A raw prawn is 15 calories. The same prawn as tempura is 85. That is the whole story of tempura in one line. The batter is flour and egg, which adds about 30 calories per piece, and then the batter soaks up frying oil, which adds another 40. Tempura is 20 to 25 percent oil by weight when it comes out of the fryer, so the light and crispy texture is doing a lot of hiding.
Vegetables are worse than they look. A slice of kabocha or sweet potato is 90 calories as tempura because the batter clings to the flat surface and the vegetable itself is starchy. Shishito peppers and shiso leaves are the cheap ones at about 30 calories each because they are thin and barely weigh anything. Kakiage, the fritter of shredded vegetables and small shrimp, is the heaviest single item on the plate at 120 to 180 calories because it is basically a puck of batter.
The way tempura is served decides the number. A 480 calorie plate with grated daikon and tentsuyu is a reasonable main. The same pieces on 280g of rice with sweet tare sauce as a tendon is 800 calories. On a bowl of udon it is 370 to 450 calories. At a good Tokyo counter, a 10 piece omakase course runs 1,000 calories and about 8,000 yen, so at least the price slows you down.