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 2024/25 here.
PY25 Annual Residual Supply Mix
The annual RSM has now been calculated for PY25 (1 April 2024 to 31 March 2025). This was the sixth 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 – 2024/25.
| NZECS Production Year | PY20 | PY21 | PY22 | PY23 | PY24 | PY25 |
| National Supply Mix (MWh) | 42,273,518 | 41,490,913 | 41,881,730 | 41,519,448 | 41,862,459 | 41,649,794 |
| NZ-ECs Redeemed (MWh) | 51,577 | 228,741 | 734,700 | 1,265,475 | 1,785,921 | 2,159,123 |
| Residual Supply Mix (MWh) | 42,221,94 | 41,262,17 | 41,147,030 | 40,253,973 | 40,076,53 | 39,490,67 |
| NZ-ECs Redeemed (% of NSM) | 0.12 | 0.55 | 1.75 | 3.05 | 4.27 | 5.18 |
| National Supply Factor (kg CO2-e/MWh) | 99.16 | 124.00 | 108.83 | 67.50 | 74.63 | 107.59 |
| Residual Supply Factor (kg CO2-e/MWh) | 99.28 | 124.69 | 110.77 | 69.62 | 77.95 | 113.47 |
| Factor Difference (%) | 0.12 | 0.56 | 1.77 | 3.14 | 4.46 | 5.47 |
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.
For PY25, 5.2% of total generation in New Zealand was attributed to NZ-ECs. The period saw relatively high generation from non-renewable sources, with the NSF higher than in the previous two periods. Coal and diesel generation were higher in PY25, making up 4.5% of total electricity generation, compared to 1.4% of total generation in PY24. As a result, the NSF was over 40% higher (74.63 kg CO2-e/MWh vs. 107.59 kg CO2-e/MWh). The increase in purchased NZ-ECs further intensified the RSF, up to 113.47 kg CO2-e/MWh.
The graph below shows electricity generation by type for each of the last six 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, BraveTrace has trialled and published a pilot monthly RSM, illustrated below. The monthly RSM is currently calculated using the same approach as the annual RSM. The rolling 12-monthly average RSF offers a flexible option for parties who wish to report on a period that does not align with the NZECS period. The monthly RSM values for PY25 are now final as the Production Year has closed
Values are updated throughout the currently open Production Year. RSM values within an active Production Year are subject to change, and are finalised at the end of the year. Read our blog article on the intent behind the monthly RSM.
Further development of Residual Supply Mix data is planned, and feedback from stakeholders and system users is welcomed.
*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.
