The Industrial CapitalistPower and energy are different things, and the industry uses different units for them. A megawatt (MW) is a rate: how much power a generator can deliver, or a load can draw, at a given moment. A megawatt-hour (MWh) is a quantity: one megawatt delivered for one hour. A plant rated at 100 MW that runs flat out for a day produces 2,400 MWh.
Why it matters
- Sizing equipment. A generator is rated in kilowatts or megawatts because the question is whether it can carry the load. Fuel is bought by the energy it produces, which is the hours matter.
- Pricing. Electricity is sold by the megawatt-hour. Capacity, the promise to be available, is sometimes paid for separately by the megawatt.
- Reading the news. A data center that needs 200 MW is describing a continuous draw. A battery that stores 400 MWh is describing how long it can supply a given rate before it is empty.
The smaller units
On a job site the same distinction shows up as kilowatts and kilowatt-hours. A light tower or a pump skid is rated in kilowatts; the rental generator feeding it burns fuel in proportion to kilowatt-hours. A crew that understands the difference sizes generators correctly and does not pay for capacity it never uses.
Capacity factor
No plant runs flat out all the time. Capacity factor is the share of a period’s maximum possible output that was actually produced. It is how a 100 MW wind farm and a 100 MW gas plant end up producing very different amounts of energy in a year, and why both numbers matter when the two are compared.