Why is a Power Plant Capacity Rated in MW and not in MVA?

Electrical Technology

Why is the Rating of a Power Plant Capacity Expressed in MW instead of MVA?

Power plant capacity is rated in megawatts (MW) instead of megavolt-amperes (MVA) because MW represents the real power. It is the actual usable energy delivered to the grid or load, which is what truly matters for energy generation and consumption. MVA, on the other hand, represents the apparent power, which includes both real power (MW) and reactive power (measured in megavars, MVAR).

For the following reasons, the capacity of a power plant is typically expressed in megawatts (MW) rather than megavolt-amperes (MVA):

In a generating station, the prime mover (such as a turbine) produces only active power, measured in watts. The generator then converts this mechanical power into electrical power, calculated as Volts × Amps × Power Factor. This electrical power is then transmitted and distributed throughout the power system.

Since the output depends solely on the mechanical power provided by the prime mover, the capacity of a power plant is rated in megawatts (MW), not MVA. In other words, regardless of how large the generator is, the actual power output is limited by the capacity of the turbine.

For example, if a 50 MW turbine is connected to a 90 MVA alternator (which is capable of supplying both active and reactive power), the plant will still only generate 50 MW of active power at full load. similarly, a 500 MW power plant delivers 500 MW of real power, but its MVA rating would depend on the power factor (e.g., at a power factor of 0.8, it would be 625 MVA). Since the real power output is the critical metric, MW is used for clarity and consistency.

Real Power is What Matters to Consumers. In simple words, real power (MW) is the actual energy that does useful work, like running appliances or industrial machinery. Consumers and utilities are primarily concerned with how much usable energy a power plant can deliver. Hence, it makes MW the relevant metric for rating a power plant.

Why is a Power Plant Capacity Rated in MW and not in MVA? — Why-Power-Plant-Capacity-Rated-in-MW-and-not-in-MVA

As stated before, a power plant’s capacity is rated based on the prime mover or turbine, not the alternator coupled to it. The turbine’s rating can usually be found on its nameplate, and it is specified in megawatts (MW) or horsepower (HP), not in kilovolt-amperes (kVA) or megavolt-amperes (MVA).

Additionally, electric power companies generate power in kilowatts (kW) or megawatts (MW), but they charge consumers in kilovolt-amperes (kVA) for electricity bill. This is because the consumer is responsible for maintaining a good power factor. If the power factor is low, the utility company must supply more current for the same amount of useful power, which increases losses in the system. Therefore, consumers are penalized for poor power factor, not the power plant.

Reactive power (MVAR) is necessary to maintain voltage stability and support the electrical grid, but it doesn’t perform useful work. MVA combines real and reactive power, but since reactive power varies with grid conditions and isn’t directly consumed, it’s less relevant for rating a power plant’s capacity.

Moreover, the relationship between MW and MVA depends on the power factor (cos ϕ), where MVA = MW / power factor. Power factor varies depending on the load and grid conditions, so MVA isn’t a fixed or consistent measure for a power plant’s capacity. MW avoids this ambiguity.

MVA = MW ÷ Power Factor (pf)

In a power plant, the power factor is typically close to 1, which means MW ≈ MVA. Since MW = MVA × Power Factor, hence, it makes more sense to rate the plant capacity in MW.

W = VA × Cosϕ

A plant’s MVA rating depends on the generator’s thermal and current-carrying limits. But MW shows how much actual, useful power is being produced.

And finally, here’s a light-hearted explanation from one of our Facebook page fans: “Powerhouse means house of power, and since the unit of power is Watt, that’s why the plant is rated in megawatts (MW), not MVA!”

Generators (in the power plant) are rated in MVA because they must handle both real and reactive power.
Transformers and transmission lines are also rated in MVA, because they carry total apparent power.
But for capacity planning and billing, MW is what utilities and operators focus on.