The concept of transformer capacity can be a bit confusing. Transformer capacity is measured in kVA (kilovolt-amps). We know that voltage multiplied by current equals power, but are kVA (kilovolt-amps) and kW (kilowatts) the same thing?
kVA (kilovolt-amp) refers to apparent power, which is the total power provided by the electrical source. In addition to providing active power (real power) to electrical devices, transformers must also supply reactive power. The reason for supplying reactive power is because capacitors inside the electrical equipment are constantly charging and discharging, occupying a portion of the transformer's capacity.
kW (kilowatt) represents active power, which is the actual power consumed by the equipment. Active power converts electrical energy into other forms of energy, such as air conditioning cooling, hot water heating, or electric vehicle propulsion.
kVar (kilovar) is the unit for reactive power, which is the portion of power that isn't actually consumed but rather temporarily stored. Electrical equipment containing capacitors/inductors will have these components continuously charging and discharging during operation, without actually consuming electrical energy. Therefore, this part of the power is referred to as reactive power.
Reactive power can be abstract; for example, a 40-watt fluorescent light requires not only 40 watts of active power to emit light but also about 80 vars of reactive power for the ballast's coil to establish an alternating magnetic field.
The relationship among these three types of power is clear, similar to familiar right triangle trigonometry:
(Apparent Power)^2 = (Active Power)^2 + (Reactive Power)^2
Therefore, kVA (kilovolt-amp) and kW (kilowatt) are not the same concept, but they are closely related. In fact, the power factor connects them together. Power factor = Active Power / Apparent Power, and it is generally denoted as cosφ. However, in everyday terms, we usually say Active Power = Apparent Power × Power Factor.
Transformer capacity is typically designed with redundancy, usually around 10%, meaning that the actual usable capacity is about 90%. At the same time, utility companies generally require the power factor to be no less than 90%. After this adjustment, multiplying the rated capacity of the transformer in kVA by two 90% factors (approximately 0.8) gives the maximum total power limit for each piece of electrical equipment.
For example, returning to the initial scenario mentioned at the beginning of this article, if a village transformer has a standard capacity of 1000 kVA and there is currently 500 kW of electrical equipment in use, how many household photovoltaic systems can be installed? Multiplying 1000 kVA by 0.8 results in 800 kW, so the additional household photovoltaic installations should not exceed 300 kW.
