Assuming you are running the 120mm fans at full speed, each one will probably use 6 to 7 watts per hour. Measely.
If you are really concerned about power consumption then you should look at the other components in your system:
GPU - ATI cards are generally more power hungry than their nVidia counterparts. For example:
Then of course there is the CPU:
However, there is one component that many people overlook that can have a huge impact on how much power your PC will consume. That component is the Power Supply.
Power Supply - As everyone knows, the PSU draws power from AC outlet and provides it to the rest of the PC. However, it's not that simple. Power supplies have maximum efficiency ratings between 65% and 85% depending on the PSU. Generally speaking, more expensive PSUs will have greater efficiency than a cheaper model, but you'll need to read the specs to find out how efficient it is.
So what the hell is efficiency, and what's the difference between 65% and 85%? Efficiency determines how much (or little) watts is wasted as heat when the PSU converts AC power to DC power. For example, let's say your PC consumes 250w of power. A 65% efficient PSU will deliver 250w and consume 384.6w (250 / 0.65) of power. That means 134.6w is wasted as heat. An 85% efficient PSU will deliver 250w and consume 294.1w (250 / 0.85) of power. Therefore, it only wastes 44.1w. That's a difference of 90.5w. However, this is a very simple explanantion because a PSU delivering 20w to 400w or higher will not be 85% efficient throughout the power curve.
A more complex view also looks at how high the load placed on the PSU is. Let's take the 250w load from the above example and use that in a 350w PSU and 600w PSU, both of which are up to 85% efficient and have the following specs:
Load %...........................................Efficiency
1 - 40.........................................................50%
41 - 50.......................................................60%
51 - 65.......................................................70%
66 - 75.......................................................80%
76 - 85.......................................................85%
85 - 95.......................................................83%
96 - 100.....................................................78%
350w PSU - The 250w load on the 350w PSU is 71.4% (250w / 350w) based on the load the PSU will be 80%. That means the PSU will be drawing 312.5w (250w / 0.80) from the AC outlet to provide 250w to the PC. 62.5w is wasted.
600w PSU - The 250w load on the 600w PSU is 41.6% (250w / 600w) based on the load the PSU will be 60%. That means the PSU will be drawing 416.7w (250w / 0.60) from the AC outlet to provide 250w to the PC. 166.7w is wasted.
The example above illustrates that going overboard on a very powerful PSU for a PC with a very small load can cost you extra money on your electricity bill.