Roughly, a CPU uses about as much power (watts) as a typical light bulb. Although power supplies on most computers are rated at 400 watts, average usage is lower. On average, a Pentium-type computer uses about 100 watts (if the monitor is off). So, the daily difference between off and running FAH is about 24×100 = 2.4 kWh. At $0.15 per kWh ( from PG&E here in California), this works out to about $0.36 per day. In general, lighting and climate control use a much larger share of household power than computers do. So the best bet for cutting costs and conserving energy would be to turn off lights, turn off your computer monitors (which use more power than a CPU), and turn down the heat.