Battery Power vs. Energy

When Tesla announced its Megapack, the spec sheet listed two numbers: 1.5 MW and 3 MWh. One measures how fast the battery can discharge; the other measures how long. Confusing the two leads to headlines that dramatically overstate or understate what a battery project can do.

A battery's power rating (MW) is its maximum discharge rate: how many megawatts it can deliver at any instant. Its energy rating (MWh) is total stored energy: how many megawatt-hours it holds when fully charged. The ratio of energy to power gives duration. A 100 MW / 400 MWh battery has 4-hour duration: it can discharge at full power for 4 hours before it is empty. A 100 MW / 100 MWh battery has 1-hour duration: same instantaneous power, but it runs out four times faster.

Match duration to application. Frequency regulation needs fast response but short bursts (minutes). A 1-hour battery works. Solar shifting needs 4 hours to bridge the evening peak. A 4-hour battery is the minimum.

Size a real project. A utility needs to replace a 200 MW gas peaker that runs 4 hours per evening. Required battery: 200 MW / 800 MWh.

Could a 200 MW / 200 MWh battery do the same job?

Check the duration. That battery can deliver 200 MW, but only for 1 hour. It covers one-quarter of the evening peak, then goes empty. Same power, insufficient energy.

Duration determines which grid problems a battery can solve, which is why every storage project is specified with both numbers.

---