Carbon Monitoring

What do the carbon numbers mean, and how are they calculated?

Live Grid Emissions panel

If you want to understand the carbon costs of your charging, the first place to look is beneath the charger status information on the Evnex app. Here you will find the Live Grid Emissions panel, which tells you information about the electricity currently available from the grid where your charger is connected.

The Live Grid Emissions panel on the charger home page

The Live Grid Emissions panel on the charger home page

% Renewable

The first number shows what percentage of the electricity is being generated from renewable sources (including Hydro, Wind, Geothermal and Solar). The grid in New Zealand obtains much of its energy from renewable sources, with most years averaging around 80% overall. However, in some seasons and at some times of day you will see this significantly higher, perhaps over 95%. Charging at such times can really help to lower the carbon impact of your driving.

CO₂ emitted per kWh

Even more detail is visible in the second figure, which shows how many grams of carbon dioxide are currently being emitted to supply each kWh of energy. The renewable percentage doesn't quite tell the whole story, since different energy sources produce differing amounts of carbon. Coal is the worst, producing 820 g/kWh, with gas significantly cleaner at 490 g/kWh. So the grid may actually be cleaner when there is 85% renewable with the remainder supplied from gas, than it is at 90% renewable with the remainder coming from coal.

In 2020, the most recent year for which the Ministry for the Environment provides guidance, the average emissions were 120 g/kWh, but it is common to see periods of the day where it is under half that amount - typically in the small hours of the morning when the wind is blowing. Again, this would obviously be a great time of day to charge.

Carbon Intensity

Expanding the information panel reveals a little more of the context about the headline numbers. The top element shows how the current rate of emissions compares to the previous 24 hours by placing it between minimum and maximum values over that period. Some days show more variation than others, but if you can keep your charging to times when emissions are in the lowest part of the range you will be making the best use of clean sources.

Expanded Live Grid Emissions panel, showing more detail

Expanded Live Grid Emissions panel, showing more detail

Generation Mix

The final chart shows the full breakdown of energy sources currently providing electricity to the grid. This is where you can find out which non-renewable sources are being used, or see whether there is much generation coming from wind.

Charging Emissions

Of course the live emissions only tell you what's going on now, not what was happening when you charged your car. For that you'll need to go into the Session History screen from the main page, and find the session you are interested in (this could be the current ongoing session).


Every 5 minutes during a charge, we look at the amount of energy that has been consumed from the grid and multiply that by the current carbon intensity to add to the total emissions. In a similar way we record what percentage of that energy came from each generation source, and accumulate these throughout the session. This includes factoring in locally generated solar energy, which we assume to have a carbon intensity of 0 g/kWh.

Carbon emitted per session

In the bottom right of the cell for each session it shows the total amount of carbon emitted in order to provide that energy.

Source of electricity

Tapping a session cell shows various charts related to that session in the top half of the screen. The tab called Gen. Mix shows a stacked chart of the energy sources so you can see what percentage of the electricity came from each different fuel type.

Session History page showing carbon emission details

Session History page showing carbon emission details

Full Emission Summary

In a similar vein, the totals from all of the charging sessions in a select period can be found in the Charging Summary screen. Here you can choose a date range, and see the total carbon cost of all charging within that period, plus a stacked chart showing the proportion of energy from each fuel source.

Charging Summary page showing your carbon emission totals

Charging Summary page showing your carbon emission totals

Data Sources

Our New Zealand grid information comes via the em6 API available from Energy Market Services (EMS). This is one of the commercial arms of Transpower.