In the circuit diagram we have a battery and three resistors R1, R2 and R3 in a closed loop back to the battery. Take note of the polarities of the voltages around the circuit and you will see that the battery’s polarity is reversed from the voltage polarities of the resistors because it is a voltage source. Using Kirchhoff’s Voltage law we get the following:
V1 + V2 + V3 – Vbat = 0, and we can rewrite it as Vbat = V1 + V2 + V3. If we think about it that totally make sense.
This principle or law is used in every electronic circuit without even thinking about it. If we use a potential divider, a Light Dependant Resistor as an input or a transistor we intuitively make use of Kirchhoff’s Voltage Law.