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 and Greenhouse Gas emissions . The higher the star rating, the better the car has performed. The star rating indicates the weighted average performance across all three areas of assessment.  The star rating for plug-in hybrid electric vehicles (PHEVs) takes account of the pollutant emissions, energy efficiency and greenhouse gases in two modes of driving: primarily using the combustion engine (so-called ‘charge sustaining’ mode) and when the battery is the main source of power (called ‘charge-depleting’ mode).  These modes are combined in a way that accounts for how the car is expected to be used in practice, based on the electric range it offers: the higher the electric range, the smaller the proportion of time it will use its combustion engine and the greater the environmental benefit. Currently, all three indexes are rated equally, signifying equal importance to health, which is affected by pollutant emissions; consumer spending, influenced by a vehicle’s energy efficiency; and the Earth’s future, which is affected by greenhouse gases. Poor performance in one part of the assessment will lower the average and result in a lower star rating.

Currently, all three indexes are rated equally, signifying equal importance to health, which is affected by pollutant emissions; consumer spending, influenced by a vehicle’s energy efficiency; and the Earth’s future, which is affected by greenhouse gases. Poor performance in one part of the assessment will lower the average and result in a lower star rating.

Green NCAP How to read the stars

Cars with a high star rating have performed well in all three assessment areas for Clean Air, Energy Efficiency and Greenhouse Gases.

Green NCAP’s test procedures 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 2020 five-star rating. Care should be taken to ensure that star ratings can be compared across years – check the ‘about the 2020’ 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.

Half stars have been added to the rating scheme from 2020 onwards. Half stars help recognise where one vehicle has performed slightly better than another and is used to partially reward and recognise those manufacturers that have made an effort to produce a greener vehicle.


The star rating indicates how well the car has performed overall. If it has a good star rating, it has performed well for Clean Air, Energy Efficiency and Greenhouse Gases. Poor performance in any one of the areas of assessment will decrease a car’s star rating. The three indexes are the second level at which cars can be compared.

For plug-in hybrid electric vehicles (PHEVs), the index in each area of assessment (Clean Air, Energy Efficiency and Greenhouse Gases) is itself calculated from two sub-indexes, derived from the performance of the vehicle in the two modes of driving that such cars are capable of: charge-sustaining where, in simple terms, the car is being driven by its combustion engine and the battery is being charged; and charge-depleting, where the primary source of power is the battery, although the engine may be used to augment the power or to heat the passenger cabin, for example.  The two sub-indexes are combined in a way which takes account of the way car is expected to be used: for cars with a small electric range (25 km or less), the results are biased towards the charge-sustaining mode; for those with a big range (100 km or more), results are biased towards the charge-depleting mode; between these ranges, a sliding scale is used to calculate the weighting towards one mode or the other.

For PHEVs it is essential that the battery is charged as fully and as frequently as possible.

For PHEVs, it is essential that the battery is charged as fully and as frequently as possible so that the car can be driven in electric mode as much as possible.  Green NCAP’s rating assumes that this is how the car will be used and the rating is not valid if the battery is not routinely charged.  For more information on this, and for other advice on driving in an environmentally friendly way, see our eco driving guide.

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 Gas Index

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

Unburnt Non-Methane 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).

Methane (CH4) in particular, contributes to the greenhouse effect, global warming and depletes the ozone layer. Its emissions are used to score the Greenhouse Gas Index.  Other unburnt hydrocarbons increase occurrences of cancer and respiratory disorders and reduce the photosynthetic ability of plants and are included here as pollutant emissions.

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 NOx formation 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.


Ammonia is a compound of nitrogen and hydrogen and is formed in the emission control systems of gasoline and diesel vehicles. Ammonia emissions can be a threat to urban air quality.

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.



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 diesel and nearly all new petrol engine types are now equipped with a particulate filter due to stricter legal requirements.

Diesel particulate filters (DPF) are very effective at trapping the particles that would otherwise be emitted into the surrounding air. However, to work properly, they need to be ‘unblocked’ every so often. This is done by a process called regeneration, and typically happens every few hundred kilometres. Under certain conditions, the engine runs at a sufficiently high temperature to burn the particles in the DPF, leaving only a small amount of ash. If the car senses that the DPF is blocked but conditions are not right to regenerate, it may artificially raise the DPF temperature by injecting additional fuel into the system. This elevates the emissions of certain pollutants but only very temporarily and, averaged over typical use of the vehicle, regeneration contributes only a tiny amount to the overall emissions, and is more than offset by the benefits the filter brings.

If regeneration occurs during one of Green NCAP’s tests, that test is deemed invalid and is not included in the assessment of the vehicle.

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, heart and 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.

Currently, the electrical energy used is measured by the amount that is needed to fully charge the battery, and not directly from what the vehicle uses.  This takes into account charging/discharging losses and better reflects the energy the consumer will have to pay for. Green NCAP expects, in the near future, to include driving resistance and driving range into its calculation of the Energy Efficiency Index with these included, electric vehicles may not be able to score the maximum Energy Efficiency points.

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).

CNG (compressed natural gas) vehicles can be either mono-fuel or bi-fuel.  Mono-fuel vehicles may have a small petrol tank for use in emergencies, but they run almost exclusively on CNG and the mode cannot be selected by the driver.  Bi-fuel vehicles may typically have a similar range on petrol and CNG and can be switched between the two fuels.  Mono-fuel vehicles are tested in CNG mode only and energy consumption is derived from the measured consumption (in kg/100 km) and the known calorific content of the fuel.

For Hydrogen fuel cell vehicles energy consumption is derived by the amount of hydrogen used. The hydrogen consumption is given in kg/100 km and 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 Gas Index in detail

Greenhouse Gases

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.

Nitrous Oxide

Green NCAP N2O

Nitrous Oxide 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.


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.