Green NCAP has developed a star rating and index system that allows cars’ environmental performance to be compared easily.

Star Rating

This is the highest and simplest level at which cars can be compared. It is a single, overall rating that summarises a vehicle’s performance in Clean Air and Energy Efficiency. The higher the star rating, the better the car has performed. Since the star rating indicates the poorer performance in the two areas of assessment, cars with a high star rating have performed well in both; those with a poor rating may have performed well in one but have done badly in another. This prevents that performance in one area is optimised to the detriment of the other.

Green NCAP How to read the stars

Cars with a high star rating have performed well on both Clean Air and Energy Efficiency level.

Green NCAP’s test protocols are constantly evolving so a car rated in one year may have been tested and scored differently to one in another year. The baseline tests remain the same but, as new technologies emerge and in order to continue to challenge the car manufacturers, additional tests may be added or the thresholds for star ratings made tougher. For this reason, the ratings are marked with the year they were issued. For the same star rating, the newer it is the better. For example, in the future, a car with a 2022 five-star rating will be better than one with a 2019 five-star rating. Care should be taken to ensure that star ratings can be compared across years – check the ‘about the 2019’ rating to see if tests have changed significantly between years.

Small differences in engine tuning can have a significant influence on emissions performance. So, for example, a vehicle may perform very differently in our tests to one with a higher or lower power/torque, even if the main engine block and components are the same. Legislation, which ensures only a minimum level of performance, applies the concept of engine ‘families’ where the result of the ‘worst-case’ engine ensures compliance of other, similar engines. However, for consumer information, where specific information is needed, this concept cannot be applied. For this reason, the star rating applies only to the specific vehicle/variant tested.

5 stars: Overall excellent performance, showing very low fuel or energy consumption and at the same time emitting low pollutants and greenhouse gasses. Well-equipped with emission abatement and fuel saving technology
4 stars: Overall good environmental performance; equipped with good and robust emission abatement and fuel saving technology.
3 stars: Average to good overall performance but equipped with regular emission abatement and fuel saving technology fitted, not outperforming competitors
2 stars: Nominal overall environmental performance lacking some emission abatement and/or fuel saving technology with room for improvement.
1 star: Marginal environmental performance showing that pollutant control and/or energy efficiency is compromised. The environmental performance design mix constituted by minimising pollutants, greenhouse gasses and fuel & energy consumption leaves considerable room for system design improvements.
0 stars: overall environmental performance just meeting the minimum regulatory standards, possibly outdated emission abatement and fuel saving technology.


The star rating indicates how well the car has performed overall. If it has a good star rating, it has performed well both for Clean Air and Energy Efficiency; if its rating is poor then it has performed poorly in one or both of these areas of assessment. The two indexes are the second level at which cars can be compared.

The star rating is based on the area of assessment in which the car has performed most poorly. If it has a good star rating, it has performed well both for Clean Air and Energy Efficiency; if its rating is poor then it has performed poorly in one or both of these areas of assessment. The two indexes are the second level at which cars can be compared.

Clean Air Index

This index shows a score out of ten for the performance of a vehicle in mitigating pollutant emissions. These are gases and particulate matter emitted from the tailpipe which are harmful to human health and to the environment.

A high index indicates good performance (i.e. low emissions). Read more below.

Energy Efficiency Index

This index shows a score out of ten for the efficiency with which energy is converted to propel the vehicle.

A high index shows that little energy is needed per unit distance, indicating an efficient vehicle.
Read more below.

Greenhouse Gases

As well as the Clean Air Index and the Energy Efficiency Index, Green NCAP monitors the emissions of so-called greenhouse gases.

Greenhouse gases absorb reflected solar energy, making the Earth’s atmosphere warmer. A lot of the sun’s energy reaches the ground directly, and a portion is reflected by the ground back into space. Some gases, when present in the atmosphere, trap that reflected energy and redirect it back to Earth as heat. The gases responsible for this are called greenhouse gases, as they play a similar role to the glass covering a greenhouse.

Read more below.

Clean Air Index in detail

Below the four emissions which are currently used to calculate the Clean Air Index.

Unburnt Hydrocarbons


Petrol and diesel fuels are derived from crude oil and contain a wide variety of compounds made up predominantly of carbon and hydrogen atoms, known collectively as hydrocarbons. When fuel is burnt in the engine cylinder some hydrocarbons may not be completely combusted and are emitted from the tailpipe into the atmosphere. This is especially true when the ratio of fuel to air is high (known as enrichment, used by car manufacturers for strategies like catalyst cooling).

Hydrocarbons contribute to the greenhouse effect and global warming and deplete the ozone layer; they increase occurrences of cancer and respiratory disorders and reduce the photosynthetic ability of plants.

Carbon Monoxide


Incomplete combustion results in carbon monoxide being produced when hydrocarbon-based fuels are burnt in an engine. This is most common when there is insufficient oxygen to burn the fuel completely, such as during enrichment.

Breathing air with a high concentration of CO reduces the amount of oxygen that can be transported in the blood stream to critical organs like the heart and brain. At very high levels, which are possible indoors or in other enclosed environments, CO can cause dizziness, confusion, unconsciousness and death.

Nitrogen Oxides

Green NCAP NOx

Nitric oxide (NO) and nitrogen dioxide (NO2) are together referred to as Nitrogen Oxides (NOx). Combustion of fossil fuels is by far the dominant source of NOx emissions. The emissions are not dependent solely on the amount of nitrogen in the fuel but also on the air-fuel mix ratio. High temperatures and oxidation-rich conditions generally favour NOxformation in combustion.

NOx contributes to acid deposition and eutrophication (over-enrichment of the soil with minerals) which, in turn, can lead to potential changes occurring in soil and water quality. The subsequent impacts of acid deposition can be significant, including adverse effects on aquatic ecosystems in rivers and lakes and damage to forests, crops and other vegetation. Eutrophication can lead to severe reductions in water quality with subsequent impacts including decreased biodiversity, changes in species composition and dominance, and toxicity effects. NOx is linked both directly and indirectly to negative effects on human health.



The combustion of heavier hydrocarbon fuels like diesel, and poor combustion under certain load conditions in petrol engines employing gasoline direct injection, results in the formation of particles. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope. Both engine types are now equipped with a particulate filter owing to stricter legal requirements.

The size of particles is directly linked to their potential for causing health problems. Small particles pose the greatest problems, because they can get deep into the lungs, and some may even get into the bloodstream and the brain.

Exposure to such particles can affect the lungs, the heart and the brain. Numerous scientific studies have linked exposure to particle pollution to a variety of problems, including nonfatal heart attacks, aggravated asthma, etc.

People with heart or lung diseases, children, and older adults are the most likely to be affected by particle pollution exposure.

The Clean Air Index uses Particulate Number (PN, as opposed to particulate mass, PM) as its measure of engine performance.

Energy Efficiency Index in detail

Until humankind derives all its energy needs from renewable sources, fossil fuels will continue to be one of the main sources of energy used for transportation: directly so for petrol and diesel cars in the form of oil-derived fuels; but also indirectly for electric and plug-in hybrid vehicles which are charged from the grid. These fossil fuels are a finite resource and are diminishing so it is important that the most efficient use is made of energy, for transportation as well as in other areas.

The energy efficiency of a vehicle does not depend simply on the power unit. Other factors can lead to a loss of efficiency when the car converts its energy source into movement. For example, powertrain losses (those losses that occur in the gearbox and transmission), aerodynamic drag, tyre friction and, most especially, vehicle mass all lead to additional energy having to be used to move the car forward. So, Green NCAP’s Energy Efficiency Index does not indicate the efficiency of the engine or power unit alone but includes all the measures taken by the manufacturer to minimise the energy needed to propel the vehicle.

Green NCAP fuel cars

For an internal combustion engine, the main fuelling types are petrol or diesel. The fuel has a known energy content so, for these vehicles, energy efficiency is equivalent to fuel consumption. However, to allow comparison with other types of power unit (electric, hybrid), a common measure of energy is needed. Using the known calorific values of petrol and diesel, fuel consumption is converted into kWh, the same unit as is used to measure electric energy. For hybrid vehicles, the total energy consumption is derived by adding the fuel used with the electrical energy used. The efficiency is calculated as the energy (in kWh) that is needed to drive a set distance (100 km).

To allow comparison, fossil fuel consumption is converted into kWh.

Green NCAP electric vehicles

Electric vehicles are much more efficient in terms of energy use than combustion engine cars. Using the scale needed to encompass all engine types, electric vehicles always score maximum points in the rating scheme as it currently stands. However, this does not mean that there are not differences in their energy efficiency, but we need to take a closer look at the range of efficiencies in which electric cars operate. To allow comparison, an additional indication is given of the energy efficiencies of electric vehicles, using a more detailed scale. In the very long term, Green NCAP will assess the whole life-cycle of the vehicle, including the energy needed to produce the car, the polluting effects of energy production (for example, in the generation of electricity for the grid) and in the destruction and recycling of the vehicle at the end of its life. When this more complete assessment is done, the benefits of electric vehicles may not be so marked.

In the shorter term, Green NCAP will include driving resistance and driving range into its calculation of the Energy Efficiency Index. Points are already allocated to these in the rating scheme but they are not yet measured so, for now, even an electric car cannot score full marks and is limited to an index of 8.5.

Electric vehicles are efficient in terms of energy use but there can be differences in their energy efficiency.

Green NCAP hybrid vehicles

For hybrid vehicles, the total energy consumption is derived by adding the fuel used with the electrical energy used. The efficiency is calculated as the energy (in kWh) that is needed to drive a set distance (100 km).

In the future, Green NCAP will assess the whole life-cycle of the vehicle.

Greenhouse gases in detail

There are many greenhouse gases but three of the most important are:

Carbon dioxide

Green NCAP CO2

Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon cycle.


Green NCAP CH4

Methane is a Green House Gas with a very high Global Warming Potential. Methane’s lifetime in the atmosphere is much shorter than carbon dioxide (CO2), but CH4 is more efficient at trapping radiation than CO2. Pound for pound, the comparative impact of CH4 is more than 25 times greater than CO2 over a 100-year period. Globally, over 60 percent of total CH4 emissions come from human activities.

Nitrous Oxide

Green NCAP N2O

N2O is a very strong Greenhouse gas with a global warming potential (GWP) around 265–298 times that of CO2 for a 100-year timescale. N2O emitted today remains in the atmosphere for more than 100 years, on average.

Greenhouse gases can be used as a third index in the future.

Currently, the measurement of some greenhouse emissions from cars cannot be done reliably under real-world conditions. Until Green NCAP can make sure it is measuring several of the most important greenhouse gases, and is doing so in a robust way, there is no index for greenhouse gases and these do not currently influence the overall rating. However, it is intended that, with improved measurement techniques these gases can be used as a third index with the same influence on the star rating as the current Clean Air and Energy Efficiency indexes.