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Explainer · Energy

Is a megawatt the same as a megawatt-hour?

Here's the difference, and why it matters for power headlines.

Updated November 1, 2026 · Numbers as published by each source on the date shown

No. A megawatt (MW) is how much power a plant can put out at one moment. A megawatt-hour (MWh) is how much energy it actually makes over time. EIA puts it simply: a 1 MW generator running at full capacity for one hour produces 1 MWh. Two plants with the same MW rating can make very different amounts of MWh in a year.

Power and energy, in plain terms

Think of a garden hose. Megawatts are how wide the hose is, the most it can deliver at once. Megawatt-hours are how much water actually came out over the afternoon. The US Energy Information Administration (EIA) defines generation capacity as the maximum electric output a generator can produce under specific conditions, and generation as the amount of electricity a generator produces during a specific time period.

EIA's own example: if a 1 MW generator runs at full capacity for one hour, it produces 1 MWh. If it runs at only half that capacity for one hour, it produces 0.5 MWh. A gigawatt (GW) is 1,000 MW.

How hard plants actually run

EIA measures how close a plant comes to its full output with a capacity factor: actual output divided by the most it could have made in that time. For 2025, EIA reports these averages for utility-scale plants:

Here is what that means for the same 100 MW rating over a year of 8,760 hours. At solar's 24.0%, it works out to about 210,000 MWh. At nuclear's 91.1%, about 798,000 MWh. Same headline size, close to four times the energy. Both are useful: solar does its work at midday when demand is high, and the grid blends sources to cover every hour.

Why batteries need both numbers

A battery has a power rating in MW and an energy rating in MWh. EIA calculates a battery's duration by using the ratio of energy capacity (MWh) to power capacity (MW). A 100 MW battery that stores 400 MWh can run at full power for 4 hours.

EIA says batteries with a duration of less than two hours are considered short-duration and mostly provide grid services that keep the system stable. Batteries with a duration between four and eight hours are typically cycled once per day, often storing midday solar power and sending it out in the evening peak.

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What is being added

US battery storage is growing fast. EIA reports the US had 43.6 GW of operating utility-scale battery capacity at the end of 2025. Operators added another 8.3 GW in the first six months of 2026, reaching nearly 52 GW. They plan to bring about 54 GW more online over the next two and a half years.

Those are MW figures. They tell you how fast the batteries can discharge, not how many hours they can run. When a headline gives only one number, check which unit it is.

The units in one line each

What to watch next

EIA's next Electric Power Monthly comes out November 24, 2026. When you see a new plant or battery announced, look for both the MW and the MWh, or the expected capacity factor, before comparing it with another.

The grid queue is also measured in capacity; see why 2,000 GW in the queue is not 2,000 GW coming. For another headline number people misread, see how the US pumps record oil with fewer rigs. Follow US energy on the live dashboard, and find past Monday Haul issues in the archive.

Common questions

What is the difference between a megawatt and a megawatt-hour?

A megawatt (MW) is capacity, the most power a plant can put out at one moment. A megawatt-hour (MWh) is energy, the amount produced over time. EIA gives the example that a 1 MW generator running at full capacity for one hour produces 1 MWh.

Does a 100 MW solar farm make 100 MW all the time?

No. US utility-scale solar panels had a capacity factor of 24.0% in 2025, according to EIA, because output depends on sunlight. Nuclear plants ran at 91.1%.

What does a battery rated in MW and MWh mean?

The MW figure is how fast it can discharge. The MWh figure is how much energy it holds. EIA calculates duration by dividing MWh by MW, so a 100 MW, 400 MWh battery can run at full power for 4 hours.

How much battery storage does the US have?

EIA reports 43.6 GW of operating utility-scale battery capacity at the end of 2025 and nearly 52 GW after 8.3 GW was added in the first half of 2026.

Sources