What Does 100 MW / 200 MWh Actually Mean?

The basic idea

A battery is usually described using two numbers:

• Power capacity (MW) — how quickly the battery can charge or discharge.
• Energy capacity (MWh) — how much energy the battery can store.

These describe two different constraints.

A 100 MW / 200 MWh BESS can, in the simplest case, discharge at a maximum rate of 100 MW and has enough stored energy to deliver 200 MWh.

The relationship between them tells us the battery’s duration.

1. Power vs. energy

The easiest way to understand the difference is to think about water.

• MW is like the size of the pipe — how quickly water can flow.
• MWh is like the size of the tank — how much water is available.

A larger tank doesn’t necessarily mean you can empty it quickly.

Similarly, a battery can have a large amount of stored energy but a relatively low maximum power output.

2. Calculating battery duration

The basic relationship is:$$E = P \times t$$

Therefore:$$t = \frac{E}{P}$$

For a 100 MW / 200 MWh battery:$$t = \frac{200\ MWh}{100\ MW}$$$$\boxed{t=2\ hours}$$

So, if the battery is fully charged and can continuously discharge at its maximum 100 MW output, it can theoretically do so for 2 hours.

This is why it is called a 2-hour BESS.

3. What if the battery is only half full?

Now suppose the battery is at 50% state of charge (SOC).

Its nominal energy capacity is still 200 MWh, but only half of that energy is currently stored:$$200 \times 50\% = 100\ MWh$$

At a 100 MW discharge rate:$$t = \frac{100}{100}=1\ hour$$

So the same battery that could theoretically discharge for two hours when full can only discharge for one hour when it is at 50% SOC.

This is our first important distinction:

Power capacity tells us how fast the battery can move energy. SOC tells us how much energy is currently available.

4. What if the battery can only discharge at 50 MW?

Now keep the battery fully charged at 200 MWh, but suppose its discharge power is limited to 50 MW.$$t=\frac{200}{50}=4\ hours$$

The battery hasn’t gained any additional energy.

It simply takes longer to release the same amount of energy.

So:

100 MW / 200 MWh → 2 hours

50 MW / 200 MWh → 4 hours

This illustrates why MW and MWh cannot be used interchangeably.

5. The same energy capacity can support different power profiles

Consider three hypothetical batteries:

Battery	Power	Energy	Duration
A	100 MW	100 MWh	1 hour
B	100 MW	200 MWh	2 hours
C	100 MW	400 MWh	4 hours

All three can discharge at 100 MW.

But they can sustain that output for different lengths of time.

This distinction becomes important when deciding what a battery is actually useful for.

A 1-hour battery and a 4-hour battery aren’t simply different-sized versions of the same asset. Their economics and potential market applications can differ substantially.

6. Why this matters economically

Now we can connect the physical characteristics to the economics.

Suppose electricity is: $30/MWh while charging

and later: $100/MWh while discharging.

A battery needs enough energy capacity to take advantage of the opportunity.

But it also needs enough power capacity to capture it within the available time window.

For example, if prices are high for only one hour, a 100 MW / 200 MWh battery has 200 MWh of stored-energy capacity, but it can only discharge 100 MWh during that one hour if its maximum output is 100 MW.

So having more MWh does not automatically mean earning more revenue.

The battery is simultaneously constrained by:

How much energy it has
and
how quickly it can move that energy.

That is the beginning of BESS economics.

7. One more constraint: this is theoretical

Our calculations above assume an ideal battery.

Real batteries introduce additional constraints:

• round-trip efficiency
• maximum/minimum SOC
• charge and discharge limits
• degradation
• auxiliary consumption
• temperature
• availability
• inverter constraints

For example, if round-trip efficiency is 90%, putting 100 MWh into the battery does not mean you can later sell 100 MWh. That will be the next layer.

The mental model

For now, remember three things:

• MW: How fast?
• MWh: How much?
• SOC: How much is available right now?

And:$$\boxed{Duration = \frac{Energy}{Power}}$$

Once you understand this relationship, a specification like 100 MW / 200 MWh stops being a piece of technical jargon.

It becomes a statement about how much energy the asset can hold, how quickly it can move that energy, and how long it can sustain a given output.