Digit Number of Segments
Partial Total 18
Partial Total 27
Note, totally off-topic, that 4, 5 and 6 are the only digits whose 7-segment representation has the same number of segments as the digit value.
Given a 24-hour clock (the worst case scenario), the sum for all 24 hours is: 6*10 for the leading zero plus 49 for the following early hours, plus 2*10 for the leading one plus 49 for the following mid hours, plus 5*4 for the leading 2 plus 18 for the following late hours. That's 216, so diving by 24 we get 9 LEDs lit on average to display the hours.
The minutes are just as easy to calculate: There's an average of 27/6+49/10 (for the first, second digits), that is 9.4 LEDs on average. The two points average 1 if they blink, 2 if they don't (worst case, which is what we choose).
In total there are 20.4 LEDs lit, that's an average current of 410mA, holy shit that's fucking much. In reality the current is significantly lower, but this is a worst-case calculation. So the display uses at most 420mA, microcontroller included. Suppose it's supplied with 5V, that's 2.1 watts. Assuming a supply efficiency of around 50%, the standby-mode clock eats about 4W averaged over a whole day, neglecting the small intervals the oven is used for cooking.
Now for the cooking. My oven has settings for 800W, 640W and 320W or something like this. I don't know if that's the useful power or the total power drawn. For the worst-case we're discussing I'll assume it's the total power. I always use it on 800W for about 1-2 minutes. So let's say the oven is used for 10 minutes a day (an average family might use it for 20-30 minutes a day). The display eats up 4W*24(hours)*60min=5760 watt-minutes. The cooking eats up 800W*10min=8000 watt-minutes, which is more. There, the display does NOT consume more energy than the oven-proper. If I only used the oven for 5 minutes a day, and the display was a really lousy one like the one described above then yes, the display might have consumed about as much energy as the cooking. Note, my oven displays a singe 0 when not in use, so that uses more than 3 times less energy. It still eats up quite a lot of energy, which sucks, but still significantly less than the oven-proper, so there goes the propaganda down the drain.