If laptops were designed to be only run by plugging them into power then Windows would not have a power save function.
Also laptop designers probably would not develop power step in the CPU's.
PC's are not designed to run off batteries and so is the reverse of laptops reguards power.
If you are SO worried of the power your PC is using... much is done by the monitor (if it is CRT type).
So, when not actually having to see nothing happening but rather check once every 30 minutes, turn it off.
Now you have a counter action
Simply turning your monitor on draws a rather large ammount of power (probably as much as saved by turning it off for 30 min).
So, unless you check every killowatt of power you use per month
and adjust the temperature setting of your refrigerator daily to match the outside temperature .......