending on the number of vehicles produced and delivered for sale in the state (see Table 1).
A ZEV is any vehicle that releases zero emissions during operation, such as a battery electric or hydrogen fuel cell vehicle. Other vehicle types can also earn credit toward the standard as indicated in Table 2.
Table 1: ZEV Credit Calculation
Option 1: Historical production volume
Option 2: Current production volume
Compliance Year 2017 or earlier (2017 as an example)
Average of 2011-2013 model year production
2017 model year production
Compliance Year 2018 or later (2018 as an example
Average of 2014-2016 model year production
2018 model year production
The program includes mechanisms to make compliance easier, such as tradable credits and different credit levels depending on the vehicle technology and capability. A single ZEV credit is not equivalent to one vehicle, as described in the table below. For example, a manufacturer that sells 100,000 cars in California had a combined ZEV and TZEV obligation in the state of 3 percent in 2012, equivalent to 3,000 credits. According to the 2012 minimum ZEV requirement of 0.79 percent, it had to generate at least 790 credits from ZEVs, and it could have generated the remaining 2.21 percent (2,210 credits) from a mix of other vehicle technologies, including ZEVs, TZEVs, Type 0 ZEVs, and NEVs. Full details of the program, including how ZEV credit requirements and allocations are calculated, are available at California’s ZEV Program website .
Table 2: ZEV Program Vehicles
This table is a high-level summary of the different vehicle classifications. More information about what constitutes a particular vehicle type and its credit level is available on California’s ZEV Program website .
Zero Emission Vehicle
Battery electric vehicle, hydrogen fuel cell vehicle
Nissan LEAF, Honda Clarity
1-9 depending on range
1-4 depending on range
Transitional Zero Emission Vehicle
Plug-in hybrid or extended range electric vehicle, hydrogen internal combustion engine vehicle
1-3 depending on technology
0.4-1.3 depending on range
Neighborhood Electric Vehicle
Short range, low-speed battery electric vehicle
Partial Zero Emission Vehicle
Advanced Technology Partial Zero Emission Vehicle
Natural gas vehicle, hybrid electric vehicle
Honda Civic GX, Toyota Prius
>0.2-3 depending on technology
Figure 1: ZEV Program Model Years 2012-2025 and Beyond
Each manufacturer must acquire ZEV credits to cover a set percentage of vehicles produced and delivered for sale in the state, as calculated by the California Air Resources Board. Note that credits are not equivalent to a single vehicle as noted in Table 2, so it is difficult to estimate the number of ZEVs or other qualifying vehicles that an auto manufacturer will produce for sale to comply with the rule. The “ZEV Floor” category refers to the minimum percentage of credits that must be for ZEVs. For all years, ZEV credits can be used as substitutes for any other categories. For model years 2012-2017 only: the AT PZEV and PZEV categories are the maximum amount of credits allowed to count toward compliance, TZEV credits can be substituted for AT PZEVs and PZEVs, and credits can also include Type 0 ZEVs, which are zero emission vehicles with less than 50 miles of range but allowed on highways (i.e., not an NEV). A manufacturer may count NEV credits towards its TZEV obligation without limit through 2017, and toward a quarter of its TZEV obligation in 2018 and beyond. Type 0 ZEVs are not eligible for credits beyond model year 2017.
As of June 2013, nine additional states have adopted California’s ZEV program. They are: Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Rhode Island, and Vermont. Other states are able to adopt the program at their discretion. Three other states – Washington, Delaware, and Pennsylvania – are following California’s Low Emission Vehicle (LEV) program , but not the ZEV program at present.