Electric car or solar panels: which reduces your carbon footprint the most?

If you want to do something to reduce your carbon footprint, you’ll probably consider both an electric vehicle and installing rooftop solar panels. But which is more effective? With overseas research often inapplicable to New Zealand, one man ran the Aotearoa-specific numbers.

Phil Jones is not your average greenie. He’s worked in the sustainability sector for over a decade, so when he purchased a Nissan Leaf and signed up to lease a solar panel array (3kW with 11kWh battery) he saw an opportunity to research the real-world effect each would have on his family’s carbon footprint.

This meant dutifully keeping tracking of daily power usage in his house and adjusting the family schedule to maximise power savings: setting their water heating – their biggest energy drain – to run during the day and moving other tasks – running the washing machine and charging the car – to times of the day when the solar panels were generating power. Of course, even with the best intentions it was hard to fully maximise the power generated by solar panels. The car was used by his partner to get to work, so could only be charged during the day on weekends and the small battery was only able to hold so much power.

Despite knowing these limitations from the start, Jones was disappointed when he looked at the impact the solar panels had on their energy usage.

“Our 3kW solar panels generated 3,860 kWh. Our household usage averaged 4,556 kWh per year. That looks like a really good match, but there’s a catch – because of system losses and the mismatch between the peak time of generation (summer daytime) and the time of highest demand (winter evenings), we had to import 3,030 kWh (67% of the household load). Only 40% of the solar generated was actually used in the house. Despite the battery storage, we exported 27% of the power generated, with the remaining 33% lost in system inefficiencies.”

This mismatch between when the solar panels were producing energy and the times when his family were using it wouldn’t have mattered as much if they were able to sell energy back to the national grid at a worthwhile rate. This certainly seemed to be a possibility back in 2013 when the panels were first installed, since the buyback rate at the time was 25c per kWh. But when it fell to 8c per KWh the end result was that – rather than saving him money – the panels were costing Jones an extra $441 per year (even at 25c per unit, the shortfall would have been over $200).

This cost might have been justifiable if using the panels meant the household’s carbon footprint was vastly reduced. However, the New Zealand power grid already draws from a large number of renewable energy sources – hydro, geothermal, wind. When Jones took this into account, he found the solar panels had only saved 196 kilograms of carbon (CO2) emissions per year – less than a return flight from Auckland to Christchurch, considering the proportional emissions for a single economy seat using the Climate Care calculator.

Gareth Shute’s son helping to charge the family’s Nissan Leaf (supplied)

What about the electric car? He’d bought a 2011 Nissan Leaf for $14,000 – more expensive than a comparable petrol model, though not by a huge margin (for example a 2011 Toyota Corolla cost around $12,000 at the time). Jones found the difference in price was negated by the savings from just over one year of electric car use.

“The EV travelled 13,375 kilometres per year, about average for a NZ car. The energy used was 2,388 kWh, or 5.6 kilometres per kWh (plug-to-wheels) – a relatively low efficiency for an EV, partly a result of living in the hilly terrain of the Kaipatiki area of Auckland. The cost of that energy was about $513, compared to $2,247 for our old ICE car. So, a net saving of at least $1,734 per year. The comparable price per litre of petrol is approximately 46 cents.”

Even using the most conservative approach, Jones estimates that the net carbon (CO2) saving per year from his electric car use was 2,218 kilograms – slightly less than a one-way trip to London travelling economy class. The actual figure would have been higher, but his figures didn’t take into account that the car was charged overnight when the electricity grid was most likely to be using hydro power. In summer, the overnight charging was often from their home solar panel-charged battery (though Jones excluded the benefit of home solar from his carbon footprint calculations for the car).

The electric car was therefore a clear winner. This matches up with a previous study from 2016 by Wellington-based consultancy firm Concept Consulting. The same piece quotes the minister of energy and resources at the time, Simon Bridges, who said that, “Across their life cycle, from resource extraction and manufacturing to driving and disposal, electric vehicles have 60% fewer CO2 emissions than petrol vehicles.

“In addition, because of New Zealand’s high renewable electricity generation, they have 80% fewer CO2 emissions when driven here. If we start to replace our fleet with electric vehicles, we can begin to significantly reduce New Zealand’s greenhouse gas emissions.”

Electric vehicles charging at a station in Newmarket, Auckland. (supplied)

While Jones’ data showed that electric cars are a cost effective way to reduce one’s carbon footprint, it is worth digging deeper into why solar panels were so ineffective in this particular case.

One obvious issue was how the specifics of Jones’ lifestyle (as part of a three-person family living in Auckland) affected the result. Consumer published a study last year that showed solar set-up (without a battery) for a high-energy-usage household in the Hawke’s Bay would pay for itself in 13-14 years, though this would rely on buying the system upfront rather than leasing it (Consumer did not calculate the carbon emissions for a leasing situation, but one would expect a slight improvement.)

Another complicating factor is the make-up of New Zealand’s electricity grid overall. While there are a large amount of renewables in the mix, gas is still essential for meeting peaks in demand. This is because power usage varies throughout the day, with peaks in the morning (around 7am when people get up) and in the evening (when they return home to cook dinner etc). Unfortunately, these bursts in activity do not match the times when solar panels are most likely to be generating power (in the middle of the day) and unless a home has a large, expensive array of batteries, then it is difficult to cover this leap in demand.

In winter, when rainfall and river flow is high, hydro power does produce a large amount of the power within New Zealand’s electricity grid (since there is a build-up of water above the dams, which can be released to generate power) and even the greater need for home heating in these colder months does little to affect this situation. Instead, it is in late summer, when the hydro lakes are low, when coal – the most emissions-intensive form of energy generation – is most often used.

This graph was created from preliminary research by Phil Jones and shows the estimated C02 footprint of the New Zealand electricity grid across the hours of the day, for each month. The raw electricity usage data was sourced from the Electricity Authority and then a C02 estimate was created using the MBIE emissions data sheet.

This doesn’t mean that solar power has no use in a New Zealand context. While many people assume New Zealand is too temperate for solar, a January 2019 study by Transpower found that our climate matches up favourably to other countries like Germany and France where solar power is heavily used (and sunny areas like Northland and Nelson are particularly promising).

The cost structure is also changing rapidly. The price of solar panels and batteries is falling dramatically, meaning that it will soon be financially viable to store solar-generated energy to cover demand peaks. Carbon pricing also looks likely to make non-renewable energy generation much more expensive, with Transpower predicting that “the cost of energy from gas-fired power stations will be double the price of energy from utility solar within a decade.” What’s more, a month after the report was released, Transpower admitted that the potential of solar was already outstripping their projections.

Nonetheless, solar currently makes the most sense for organisations who use large amounts of power during the daytime. For example, Yealands Wine Group put in the largest solar array in New Zealand in 2016, which provides 30% of their power needs. In contrast, the New Zealand government has decided not to be a leader in this regard, cancelling plans for a solar array on the rooftop of parliament this month.

The solar panels at Yealands vineyard (supplied)

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One of the biggest uses of coal in New Zealand is to create power for our most energy-intensive businesses. Bluescope NZ Steel at Glenbrook and Fonterra are the two biggest users of coal in New Zealand, ahead of even Huntly Power Station (it has occasionally been the second-biggest user in years when the hydro lakes were low, but is gradually phasing out coal). Last year, Fonterra announced a move towards less polluting fuel types, though it won’t phase out coal for many decades. Another business with a huge impact is Tiwai Point aluminium smelter, which is seen as a major obstacle to New Zealand reaching a 100% renewable electricity grid given that it uses roughly 13% of the power created by the grid and needs to run 24/7, 365 days of the year.

The huge carbon footprint of businesses in comparison to individual consumers is often used as a reason why political action is more important than making changes in your personal life – especially given that 100 companies create 71% of the world’s carbon emissions. While we do urgently need more political action, it’s also true that consumer action has some sway. In fact, that 71% figure includes the emissions released by fossil fuels after they were sold onto customers and then released by their personal usage. If consumer demand changes, then so will the business conditions of those companies.

Meanwhile, the era of the electric vehicle is steadily coming upon us. Phil Jones’ Nissan Leaf has already shown its worth and, despite some battery decay, the resale value for this model of car remains high (around $10-11k). There is also research happening into using the power held by electric cars to meet demand within the wider grid, which is one possible solution to the peak-demand problem. Certainly the increasing numbers of electric vehicles will place new demands on the electricity grid and solar will be a part of that solution. Electric vehicles might be coming into their own first, but solar will play an equally important part in New Zealand moving toward a low carbon economy.


The Spinoff’s science content is made possible thanks to the support of The MacDiarmid Institute for Advanced Materials and Nanotechnology, a national institute devoted to scientific research.

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