Nuna8 of Nuon Solar Team Netherlands is seen here on day four of the 2015 World Solar Challenge. Forty-five cars from 25 countries are taking part in this 3,000km race across the Australian outback. All have one thing in common - they are powered by the sun. The biennial race started in Darwin on Sunday and the winning cars were expected in the south Australian city of Adelaide late yesterday or today. One of the competition's main rules is that the solar cars are allowed to store only 5kWh of energy at a time. The rest of the energy must be reaped from the sun or from the motion of the car.