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We calculate emissions using the emissions factors from a number of sources, as no single source exists for all generation types.

• The 2019 emissions factor for geothermal, coal and gas is published by MBIE.

• The factor for diesel is unpublished but is based on average diesel emissions over the past 10 years, also supplied by MBIE.

• The emissions factor for co-gen comes from EnergyLink, and excludes Kinleith’s wood fired station.

• Hydro and wind are rated at zero.

The classification of each generation station and unit as to generation type is undertaken by Transpower, based on the information available from generators. More information is available here (PDF).

To get your personalised carbon emissions, we calculate the portion of your usage attributed to each energy source for each half hour period, and we multiply these by relevant emissions factors. These are added together to give the total emissions for each half-hour period, and each half hour is added up to give a daily total.

So, you'll work out my individual carbon emissions from NZ-wide information?

That's right. The electricity market operates nationally - not locally or individually - so we believe that calculating your individual carbon emissions using the average emissions at a national level is most appropriate. Plus, we're all responsible for how much power we use as a country. That's because our electricity supply is influenced by national demand, or how much we're all using at any given time. If demand exceeds the supply of cleaner, renewable electricity, our non-renewable energy sources (like coal and gas) are fired up to meet it, and they produce lots more carbon emissions.

Are there things that might be different for each person, though?

You betcha! Once it's in the national grid, electricity discharges at the first available grid exit point, so if you live close to a renewable generation station, you're more likely to use electricity from that source than any other. Here are a few factors that can have an impact on the generation mix, and might mean your carbon emission profile is different to the national profile:

• Where you live - most of our hydro generation is in the South Island so if you're a Benmore local it's likely that more of the electricity you use comes from hydro generation. When there are constraints in the market, like power outages and maintenance on the HVDC cable (which transports electricity from the South Island to the North Island) it can mean that the power generated down south can't be moved north and will stay, and be used, down south.

• Whether you're near any embedded generation - NZ has generation stations which are embedded within local networks rather than connected to the National Grid. If there is embedded generation near your property then the electricity you use is more likely to be generated there.

• How much power is lost in the lines on its way to your place - we calculate your emissions using the volume of electricity recorded at your meter. To supply you that electricity, the generators have to generate slightly more electricity than what's needed, because some is lost in the process of getting power from sites of generation, through the National Grid and into the local distribution network. It's usually around 7% that's lost, though this can vary depending on where you live.

It sure is, which is why we say that emissions are ‘relatively’ low. We acknowledge that to have security of electricity supply we have to generate electricity from a range of sources, and some of those will create emissions. But we are better to move our demand to times when renewables are more bountiful, and avoid using power at times when high emissions sources like coal and diesel are in the mix.

At the moment the thresholds are set at 350 CO2e for low, 450 CO2e for medium and 550 CO2e for high, but we don’t have a lot of historical data to base these thresholds on, so we might shift them over time, or even seasonally. Our goal is to give users a realistic view of when emissions are at their worst and we should be working together to reduce our demand, and times when emissions are relatively low so we have as small an impact as possible when we use electricity.

The current mix of generation in the grid comes from Energy Market Services – a service provided by Transpower.

Current emissions shown are only generation emissions, and exclude embedded emissions.

We calculate emissions using the emissions factors from a number of sources as no single source exists for all generation types. The 2019 emissions factor for geothermal, coal and gas is published by MBIE. The factor for diesel is unpublished but is based on average diesel emissions over the past 10 years. The emissions factor for co-gen comes from EnergyLink, and excludes Kinleith’s wood fired station. Hydro and wind are rated at zero.

The spot price of electricity is largely determined by the market dynamics of supply and demand. Lots of things affect supply and demand, including the availability of different types of generation.

For example, when there is abundant supply – there is lots of wind, which can't be stored so has to go straight into the grid; or bountiful stores of water at the hydro stations - prices tend to be at their lowest.

On the other hand, when there is heavy demand that can't be met from renewables, generators run coal and occasionally diesel. They're expensive to run so the market price goes up.