Long answer: My refridgerator is about 5 years old, has a volume of 103 liters and a freezing compartment of 16 liters. Its energy class is “A”, the label states a power consumption of 219 kWh per year.
I borrowed a power meter, and measured consumption before and after defrosting. Here is a chart of the measured data. I measured only 5 values, one week before defrosting, 1 hour after turning the fridge on, and then 8, 23 and 47 hours after defrosting. The y-axis shows the average power consumption, i.e. the intensity the fridge sucks energy.
The data suggests that the power consumption is on average at 15.9 Watt. The difference before and after defrosting is 0.5 Watt. I do not want to attribute this to the defrosting. The outside temperature in the week before defrosting was higher, also I used the fridge more often because there were holidays and I was at home. But even if defrosting would have this effect, these 0.5 Watt add up to only 4.38 kWh or 1.50 EUR per year.
Another insight was that 15.9 Watt project to 139 kWh per year and this is quite below the value 219 kWh stated on the energy label. Even after 5 years! Maybe this is because I put the thermostat a rather low level (2 out of 5), or maybe the consumption in the summer will be much higher.