Residual Supply Mix for electricity certification
At BraveTrace, we strive to increase public understanding of the attributes and characteristics of energy purchased and consumed in New Zealand.
To this end, we ensure that energy purchases tracked through our system are not also claimed by non-participating energy users. The mechanism by which we do this is called the Residual Supply Mix (RSM).
The RSM describes the nature of the electricity received by energy consumers who do not purchase New Zealand Energy Certificates (NZ-ECs) directly from suppliers. It is a key part of how greenhouse gas emissions are accounted for in New Zealand. It is important that all explicit transfers of energy attributes are tracked on the RSM to ensure no double-counting occurs. BraveTrace seeks to include all contractual market-based instruments into the RSM so that non-certifying parties face more carbon intensive emissions factors.
Parties wishing to report on the emissions intensity of consumed electricity can now rely on either the RSM, or the specific information contained within NZ-ECs provided to them by their supplier. These are both market-based emissions factors, in line with requirements for dual reporting as laid out by the Greenhouse Gas Protocol.
It is important to utilise the Residual Supply Factor (RSF), in order to recognise the growing amount of explicit energy attribute purchase taking place in the New Zealand electricity market.
Interested in the detail? Download our Annual RSM Publication for 2023/24 here.
PY24 Annual Residual Supply Mix
The annual RSM has now been calculated for PY24 (1 April 2023 to 31 March 2024). This was the fifth year in which NZ-ECs were transacted in New Zealand, and the difference of the RSM from the National Supply Mix (NSM) continues to grow in line with growth in the number of transacted NZ-ECs. The table below compares the RSM figures between the Production Years 2019/20 – 2023/24.
| Production Year | 2019/20 | 2020/21 | 2021/22 | 2022/23 | 2023/24 |
| National electricity generated (MWh) | 42,273,518 | 41,490,913 | 41,881,730 | 41,519,448 | 41,862,459 |
| Total volume NZ-ECs transacted | 51,577 | 228,741 | 734,700 | 1,265,475 | 1,785,921 |
| National Supply Factor (kg co2-e/MWh) | 99.16 | 124.00 | 108.83 | 67.50 | 74.63 |
| Residual Supply Factor (kg co2-e/MWh) | 99.28 | 124.69 | 110.77 | 69.62 | 77.95 |
| Difference (%) | 0.12 | 0.56 | 1.77 | 3.14 | 4.46 |
National Supply Mix (NSM): the total mix of electricity generation supplying New Zealand’s domestic demand.
National Supply Factor (NSF): the emissions factor of the National Supply Mix.
Residual Supply Mix (RSM): the mix of electricity generation minus the volume of NZ-ECs redeemed.
Residual supply Factor (RSF): the emissions factor of the Residual Supply Mix.
Electricity emissions for the PY24, although higher than last year, are still down significantly from previous years with a National Supply Factor (NSF) of 74.63 kg/MWh. Strong renewable generation has helped to keep the carbon intensity of the system down compared to previous years. The NSF should be used as a location-based emissions factor, as part of dual reporting requirements.
NZ-ECs increased in popularity again in PY24, resulting in a Residual Supply Factor (RSF) that is 4.46% more carbon intensive than the NSF (up from 3.14% last year), to 77.95 kg co2-e/MWh. The RSF should be used as the market-based emissions factor, as part of dual reporting, and reflects the emissions associated with grid-supplied electricity minus the volume of NZ-ECs redeemed.
PY24 also saw a slight increase in the national electricity generation. This appears to be an increase in demand partly driven by increased irrigation and electrification.
The graph below shows electricity generation by type for each of the last five Production Years.
Annual Residual Supply Factor
*Co-generation is an energy efficient technology in which heat and electricity are by-products of the process. However, its primary functionality is not to produce electricity. Thermal generation uses heat produced by burning coal, diesel or natural gas as fuel to produce electricity. Cogeneration is a subset of thermal generation.
Monthly Residual Supply Mix (update)
In addition to the annual RSM calculation shown above, BraveTrace has trialled the development and publication of a monthly RSM throughout all the Production Years. These monthly figures have now been updated with final end of year NZ-EC transaction volumes for months within previous production years. Monthly figures within PY25 (1 April 2024 – 31 March 2025) are subject to change and will be finalised at the end of the production year.
The interactive chart below shows the RSM for each month starting from 1 April 2020. This chart will be updated each month with the RSM data of the preceding month.
Further development of Residual Supply Mix data is planned, and feedback from stakeholders and system users is welcomed.
Read a blog article on the intent behind the monthly RSM.
*Cogeneration is an energy efficient technology in which heat and electricity are by-products of the process. However, its primary functionality is not to produce electricity. Thermal generation uses heat produced by burning coal, diesel or natural gas as fuel to produce electricity. Cogeneration is a subset of thermal generation.
**Rolling 12 month average values are inclusive of the month of display.
Monthly RSM – calculation methodology
The Monthly RSM is calculated using the same basic approach as the annual RSM – building up a model of supply by adding up generation and emissions data from each of the generation facilities in New Zealand, and adjusting for the volume of certificates redeemed within that month.
This method is highly transparent, and flexible to improve over time, as we will seek to improve individual assumptions in order to improve the accuracy of the output figures.
The calculation of the underlying national supply mix is performed by EnergyLink to our specifications, and incorporates transaction data produced and held within the NZECS Registry. For this reason, these national supply factors will vary slightly from numbers produced by the Ministry of Business, Innovation and Employment (MBIE), due to differences in underlying inputs and assumptions.
A description of the methodology taken is provided in the link below. This document also discusses potential approaches to improving the RSM methodology.
NZECS Residual Supply Mix methodology – approach review and detail (Prepared Dec 2019, current for 2021/22 calculation)
Additionally, further information can be found within the NZECS RSM technical note.
