1 About

1.1 Citation

If you wish to use any of the material from this report please cite as:

  • Ben Anderson (2019) NZ GREEN Grid Household Electricity Demand Data: EECA Data Analysis (Part C) Upscaling Advice Report v1.0_Final, Centre for Sustainability, University of Otago: Dunedin.

This work is (c) 2019 the authors. Usage rights are specified in the License section (1.4).

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2 Introduction

Previous work conducted as Part B of this project has calculated the % contribution of different appliances to peak power load (kW) and to overall annual consumption (kWh) for the ~ 40 New Zealand GREEN Grid household electricity demand study dwellings (Stephenson et al. 2017).

EECA have expressed an interest in understanding how these results and/or this sample of dwellings can be scaled up to provide estimates of the same values for the overall New Zealand residential dwelling stock/household population.

Unfortunately, due to their recruitment method the New Zealand GREEN Grid household electricity demand study sample dwellings are unlikely to be representative of either the two regions from which they were drawn (Taranaki and Hawke’s Bay) or New Zealand as a whole as we discuss below. In addition these households form an extremely small sample so that the resulting confidence intervals and margins of error around derived estimates are large (see Section 3.3 below). In combination, these two issues mean that upscaling the sample to represent either the regions from which they were drawn and/or the national household population will produce estimates that are neither representative nor precise.

If we assume that the GREEN Grid sample adequately represents the regional or national distribution of electricity-using devices and appliances and that the way the occupants of the GREEN Grid dwellings use these devices and appliances represents the usage habits and patterns of all New Zealand households then we could make generalisable inferences. However we can do very little about the lack of precision caused by the small sample size.

Whilst these assumptions may be plausible in the case of, say, lighting, it is likely to be less plausible in the case of space or hot water heating due to the absence of reticulated gas and coal/wood as main energy sources from the GREEN Grid sample. Further, the very small size of the GREEN Grid sample means that unusual patterns may be unrealistically inflated in the upscaling process.

Nevertheless it is still useful to review potential upscaling methods should we wish to make assumptions of this kind or to guide future work when a larger and representative sample of New Zealand households and their energy consumption is available.

The remainder of this report therefore introduces three potential upscaling methods both of which have been used with the GREEN Grid household electricity demand data in different ways and for different purposes. The first is a simple multiplication method used to estimate overall residential electricity consumption for comparison to the GXP level data used in Part B. The second is a simple proportionate upscaling method used to estimate energy demand for heat pumps in New Zealand (Dortans 2019). The third is a more complex re-weighting approach that uses iterative proportional fitting (Simpson and Tranmer 2005; Anderson 2012) to re-weight households on multiple dimensions to match known Census distributions (Anderson 2019).

3 Data

A number of data sources are used in this report.

3.1 Census 2013

This data comprises family, household or dwelling counts (depending on the variables of interest) at area unit level. The data was downloaded from the Stats NZ census table download tool and processed using code developed as part of the EU H2020 Marie Skłodowska-Curie scheme-funded SPATIALEC Global Fellowship hosted by the University of Otago.

3.2 Grid exit point kWh export (GXP)

This data comprises the half-hourly electricity export (kWh) data for each New Zealand Grid Exit Point for 2015 downloaded from the Electricity Authority EMI grid export database. We use this data to provide total electricity consumption levels against which we can compare our various residential consumption estimates.

3.3 NZ GREEN Grid Household Electricity Demand

## Date in ISO8601 format; converting timezone from UTC to "Pacific/Auckland".
## Date in ISO8601 format; converting timezone from UTC to "Pacific/Auckland".

This sample comprises 44 households who were recruited via the local electricity distribution businesses (EDBs) in two areas: Taranaki starting in May 2014 and Hawkes Bay starting in November 2014 (Anderson and Eyers 2018).

Recruitment was via a non-random sampling method and a number of households were intentionally selected for their ‘complex’ electricity consumption (and embedded generation) patterns and appliances (Giraldo Ocampo 2015; Stephenson et al. 2017; Jack et al. 2018; Suomalainen et al. 2019).

The EDBs invited their own employees and those of other local companies to participate in the research and ~80 interested potential participants completed short or long forms of the Energy Cultures 2 household survey (Wooliscroft 2015). Households were then selected from this pool by the project team based on selection criteria relevant to the GREEN Grid project. These included:

  • having the majority of their energy supply from electricity (i.e. not gas heating);
  • presence of children;
  • family and dwelling size;
  • types of appliances owned.

As a result of this process the sample cannot be assumed to represent the population of customers (or employees) of any of the companies involved, nor the populations in each location or New Zealand dwellings as a whole (Stephenson et al. 2017).

3.3.1 Annual electricity consumption

Table 3.1 shows the mean annualised net electricity consumption for all dwellings in the data by presence of photovoltaic panel (PV) inverter. For those that do not have PV the mean annual consumption of ~ 8,000 kWh is ~ 14% higher than the ~ 7,000 kWh reported for 2018 (Electricity Authority/Te Mana Hiko 2018). As we discuss below, this is likely to be due to the social composition of these dwelling.

Table 3.1: Summary statistics for annualised net electricity consumption by presence of PV Inverter (NA indicates unknown)
PV Inverter min mean max sd nHouseholds ci_lower ci_upper
NA 6157.09 10329.84 14502.59 5901.16 2 2151.40 18508.28
2298.76 7971.57 16762.36 3262.85 39 6947.53 8995.60
yes -5144.71 945.10 9423.95 6151.52 4 -5083.28 6973.48
Mean annual net electricity consumption by presence of PV inverter

Figure 3.1: Mean annual net electricity consumption by presence of PV inverter

Figure 3.1 plots these values and the width of the 95% confidence intervals illustrates the degree of uncertainty in the mean estimates which is largely driven by the small sample sizes reported in Table 3.1 together with inter-household heterogeneity. It is worth noting however that the ~ 7,000 kWh reported for 2018 (Electricity Authority/Te Mana Hiko 2018) lies just within the lower 95% confidence interval for the dwellings with no PV inverter.

3.3.2 Heating

As an example of the sample bias, Table 3.2 shows the main heat source used for space heating in the GREEN Grid sample. As we can see heat pumps were the main heat source in 55% of dwellings and wood burners in 25% with very little use of gas in any form and no record of coal use.

Table 3.2: Main method of heating, n dwellings = 44
Appliance N %
NA 0 0.00
2 4.55
Enclosed wood burner 11 25.00
HRV or other ventilation system 2 4.55
Heat pump 24 54.55
Open fire 1 2.27
Other 1 2.27
Portable electric heaters 1 2.27
Portable gas heater 1 2.27
Underfloor gas heating 1 2.27

For comparison, Table 3.3 and Figure 3.2 compare the ‘main method of heating’ for the GREEN Grid sample with the results from the BRANZ Housing Conditions Survey 2015 (Vicki White and Mark Jones 2017) where comparable. As we can see, the GREEN Grid sample has a higher incidence of heat pumps compared to the HCS 2015 estimate of 40% heat pumps for owner occupied dwellings (the most similar to the GREEN Grid sample) and 25% for rentals. On the other hand, only 25% of the GREEN Grid sample used enclosed wood burners as a main method of heating compared to HCS 2015 estimates of 39% and 23% for owner-occupied and rental dwellings respectively. Relatively few GREEN Grid dwellings used portable electric and none used portable gas heaters compared to the HCS 2015 sample. These results suggest that the pattern of electricity use for space heating in the GREEN Grid sample is unlikely to be representative of all New Zealand dwellings.

Table 3.3: Main heat source
sample Appliance % Lower 95% CI Upper 95% CI
GREEN Grid sample Enclosed wood burner 25.0 -0.6 50.6
GREEN Grid sample Heat pump 54.5 34.6 74.5
GREEN Grid sample Portable electric heaters 2.3 -26.9 31.5
HCS 2015: owner occupied Heat pump 40.0 33.9 46.1
HCS 2015: owner occupied Enclosed wood burner 39.0 32.9 45.1
HCS 2015: owner occupied Portable electric heaters 25.0 18.9 31.1
HCS 2015: owner occupied Portable gas heater 4.0 -2.1 10.1
HCS 2015: rental Heat pump 25.0 14.2 35.8
HCS 2015: rental Enclosed wood burner 23.0 12.2 33.8
HCS 2015: rental Portable electric heaters 33.0 22.2 43.8
HCS 2015: rental Portable gas heater 15.0 4.2 25.8