The basic idea

A battery’s energy capacity tells us how much energy it can store. But that doesn’t tell us how much energy is available right now. For that, we use State of Charge (SOC).

SOC tells us how full the battery is, expressed as a percentage of its usable energy capacity.

So:

• 100% SOC → battery is fully charged
• 50% SOC → half of its usable energy is stored
• 20% SOC → one-fifth is stored
• 0% SOC → no usable energy remains

This sounds simple, but SOC becomes one of the most important variables in battery operation because it changes what the battery can do next.

1. Start with our 100 MW / 200 MWh battery

From the previous page, we established that a 100 MW / 200 MWh BESS

has:

• Maximum power = 100 MW
• Energy capacity = 200 MWh
• Theoretical duration = 2 hours

Now let’s introduce SOC.

If the battery is at 100% SOC:$$200 \times 100\% = 200\ MWh$$

So there are 200 MWh of stored energy.

At 50% SOC:$$200 \times 50\% = 100\ MWh$$

At 25% SOC:$$200 \times 25\% = 50\ MWh$$

We can generalize this as:$$\boxed{Available\ Energy = Energy\ Capacity \times SOC}$$

2. How long can the battery discharge?

Suppose the battery is at 50% SOC.

We have:$$200 \times 50\% = 100\ MWh$$

If the battery is discharging at its maximum 100 MW:$$t = \frac{100}{100}=1\ hour$$

So the battery can theoretically deliver:

100 MW for 1 hour.

Now suppose SOC is 25%:$$200 \times 25\%=50\ MWh$$

Therefore:$$t=\frac{50}{100}=0.5\ hours$$

or:

100 MW for 30 minutes.

The same battery therefore has a different remaining discharge duration depending on its SOC.

3. But 0% and 100% aren’t always the operating limits

This is where real batteries become more interesting.

A BESS may not operate across the entire nominal 0–100% range.

For example, suppose a battery is operated between: 10% SOC and 90% SOC.

The battery has a nominal capacity of: 200 MWh

But its usable operating range is:$$200 \times (90\%-10\%)$$$$=200\times80\%$$$$=\boxed{160\ MWh}$$

So although the battery is described as having 200 MWh of capacity, only 160 MWh may be available within the chosen operating window.

This distinction matters when we start calculating actual revenues.

4. SOC is not just a measurement

SOC is also a constraint on what the battery can do next.

Imagine electricity prices look like this:

Time	Price	Battery SOC
4 PM	$60/MWh	80%
5 PM	$90/MWh	60%
6 PM	$150/MWh	40%
7 PM	$180/MWh	20%

At 4 PM, the battery might want to discharge because prices are rising. But every MWh it discharges reduces its SOC. By 7 PM, when prices are highest, it could have very little energy left.

So the question for an optimizer becomes:

“When should I use my limited stored energy?”

5. Charging changes SOC too

SOC works in both directions.

Suppose our battery starts at: 20% SOC

Its stored energy is:$$200\times20\%=40\ MWh$$

Now suppose we charge it with 50 MWh.

Ignoring efficiency for the moment:$$40+50=90\ MWh$$

New SOC:$$\frac{90}{200}=45\%$$

So charging 50 MWh increased SOC from: 20% → 45%

Similarly, discharging 50 MWh would reduce SOC from: 45% → 20%

This gives us a basic state equation:$$SOC_{t+1}=SOC_t+\text{charging}-\text{discharging}$$

The real equation will later need to account for efficiency and other constraints.

6. Why SOC becomes important for optimization

Now imagine the battery has two possible opportunities.

Opportunity A

Discharge now at: $100/MWh

Opportunity B

Wait two hours and potentially discharge at: $180/MWh

If the battery had unlimited energy, this would be easy.

But it doesn’t.

Its stored energy is finite.

Using energy for Opportunity A means that energy cannot also be used for Opportunity B.

This is where SOC creates an opportunity cost.

The value of discharging 1 MWh isn’t just the revenue earned now. It can also be the value of the opportunity you give up later.

That’s one of the reasons battery optimization becomes more complicated than simply:

Buy low → sell high.

7. SOC + power + energy

We can now combine the concepts from BESS Mechanics 1 and 2.

A battery is constrained by at least three things:

• Energy capacity: How much energy can it store?
• Power capacity: How quickly can it charge or discharge?
• SOC: How much energy is available right now?

For our 100 MW / 200 MWh battery:

200 MWh tells us the size of the energy reservoir.

100 MW tells us how quickly energy can move in or out.

SOC tells us how full that reservoir currently is.

These constraints interact. A battery could have 100 MW maximum discharge power but if it is only at 10% SOC, it doesn’t have enough stored energy to sustain 100 MW for very long.

8. The next complication: efficiency

So far we’ve assumed:

50 MWh charged → 50 MWh stored

and:

50 MWh discharged → 50 MWh delivered.

Real batteries aren’t perfectly efficient.

If you put 100 MWh into a battery, you might get something like 90 MWh back over a complete charge-discharge cycle.

So the amount of energy entering the battery and the amount eventually delivered to the grid are not necessarily the same.

That means our SOC calculation needs to account for charging and discharging efficiency.

That’s the next piece of the mechanics.

The mental model

For now, think of SOC as the battery’s current energy state.$$\boxed{Available\ Energy = Capacity \times SOC}$$

And remember:

SOC isn’t just telling us how full the battery is. It determines what the battery can do next.

Once you add electricity prices, that becomes an optimization problem: Do I use the energy now, or preserve it for a potentially more valuable opportunity later?