A 4 hour battery, what does that even mean? A human example.

There's a lot of confusion surrounding the power and energy of batteries. It's gotten worse since grid operators started talking about batteries in hours. I explain it in simple terms.

Batteries are transforming the energy system, but for grid operators they are a new phenomenon. That’s part of the reason why there’s a lot of confusion going on.

For example, I often read sentences like: “In Texas we regularly have periods of 20 hours with not enough solar and wind, so a 4 hour battery is not going to cut it.” Which is nonsense because the “4 hour” in a “4 hour battery” has nothing to do with how long the periods are that it can bridge.

Fortunately, this is simple once the penny drops. So let me explain it real quick using a human being as an example.

Imagine you are moving and there is a truck full of your stuff standing in front of your new house that you have to unload.

First we must explain the difference between energy and power.

ENERGY
It’s gonna take a lot of hard work for many hours to unload that whole truck and you will probably have to get something to eat along the way to replenish your own energy stores. The total amount of work it takes to unload the truck is called energy.

POWER
Power is how fast you can do the work needed. For example: if you would be twice as strong and could carry two boxes at the same time, unloading would go faster. Or if there was two of you, unloading would go twice as fast. Because together you would have twice as much power. So power is about how fast you can use energy.

I hope you now have a feeling for energy and power. Energy is what it takes to unload the truck. Power is how fast you can do that. Now let’s introduce a battery.

Imagine you have two robots that can do the unloading for you.

Each robot can do a certain amount of work that is determined by the size of the battery. When you talk about battery size that way, it’s the same as talking about battery ENERGY. And that size/energy is expressed in kilowatt-hour or kWh. For example, a nice size for a robot would be a battery of about 10 kWh.

Now imagine it would take the robot 10 hours to unload the van. And at the end of that the robot would be completely empty. In that case it would have used 1 kWh of energy per hour. 1 kWh per hour is the same as 1 kilowatt (kW) of POWER. So if you use 1 kilowatt or kW of power for 10 hours, you have used 10 kWh of energy.

If you used both robots, things would go twice as fast. And both robots together would have a power of 2 kilowatt. If they use that for 5 hours, they would use 10 kWh. So two robots for 5 hours use the same amount of energy as 1 robot for 10 hours.

Back to grid operator batteries

Grid operators are used to talking in power. They express everything in kilowatt (kW) or for higher power levels megawatt (MW). That’s how they’ve always done it.

Now along comes this new thing: the battery. So imagine someone has bought a big battery of 100 megawatt-hour (MWh) and wants to connect it to the grid. They contact the grid operator and say: “I want to connect a battery to the grid. It’s a large one. 100 MWh.”

To which the grid operator answers: “Sorry, but that’s now how we talk. We express everything in power. In MW. How many MW is your battery?”

And for the grid operator this does make sense. Because they want to know how fast you can charge or discharge this battery. That determines how big the power cable needs to be that’s going to be attached to your big battery.

Now you know that your battery cells can be charged to one fourth of their charge in one hour. People who know about electronics call this the C rate. If you can charge one fourth of the battery in an hour, the C rate is 0.25. If you charge one fourth of an 100 MWh battery in one hour, you need 25 MWh per hour and that’s the same as 25 MW of power. You tell this to the grid operator.

Now the grid operator replies: “Aha! So you have a 25 MW, 4 hour battery.”

You and I probably think that this is still a 100 MWh battery and the fact that it can be charged or discharged in 4 hours doesn’t change that. But at least we are now speaking the same language as the grid operator.

It also makes clear why it’s an error to think that a 4 hour battery can only cover energy demand for 4 hours as many people claim. A 4 hour battery really means nothing else than that it takes a minimum of 4 hours to charge or discharge, but of course you can charge it longer and slower.

I hope that for a few people reading this, it will have cleared up some confusion. And I hope you share it with people who are confused.