Sounds tempting: supplying yourself with electricity, including the electric car parked in the garage, with solar energy generated on your own house roof. This is not only sustainable and thus good for the ecological conscience. Because if you also charge your electric car with the energy of the sun in addition to the consumption of your house, you increase your own consumption and thus the economic efficiency of your photovoltaic system (PV system), as a small calculation example illustrates: A PV system with an output of 5 kWp can supply up to 5000 kWh of electricity annually. An average electric car with an energy consumption of 17 kWh per 100 kilometres could cover almost 30,000 kilometres with this amount, a good 2500 kilometres per month.
At the same time, according to a study by the Fraunhofer Institute for Solar Energy Systems, the savings for each kilowatt-hour of electricity consumed by the car itself compared to the usual tariffs is currently about 20 cents – even slightly more in the sunny south and slightly less in the not-so-sunny north. Roughly speaking, if you consume all of the 5,000 kWh of electricity yourself, perhaps with the help of an additional stationary storage unit, the purchase of which can also be worthwhile according to the study, you save a good 1,000 euros per year and thus also shorten the time it takes for the investment in the solar modules on the roof to pay for itself.
As a rule, even without the additional charging of an electric car with PV electricity, it only takes about eight to twelve years for the cost balance of a solar system to become positive. From then on, the PV system only pays for itself, and for quite a long time: the general service life of a modern PV system is a good 25 to 30 years. Some manufacturers even give a 30-year guarantee on the durability and performance of the PV system.
Some people may wonder whether the amount of electricity they generate themselves is enough to power their house and car. Here, too, a brief calculation example: the average commuter covers between 20 and 40 kilometres a day and needs only about 3.5 to 7 kWh of electricity to do so in his electric car, which consumes 17 kWh per 100 kilometres. Calculated over 200 working days, the electric car requires about 700 to 1400 kWh. The average electricity consumption of a single-family house with 4 people is about 4000 kWh per year. Depending on the mileage of the electric car, that could be tight with a 5 kWp system. If so, simply add one or two more kWp to the roof.
A field report published just a few weeks ago on the tech portal t3n confirms that two electric cars in combination with a photovoltaic system including electricity storage is also worthwhile. With a 10 kWp photovoltaic system and an 8 kWh battery storage unit, the author achieves an annual degree of self-sufficiency of 81 per cent, saves electricity costs of a good 1700 euros per year and assumes that the investment will have paid for itself within about nine to 13 years.
How PV surplus charging works with an electric car
There are three ways to transfer surplus electricity, i.e. electricity not currently needed by the household, to the electric car.
- In the case of PV charging without controlling the wall box, the electricity flows directly into the electric car at a predefined power level. This variant, which does not take real-time information from the PV system into account, increases self-consumption, but not optimally.
- With PV charging via an enable signal, the electric car only charges when the solar system has reached a predefined surplus value. If the power generation falls below the value again, the charging process is automatically paused until the value is reached again. It is true that the PV’s own share can be increased more in this way. But it can be even better.
- The best value for self-consumption can be achieved by PV charging with dynamic control. In this case, the domestic power plant permanently adjusts the charging current so that the best possible value for self-consumption is achieved. Such a solution initially costs somewhat more than the two previously mentioned variants. However, these costs are offset thanks to the more efficient utilisation of the PV system.
Technological developments turn the electric car into an intermediate storage unit
In a few years’ time, as is currently becoming apparent, electric cars will not only be passive power receivers, but also active energy providers. Currently, electricity only flows in one direction: from the PV system to the wall box and then into the electric car, ideally controlled by a charging management system for the most efficient use of the self-generated energy. Since in many countries the government subsidises the purchase of a privately used wall box, the installation of a charging option at home is all the more worthwhile.
Bi-directional charging stations and the regulatory requirements for using an electric car as a temporary storage unit are currently still being developed or agreed upon. However, the first pilot projects have already been very promising – industry experts expect the first products and solutions for end customers in two to four years. Then the electric car in the garage can not only improve self-consumption, but also provide the household with energy when the sun is not shining or the PV system cannot deliver enough power.
In addition, the e-car battery can also stabilize the power grid by being integrated into the balancing energy market as part of a huge swarm storage consisting of many electric cars. After the first pilot projects on the V2G (Vehicle-to-Grid) technology, it appears that owners of an e-car who make a handful of kilowatt-hours of the e-car battery available for grid stabilization can earn a good 700 to 1000 euros a year. Ideally, with completely CO2-free electricity generated on their own roof.
Michael is a freelance journalist and has been writing almost exclusively about sustainable mobility, electric and hybrid cars, energy issues and environmental protection since 2012.
Edited and translated by Christian Pfäffli