Arguably the most important basic electrical formula to know is that watts (power) is equal to amps (current) times volts. It helps us to think of electricity as water running through a pipe. In this scenario, the voltage is equivalent to the water pressure or how hard it is pushing, the current (amperage) is equivalent to the diameter of the pipe, and the power (wattage) is equivalent to the force which the water comes through the pipes--a function of the pressure and size of the pipe.
Q: How does this apply to the fan I'm looking for? DC fans will often give you the voltage and amperage. As long as you have two of these, you can determine the third. For instance, a 12v fan that has a 0.25A current will draw 3 watts of power. Amp draw will compound linearly as you add more devices into a system (think adding a new water pipe with a consistent pressure); voltage will not change unless the power supply is changed. If you use eight of the 12v/0.25A fan from the example above, you will be drawing 2A total (or 24 watts).
Q: What voltage fan can I use with my system? A 12v rated fan can usually be powered with a 5v power supply, but it will spin at ~42% (5/12) of the rated speed. Some fans will not start up at all if voltage is too low--this is normally at 35% of the rated voltage. Inversely, running a 5v rated fan on a 12v power supply will cause the fan to run at more than twice the speed and the fan will overload/die quickly. Never run a fan at a significantly higher voltage than what it is rated (normally a safe margin is +/- 15% of the rated voltage). The lifespan will drop off significantly as you get further you get away from the rated voltage.
Determining Power Supply Requirement
Q: Which of your power supplies do I need? AC (wall outlet) to DC (most small electronics) power supplies are normally sized in amps and volt outputs. They will take in AC power (110v-120v in the USA) that's throughout your home or office and transform it down to DC power (ie. output: 12v-2A). We recommend that you leave a 10% margin for safety when calculating the power supply or it may overheat if soldering is subpar. For instance, only power 1.8A worth of devices off a 2A power supply
Determining Energy Cost
Q: I looked up energy costs in my area and the show kWh--what's that? Kilowatt hours (kWh) are a measurement of energy used at a constant rate over a period of time. Most power companies bill in terms of kWh. One kilowatt is equal to 1000 watts.
For example, it would take a 2 watt fan 500 hours (20.83 days) of run-time to reach 1kWh. Washington state averages $0.10/kWh at the time of writing this (feel free to look up your state averages here). Our 2 watt fan example would cost approximately $0.0048 per day in Washington ($0.10/20.83). This means that this fan would cost approximately $1.75 in power annually if it ran 24/7 for the whole year ($0.0048*365).
Q: How can I cut energy costs? If your fan is on one of our thermal controllers, or is not being used continuously, you can discount this annual cost significantly. Let's say that you have four of those 2 watt fans on an electronics cabinet in your office that is being used 8 hours per day/5 days per week. The original 24/7 annual cost of those 4 fans would be $7/year, but the new energy cost would be $1.67 for the fans. The cost of the entire controller can be made back in a few years due to energy savings. These savings are magnified as you use higher amp draw systems or more tailored on/off periods.
It is important to note that there is additional energy lost in converting AC to DC power, but it is somewhat marginal and much more efficient the the inverse conversion from DC to AC power.