The policy has boosted brands such as Tesla, Nissan , Hyundai and BMW, which all offer fully electric vehicles, rather than hybrids that use electric motors to drive the car but also have a combustion engine.
Brands without fully electric offerings, such as Ford and Daimler's Mercedes-Benz, have seen sales drop, although Ford and Mercedes are among several automakers that have promised to offer electric cars in Norway from 2020.
California-based Tesla sold 3,760 vehicles in Norway in June, for a 24.5% share of all cars during the month, and was also the top-selling brand for the first six months.
Most of its sales were of the mid-sized Model 3, while the bigger Model S and Model X have seen lower year-on-year volumes.
The International Energy Agency (IEA), which includes the more widely-sold plug-in hybrids when counting electric cars, measured Norway's share at 39% of sales in 2017, far ahead of second-placed Iceland on 12% and Sweden on 6%.