There is no one-size-fits-all answer to how warm a ski jacket will keep you, as people's perception of warmth varies from person to person and the area of use often differs as well.
For downhill-oriented skiers, a simple padded ski jacket is probably more recommended, whereas downhill-oriented freeriders and tourers prefer a hardshell jacket. What can be said, however, is that a ski jacket is basically designed to build up and store warmth. Various materials are used for this, from synthetic fibres to down and wool.
It is not only the ski jacket you wear that is important, but also what you wear underneath. The basis for solid ski equipment is thermal underwear. Consisting of long pants and a long-sleeved shirt, they lie directly on the skin. Thanks to their breathable function, they wick sweat away from the body and keep you warm and cosy.
Then put on the ski pants on your legs and you're ready for the slopes. To keep the upper body warm, one layer is usually not enough and it is advisable to put on a jumper or a fleece jacket before putting on your ski jacket.