What does a 200 MW capacity power plant mean?

Question

1 W is 1 J of energy transferred in 1 s. So what does a 200 MW capacity power plant mean? Does it mean it generates 200 MJ of energy in one second? I have also read it can mean 200 MW of power in any time, 1 minute or 1 hour. It is confusing me a little.

So what does 200 MW capacity power plant mean w.r.t. time?

Answer 1

It sounds like you're misunderstanding power.

Power is a rate of energy generation or consumption. If my circuit is using 1 W of power, it needs 1 J of energy to run for 1 s, 10 J to run for 10 s, etc... but the power consumption is still a constant 1 W.

In your case, you have a power plant that can generate 200 MW. It can do this for 1 s (generating 200 MJ of energy), 10 s (2000 MJ), or any greater length of time - but it cannot create more than 200 MW of power.

Answer 2

It means that it can supply 200MJ per second in any second, or 200kJ per millisecond in any millisecond, and so on.

Don't think of the definition 1W = 1J/s. Rather think of the load it can handle. 200MW is 2 million good old 100W bulbs. The plant can light them anytime, for as long as you want.

Answer 3

Simply, it means the plant's capacity is that it can generate 200 MJ in 1 second.

So you don't need to state time every time. That's why they converted J/s to W so that we get rid of stating time every time we talk about the plant's capacity.

Answer 4

You're right about 200MW of power means that the plant is capable of delivering 200 million Joules of energy, every second, to the town.

That's every second. And that's how much it could generate, at maximum demand, which we will assume is the case.

Written another way, \$200\text{ MW} = 200\text{ MJ/s} = 200,000,000\text{ J/s}\$

In one single second, then, it generates and delivers \$200\text{ MJ}\$ of energy to the town. In ten seconds it will deliver \$200\text{ MJ} \times 10\text{ s} = 2000\text{ MJ} = 2\text{ GJ}\$ of energy.

In one minute, being 60 seconds, the plant will generate \$200\text{ MJ} \times 60\text{ s} = 12\text{ GJ}\$. In one hour, which is 3600 seconds, the energy delivered will be \$200\text{ MJ} \times 3600\text{ s} = 720\text{ GJ}\$.

This seems easy, and it is. There's perhaps a clue to your confusion in the words you chose to use. You said "200MW of power in any time", suggesting that you think that a certain amount of power can be generated/delivered within a certain amount of time, which doesn't make sense. It's as if power is a commodity. It's not. Energy is the commodity, each Joule being a unit of that commodity. Power is the rate at which that commodity can be produced, and delivered.

You'd understand if I explained that some factory is able to produce 100 Sony Playstations per day. If I then told you that "the factory can produce 100 Playstations per day in a month", what does that mean? That would be saying "100 units per day per month", which makes no sense. It can produce Playstations at rate of 100 units per day, whether that day be Tuesday or Friday, or whether the month has an "A" in it, or if it operates for 1 day or 3 years. What comes out of the factory is 100 units, every day, and that's all there is to know.

If we assume the electrical generating plant is operating at maximum output (200MW) all the while that is operating at all, then it was producing 200MW of power at every single instant in time from the moment it was first switched on, to the moment it gets shut down, if ever. 200MW of power.

It's redundant to say "it produced 200MW in September", we know that, because it's a 200MW plant, all the time. It might be true, though, to say "it output 200MW during September, but operated at a reduced capacity of 150MW during October".

If we are told that it operated at maximum capacity, 200MW, for a full year, without pause, then we can make the calculation of total energy \$E\$ in Joules, that was produced:

$$E = 60\text{ s}\times 60 \times 24 \times 365 \times 200,000,000\text{ J/s} = 6.3\times 10^{15}\text{ J}$$

Just to drive the point home, you will have noticed that power companies do not sell units of power. Nobody ever says "electricity costs $0.1 per kilowatt" (except in error), because a kilowatt is a unit of power, the rate at which the commodity is produced/consumed, not the commodity itself. The commodity is energy, in Joules, but is often quoted in "kilowatt-hours", or \$\text{kWh}\$. Look at that unit; it is the product of time (hours) and power (kilowatts), the same multiplication I performed at the top of this answer, for various periods of time.

It is correct to say something like "electrical energy costs $0.1 per kWh", because \$\text{kWh}\$ is a unit of energy, not power.