Frequency

CPU is just after the screen most power consuming part of laptop. The higher frequency the greater power consumption, each cycle uses a given amount of power, even there is nothing to do. Usually when you working on laptop you needn't full speed of processor, so why not decrease it? Each laptop's CPU supports technology called frequency scaling. Available frequencies depend on the model. You may check it by entering in terminal as root below commands.
Commands:

available frequencies
     cat /sys/devices/system/cpu/cpuN/cpufreq/scaling_available_frequencies

current frequency
     cat /sys/devices/system/cpu/cpuN/cpufreq/scaling_cur_freq

set frequency to X (only when 'userspace' governor is enabled)
     echo X > /sys/devices/system/cpu/cpuN/cpufreq/scaling_setspeed
 

 *where cpuN is processor/core number counted from 0.

Next thing are scaling governors which sets frequency depending on CPU usage.

Here is list of scaling governors:

• Performance sets the speed to maximum, essentially disabling frequency scaling. 
• Powersave drops CPU to its lowest supported frequency.
• Userspace allows the CPU speed to be set manually or dynamically by software. 
  
• Ondemand runs at the lowest possible frequency and increases to maximum when CPU usage hits 100%.
• Conservative runs at the lowest possible frequency and increases incrementally as more power becomes necessary.

Commands:

available scaling governors
     cat /sys/devices/system/cpu/cpuN/scaling_available_governors

current scaling governor
     cat /sys/devices/system/cpu/cpuN/scaling_governor

set scaling governor
     echo 'governor' > /sys/devices/system/cpu/cpuN/scaling_governor
 

 *where cpuN is processor/core number counted from 0.

Attention

If above functions doesn't work, probably:

• you haven't installed acpi or acpid or both packets
• acpi-cpufreq module isn't loaded 
• you haven't compiled this features into kernel
  
• your processor doesn't support frequency scaling

Research:

Hardware: Intel Pentium Dual-Core Processor T2390 1,86 GHz

Available frequencies: 1867 1600 1333 1067 800 MHz