There are only 70 calories and just 0.01 grams of fat in 100 grams of potatoes. The tuber only becomes a calorie bomb when it is prepared with a lot of oil or fat.
Fries fried in oil are of course higher in calories than cooked jacket potatoes. There is a useful trick to make potatoes healthier: eat potatoes that have been warmed up the day before. When it cools, the structure of the starch changes and what is known as resistant starch is created. Resistant starch acts like fiber and is also found in other foods.