Let say desktop 300W x 4 = 1200 x 18 = 21.6KWH
Laptop 200W x 2 = 400 x 18 = 7.2KWH
1 hp = 750 W x 16 = 12KWH

21.6 + 7.2 + 12 = 40.8 x 30 days = 1224 KWH.

Above just rough calculation as desktop power consumption is always fluctuate due to usage, it is not possible that your computer always run at peak load.

So answer, yes, could reach 1200KWH

Flourescent light is considered energy efficient already unless one is using old type bulb one.

Water boiler is high energy usage one, at least >1KW one.

1 HP power air-con generally consumes about 20~30 cents per hour

Compressor will be shut down when reaching preset temp by you. So if room is too big, it won't able to reach the temp preset ley day 24, so compressor will run non-stop.

I forget the exact figure, but roughly 1 HP is recommended around the size of 12x12, and 1.5HP is suit at around 18x18

I would say 1.5 HP is the optimum for your case.

Correct me if I am wrong.

I would think if one is using near the capacity of the cable, by using larger cable, will cause less heat and saving on some resistance as well as bigger size cable having less resistance in the first place. But whether the saving is significant or not, still a lot can be studied and explored.

As for ordinary household the saving a few cents might not able to recoup back what had invested in bigger cable. Just like inverter air-con will save electricity. But if you are using just a few hour per day, the amount of power bill saved might not able to recoup back the extra money fork out to purchase a inverter air-con compared to ordinary one.

Inverter air-cond save electricity but it depends how it is being used. If not the extra money fork out is not worthwhile. If your compressor need to run 100% all the time (under sized), inverter air-con won't able to save you a single cent.

The saving power bill changing from smaller to bigger size cable as stated by forumer could be the previously old smaller cable had crack or deteoriated due to whatever reason in the first place, so by changing the new and bigger cable lead to power bill save.
Just my wild guess.

Err.... flourescent light got use inverter? Sorry noob in this area.

What the use of inverter in flourescent then?

As far as I knew, inverter is some device that control the current flow once the motor doesn't need so much power. So how this applied on flourescent?

I used it before, those electronic ballast can be not realiable as well, blown 4 of out 6 in one year time(in my case). I noticed all those electronic ballast are mainly from Chapalang brand, seldom got well known producers one. Just my experience.

I wonder why less well known producers want to venture in this area.

Benefit of electronic ballast, your light won't blink blink when switch on, just light up. Other than that, don't see other major benefit of it, especially the reliability side which is the major disadvantage.

This is being discussed before. http://forum.lowyat.net/index.php?showtopic=973128

Those are hours of usage and number of desktop/laptop stated by the origin question.

Electricity bill is calculated in power aka KWH, now is about 30+ cents per KWH

So if your equipment consume 100W and you use it for 10 hours then, 10 x 100 = 1KWH.

In scientific term 1HP = 0.75KW.

But for actual case and actual consumption, it could varies from manufacturer to manufacturer, depended how the motor setup etc issue.

There is no straight accurate answer can be given.

Just a rough guide only.