MW is the engine, MWh is the distance driven — how electricity's power differs from its amount

"100MW solar farm completed." "Work begins on 100MW gas power plant." The two headlines share a number. But the electricity the two plants make over a year is very different. That is because the number does not say "how much is made." It says "how much can be put out at a single moment."

Know just this one difference, and energy news reads quite differently. In letters it is a single h; in meaning it is the difference between "a moment" and "an amount piled up."

How are MW and MWh different?

In car terms, MW is engine power and MWh is the distance actually driven. Even a powerful car has driven 0 if it sits in a parking lot.

MW (megawattMW and MWh MW (megawatt) and MWh (megawatt-hour) are kW and kWh scaled up to the mega unit. They are used for large values such as plant size or power market volumes. Glossary →) is the size of the power a generator can put out at a single moment. MWh (megawatt-hour) is the amount of electricity made with that power over a stretch of time. 1 MW is 1,000 kW, and running a 1MW generator for a full hour gives 1MWh, that is, 1,000 kWh.

The unit printed on an electricity bill is the kWhkW and kWh A kW (kilowatt) is a unit of power: how hard electricity is made or used at a moment. A kWh (kilowatt-hour) is a unit of energy: how much is made or used over time. Glossary →. Take a household that uses 300kWh a month (an assumption): 1 MWh is the electricity three such homes would use in a little over a month. A 1MW generator makes this amount in an hour.

How much can 1 MW make in a year?

A year has 8,760 hours. A 1MW unit running at full output all year without a break makes 8,760 MWh. That is the ceiling on the electricity 1 MW can make in a year. Real generators do not reach this ceiling. They stop for maintenance, run less when demand is low, and solar stops when there is no sun. The share of that ceiling actually made is the capacity factorCapacity Factor Capacity factor is the ratio of how much a plant actually generated to the maximum it could have produced. Plants of the same size can deliver very different output depending on their capacity factor. Glossary →. The table below counts traded volume on the market rather than the amount made.

If a shop that could be open 24 hours a day served customers for only 8 hours, its capacity factor that day is 1/3. Even with the same size shop, different opening hours mean different sales. Power plants are the same.

How much was actually made?

Working out 2025 capacity factors from the installed capacity and traded volume in KPX's (the Korea Power Exchange's) *2025 Electricity Market Statistics* gives this. A GWh (gigawatt-hour) of traded volume is 1,000MWh.

For the same 1 MW, the electricity traded on the market in a year was close to six times as much for nuclear as for solar. Read the gas power plant from the opening headline against the LNG row of the table (the average of the existing LNG plants in that table). A new plant may run differently.

Is solar a lazy generator?

From the table alone, it looks that way. But it is not. Capacity factor is not a ranking of good and bad; it shows the place each generator holds in the power system. Nuclear takes the role of running on without stopping. LNG often takes the role of switching on and off in the hours when demand is high. Solar makes power only in the daytime, while the sun is up.

At the end of 2025, the plants taking part in the power market, including sources not shown in the table above, added up to 146,157MW, about 146 GW (gigawatts; 1 GW is 1,000 MW). That is more than five times all the nuclear capacity. The electricity traded in the market that year was 547,163GWh (provisional). That is less than half of what all the capacity would have made running at full output all year (capacity factor about 43%). There is a reason for so much capacity.

Electricity is hard to store, so there must always be enough plant to get through even the single hour of highest demand. It is like keeping seats free for the busiest moment, which comes for only a few hours a year. The size of the plant fleet is set not by normal times but by the single busiest moment.

How many homes is 100 MW?

Let's go back to the 100MW solar farm in the opening headline. Take a household that uses 300kWh a month (an assumption).

• Electricity sold on the market (traded volume) by 1 MW of solar in 2025 → about 1,150MWh
• A 100MW farm → about 115,000MWh = 115 million kWh
• Electricity one household uses in a year → 300kWh × 12 = 3,600kWh
• 115 million ÷ 3,600 → about 32,000 homes

Do the same sum with 100 MW of nuclear and you get about 187,000 homes.

But this only sets the amount made over a year against the amount used over a year. Solar makes nothing at night, so it does not mean "it can supply power to that many homes at any time."

What should you watch for when handling these numbers?

If you handle these numbers in a report or a contract, it helps to check three things.

• Capacity is a value at one point in time, such as "as of year-end," while traded volume is a value added up over a year. As in the table above, dividing traded volume by "capacity × 8,760 hours" gives a value close to the capacity factor. But if a lot of new plant was built partway through the year, the result comes out lower than the real figure, because that plant counts toward capacity for the whole year but ran for only part of it.
• The 2025 traded volume in the table above is provisional. When the final figures come out, the capacity factors may change a little.
• "Traded volume" is the amount bought and sold in the power market. Electricity used without going through the market is left out.

So when you see a "100MW" story, what do you check first?

If a story gives only "MW" and no "MWh" or "annual generation," check the type of power plant first. Even for the same 100 MW, nuclear and solar make close to six times different amounts of electricity in a year. To convert units yourself, try the unit converter.

The same 100MW splits into 32,000 homes' worth if it is solar and 187,000 homes' worth if it is nuclear. Then one more question comes up. Nuclear capacity is smaller than LNG capacity, so why does it make more electricity? The answer is in *Nuclear makes it, LNG sets the price*.

Words in this piece

• MW · MWhMW and MWh MW (megawatt) and MWh (megawatt-hour) are kW and kWh scaled up to the mega unit. They are used for large values such as plant size or power market volumes. Glossary → — MW is the power that can be put out at a single moment; MWh is the amount of electricity made with that power over a stretch of time.
• kW · kWhkW and kWh A kW (kilowatt) is a unit of power: how hard electricity is made or used at a moment. A kWh (kilowatt-hour) is a unit of energy: how much is made or used over time. Glossary → — the smaller units of the same pair. A household electricity bill counts in kWh.
• capacity factorCapacity Factor Capacity factor is the ratio of how much a plant actually generated to the maximum it could have produced. Plants of the same size can deliver very different output depending on their capacity factor. Glossary → — the ratio of electricity actually made to the ceiling of running at full output all year.
• peak demandPeak Demand Peak demand is the highest level of electricity demand during a given period. KPX's rules call it 'peak load' and measure it as the largest of the hourly average values. Glossary → — the size of the single hour of the year when the most electricity was used. Plants are kept ready to match this moment.