There are a few methods to quickly determine wattage on a part that doesn’t really list the parameter in the product attributes or the data sheet. Many parts have this rating because they absolutely need it in the description due to it being critical to their design. A few examples include power supplies, resistors, most AC or DC fans, most AC or DC motors, and anything else that deals with power directly. However, there are many electronics parts out there without the rating and this may cause concern. The answer will depend on the type of item asked about. There are two fundamental formulas for power that should be utilized.
Where “I” is the current measured/calculated, V is the voltage measured/calculated, and R is any resistance or resistor on the path that current is traveling on.
Three Types of Power Ratings
There are three types of power ratings to consider. There is power generated (supply side), power consumed (load side), and maximum power rating which is the absolute maximum a part can handle before it burns out which can be either load side or supply side depending on the type of part. For example, a power converter will have maximum amperage input requirements and maximum amperage output specifications. If the input amperage is exceeded the load of the power supply will be overloaded and burn out. If the output amperage required is exceeded by the load, the load will not function properly and the power supply may not last as long.
Believe it or not, capacitors and inductors are not perfect devices. The reason that there are ESR equivalents and DC Resistance equivalents is that there is a factor called leakage. Leakage is based on the fact that all materials in devices will have some sort of resistive loss based on how resistance is determined. Devices will typically list what their equivalents are in the datasheets.
Our list of capacitors all have voltage ratings, but no current or power ratings. Capacitors are highly dependent on the maximum voltage used in the circuit design. The context for missing current data is based on the fundamental operation of a capacitor. If the capacitor is used in a non-oscillating, there will be hardly any power drawn by the component because leakage is so low. Determine the parasitic power loss through the following formula:
Where “I” is the current measured/calculated on the wire and RESR is the series equivalent resistance. Remember Ohm’s law is V = I x R and can be manipulated using algebra (so can the power equation). It is easier to determine power loss in an AC application or oscillating DC application for combinations of these three parts working together based on resistive values. Technically, the current isn’t passing through the capacitor based on electric field isolation between layers of a capacitor, but current can be conducted on both sides because the voltage is changing rapidly enough to cause a difference in potential on either side.
Our inductors list current ratings, but no voltage or power ratings. Inductors are dependent on the maximum current used in the circuit design. To determine how much power an inductor can handle before burning out, take the current rating times the voltage used in the circuit. To determine power loss from resistance in the inductor, take the DC Resistance (DCR) and use it in the P=I^2*R formula to determine if there is a significant loss.
Cables and Wires
The special note about wire is that the maximum current is determined by the wire gauge used. Here is a table for single core UL-CSA tested cables for amperage for 4/0 to 24 AWG at 30º C. Data is bound to change based on how wires are tested.
Wire Gauge Ampacity Chart
|AWG Size||Ampacity (Amps)|
After the current is either measured or compared to charts or calculated, just multiply the rated voltage times the current for the rated power the cables can handle.
The same formulas apply to active components to find wattage. The only thing I should mention is most items give maximum current and maximum voltage ratings. So if the wattage is calculated this way, it will be maximum wattage consumed or generated (depends if it is a load or supply). Also, some components like motors or fans have a variable driving current and voltage because of the conditions that the fan is operating in, typically manufacturers will give a nominal value and one can calculate a nominal power. This will be around what the fan/motor should consume in optimal conditions, expect a change if the conditions are not met. Items that require heat-sinks also may have varying voltage or current and it is recommended to use the appropriate heat-sink or there will be variance.
I will cover an example of how to find wattage for COB LEDs in a separate post because there is more detail to that example. Here is the post:
The final topic for wattage are items that have Root Mean Square (RMS) ratings for voltage and current. These apply to alternating current applications and some oscillating DC circuits. The power can still be calculated from these values, but the answer will also be in RMS terms. Most manufacturers will only be concerned about the RMS values because finding absolute power gets complicated in elaborate AC grids.