One unit confusion explains more bad energy reporting than almost anything else: the difference between power and energy. For solar and wind, the headline number is power — megawatts. For batteries, the number that actually matters is energy — megawatt-hours, or at scale, gigawatt-hours. Tesla's Form 10-K for fiscal 2024 reports its storage business in energy terms, and that choice is not arbitrary.
The analogy that makes it click is a water tank. Power (megawatts) is how fast water can flow through the pipe at any instant. Energy (megawatt-hours) is how much water the tank actually holds. A fat pipe with a tiny tank empties in seconds; a modest pipe on a huge tank runs for hours. For a battery, the pipe is its power rating and the tank is its energy capacity — and what makes a battery useful for the grid is how long it can keep delivering, which is the tank.
That is why 'a 100 MW battery' is an incomplete sentence. A 100 MW battery that holds 100 MWh runs at full power for one hour. A 100 MW battery that holds 400 MWh runs for four hours. Same power rating, very different machines — and the four-hour system is far more valuable for shifting solar from midday into the evening peak. The duration is the energy divided by the power, and you cannot know it from the megawatt number alone.
This is the reasoning behind how Tesla reports its energy storage products — Powerwall for homes and Megapack for the grid. The company describes its storage deployments in energy terms, because gigawatt-hours deployed is the figure that captures how much actual storage capability was added to the world in a period. A megawatt figure would tell you the instantaneous power but hide the thing customers are buying: the capacity to store and shift energy over time.
For anyone reading the storage market, the practical rule follows directly. When a deployment is quoted only in MW, treat it as half a number and ask for the MWh or the duration. When it is quoted in GWh, you are looking at the metric that reflects real storage added. The energy figure is the honest one because energy — not power — is what a battery is for.
To see how Tesla itself frames and quantifies its storage deployments, the annual report is the source. The filing — retrievable through tools such as EdgarBeast — is where the energy-storage segment and its deployment figures are reported directly, rather than in a rounded headline.