Based on current tariff should be around 600kwh

For the first 200 kWh (1 - 200 kWh) per month sen/kWh 21.80
For the next 100 kWh (201 - 300 kWh) per month sen/kWh 33.40
For the next 300 kWh (301 - 600 kWh) per month sen/kWh 51.60
For the next 300 kWh (601 - 900 kWh) per month sen/kWh 54.60
For the next kWh (901 kWh onwards) per month sen/kWh 57.10
The minimum monthly charge is RM3.00

The new tariff is also going to be revised monthly. So it is going to follow oil price trend to go up and down. Really no point to estimate how much you gonna pay in a year because oil price nowadays performing wild swing up and down in a single year. No way to truly estimate the base cost.

Desktop is nothing compared to aircon. You don't run at full power all the time. And what is your PSU draw? It most likely won't reach 1HP like the smallest aircon. Most likely the cost to run that desktop is like 0.1 1HP aircon only.