ETH study: rooftop home solar panels can supply up to 90 per cent of the electricity for your electric car

Martin Jungfer

28.1.2022

Translation: Veronica Bielawski

According to a study carried out at the ETH, anyone who has a solar panel on their roof can use it to charge their car. It alone can provide almost all the electricity needed to charge your car – and you can drive it as normal.

Registrations of electrically powered vehicles (EVs) are rapidly rising. At the same time, there’s growing concern about the reliability of the electricity supply in Switzerland. The two are only partly related, but are often muddled in public discourse. Especially critics of electric vehicles are susceptible to half-baked arguments. But what if the electricity required came from your very own rooftop photovoltaic power generation – at least for those lucky enough to own their home? What proportion of an EV’s electricity requirements could be covered in this way? A team from ETH Zurich investigated this question and has now published the results of their study.

«Many people overestimate the electricity required for e-mobility,» says Henry Martin, a PhD student involved in the study. And it’s easier than you might think to meet a high proportion of the energy needs yourself for a Tesla, ID.3 and the like. That’s why he considers the expansion of photovoltaics to be the «the right way forward» and believes the incentives could be strengthened. Perhaps the study results will provide further impetus for policymakers.

Martin Raubal, Professor of Geoinformation Engineering at ETH Zurich, and his research team wanted to find out in the study exactly how much of a person’s own solar power can be used to charge the EV without sacrificing comfort or flexibility. Many people seem to fear exactly this – that charging their car with solar power will be limiting. A common question is: «How am I supposed to use my electric vehicle if it has be hooked up to the charger during the day, when the sun is shining?»

The ETH researchers’ finding is an all-clear: power generated from a rooftop photovoltaic installation can cover over 90 per cent of the EV’s electricity consumption – and that’s just by making minimal adjustments to the charging process. This doesn’t even require having a second battery at home to temporarily store excess energy.

EV driver with a virtual solar system

How did the researchers arrive at this result? They based their study on the usage behaviour of 78 people with EVs, who were participants in the «SBB Green Class» project. Here’s how it works: you pay a surcharge for a GA Travelcard, and in return you also get an electric car – including a charging station for your home. ETH received information on when the electric car was being used, when it was left at home unused, when it was being charged during this time and with what power and up to what battery level it was charged. As the authors of the study state, these 78 people are not representative of the whole of Switzerland. The participants in the study are likely to have used their vehicle more frequently and for longer distances than average.

However, the dataset of 78 households allowed for a number of theoretical scenarios to be run. First, a solar system was virtually installed on each participant’s roof. The researchers calculated the potential output based on the size of the roof and weather data at the specific location. In reality, however, the «Green Class» participants charged their EVs using mains electricity.

Next, the researchers superimposed the following data: the times when the electric car was at home and connected to the charging station; and the electricity supplied by the virtual solar system. In this baseline scenario, the solar system generated an average of 15 per cent of the electricity needed to charge the EV. «The value is so low because various factors haven’t been optimised,» explain Professor Raubal and PhD student Henry Martin. The EV was not always plugged into the charging station; or it was being charged at the wrong time, or with too much power at the wrong time.

Smart EV chargers transfer solar power to the car

The second scenario was – like all scenarios in this study – calculated purely theoretically on the basis of the available data. The aim here was to charge in a smarter way, without interfering with the use of the EV. To do this, the researchers relied on so-called smart charging. They simulated that the battery of the electric car should preferably be charged when solar power from the roof is available. This is something almost all EV chargers these days can do. Controlling charging is also easy thanks to simple software solutions like Solarmanager, which was developed in the canton of Aargau.

In the study, the smart charging station increased the percentage of solar power used to charge the EVs from 15 to 56 per cent. «We were surprised by this large proportion,» says Henry Martin. He adds, «Smart charging can significantly increase the individual’s consumption of their own solar power – and the vehicle can be used with the same flexibility as if it were being charged from the mains current.»

A third scenario showed what happens when other parameters change: the researchers set the charging station to allow for partial charging – for example, up to only 60 or 80 per cent instead of 100 per cent – or to match the driver’s actual required range. They also set the EV chargers to not necessarily start charging the car immediately, but to wait, for example, until the sun provided an appropriate amount of energy through the solar system. The result: on average, 90 per cent of the electricity required by the EV in one year came from the solar power system. Here, too, the drivers reportedly experienced no limitations due to solar charging.

It’s also possible to «fuel» EVs with 100 per cent solar power; this would require a home battery to store excess electricity, which would act as a buffer. The ETH researchers, however, are ambivalent about the use of a battery for this purpose. On the one hand, this would allow for a slight increase in the proportion of energy needs covered by private electricity production. On the other hand, the sustainability balance of this charging system as a whole is unclear; according to a press release, the production of home batteries generates relevant quantities of CO₂. And, for most people, electricity storage systems aren’t economically worthwhile. They’re expensive to purchase and install, and the price of electricity is comparatively low – especially in Switzerland.

Great potential thanks to intelligent algorithms

In any case, Professor Raubal sees much more potential in expanding the possibilities offered by smart charging. His department develops machine learning strategies with the aim of predicting solar yield and usage behaviour as accurately as possible. Things like surplus control, which means the car charges only when solar power is available, could be supplemented by further data, Raubal explains. For example, it would be helpful to integrate your personal calendar, he says. Assuming it contains information on when future trips – be it to the office or the mountains – the charging station could charge the car’s battery to suit the time and length of the upcoming trip. Another relevant piece of data is whether there’s an EV charger at the destination.

Raubal is currently conducting research in Singapore. Starting in 2030, only fully electrically powered cars will be sold there. And from 2040 on, only fully electrically powered cars will be on the roads. The EU is giving the industry and buyers a slightly longer grace period; cars with a combustion engine can be sold until 2035. In Switzerland, no end date has been set yet.

But regardless of the specific «end year», the trend seems clear: electrically powered vehicles are taking the sales charts by storm. The study carried out by researchers at ETH shows that the option of having a rooftop solar system offers a good starting point to homeowners who already drive an electric car or who will do so within the next few years. And while it doesn’t allow them to close the electricity gap, it does allow them to contribute to grid security and stability.

Martin Jungfer

Head of Content

Martin.Jungfer@digitecgalaxus.ch

Journalist since 1997. Stopovers in Franconia (or the Franken region), Lake Constance, Obwalden, Nidwalden and Zurich. Father since 2014. Expert in editorial organisation and motivation. Focus on sustainability, home office tools, beautiful things for the home, creative toys and sports equipment. more

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