New research is a sharp reminder that if New Zealand wants a stable, affordable, 100% renewable grid, reducing demand is just as critical as increasing supply. The choices we make—about the efficiency of the cars we buy and, even more importantly, the performance of the buildings we live and work in—are all connected.
While New Zealand’s uptake of electric vehicles obviously means more demand on the national grid, a lot of the discussion has focused on managing the daily peaks. There’s talk of how to shift charging patterns, so cars are charged overnight instead of at 5:30pm when people arrive home from work, right in the middle of the evening demand peak. Smart chargers and time-of-use tariffs are among the tools that can bring about that behaviour change.
However, an excellent research paper just released from University of Otago highlights a much bigger problem that is harder to solve and that’s seasonal variation. EV charging demand is significantly higher in winter. Batteries are less efficient at lower temperatures; range reduces for the same amount of energy input as energy is used to keep the battery pack at an optimal operating temperature. In addition, unlike a fossil fuelled car that uses waste heat to keep the interior toasty warm, an EV powers its heaters directly from the battery. EV drivers are quick to learn that running the heater or air conditioning has an immediate impact on range.
The research suggests that electricity consumption for vehicle charging could be up to 16% higher in winter in colder parts of the country. This could increase a region’s total monthly electricity consumption by up to 30% by 2050. That’s a massive lump of extra demand to plan for, during the winter period of peak demand from residential heating, which is already straining New Zealand’s grid.
Figure from Estimating the seasonal variation in electricity demand of future electric vehicle fleets, Michael Jack et al. Titles of individual graphs identify location – lower, upper or central areas of the North or South Island respectively.
It’s another powerful argument for why we have to look at the energy system as a whole. We can’t just focus on generating more and more renewable electricity to meet ever-increasing demand. We have to get serious about reducing our existing loads.
This is where Passive House comes in. Every new home built to the Passive House standard drastically cuts its heating demand, flattening that residential winter peak. Every deep energy retrofit does the same. By making our buildings radically more efficient (as well as healthier and more comfortable), we free up huge amounts of grid capacity. That can then be used to power the electrification of the transport sector without having to build expensive new generating capacity that will only be needed for a few months of the year. Hydro-electricity may be considered “green” but the embodied carbon emissions (as well as environmental impacts) of building new hydro-electric dams are enormous.
An important factor to consider when purchasing an EV
The study is based on a fantastic real-world dataset of over a thousand EVs in the Flip the Fleet citizen science project. Huge performance differences were found between vehicle models.
The amount of energy a car uses per kilometre (Wh/km) is its energy intensity. This varies with driving style and terrain and a bunch of other factors, plus some models are just inherently more efficient. Energy intensity also varies significantly with the outside temperature: some cars are a lot more sensitive to the cold than others.
Running climate control is resource hungry for EVs. Some models use heat pumps for heating and cooling and this paper highlights the impact on efficiency. The authors compared an older Nissan Leaf model with a basic resistive heater to a newer version with a more efficient heat pump. A heat pump saw the car’s energy demand increase by 15% in winter compared to summer. Using resistive heating saw a jump in demand of 25%.
That’s a huge difference and highlights a very practical takeaway: if you live in the bottom half of the South Island, make sure to buy an EV with a heat pump! Some detailed research may be required as even within a single make and model, the year of manufacture, trim level and country of origin if imported second hand may determine whether a specific car has a heat pump or resistance heat source.
Abstract and reference
Estimating the seasonal variation in electricity demand of future electric vehicle fleets
Large-scale uptake of electric vehicles is crucial for reducing emissions but poses a challenge for highly-renewable electricity grids. While daily charging demand can be managed through storage and demand shifting, seasonal variation is more difficult to resolve. This study develops a statistical model of monthly vehicle energy intensity (Wh/km) using a citizen-science dataset of over 1000 New Zealand EVs. The model, which considers vehicle model, heating and cooling degree days, and location, is used to predict seasonal variation in EV electricity demand. The findings show that charging consumption can vary seasonally by up to 16% in some regions and increase winter monthly consumption by up to 30%. These results highlight the importance of considering vehicle efficiency and climate in future electricity infrastructure planning, noting that the largest uncertainty in projections comes from the unknown energy intensity of the future EV fleet.
Reference: Jack, M. W., Paulsen, P., Xiao, X., Parker, R., & Myall, D. (2025). Estimating the seasonal variation in electricity demand of future electric vehicle fleets. Energy, 333, 137089. https://doi.org/10.1016/j.energy.2025.137089