Gigawatts to Megawatts: Reactor Unit Output Ratings

Splitting a Nuclear Station's Gigawatt Headline into Reactor Units

Press releases describe nuclear sites in gigawatts: a 3.2 GW project, a 5.6 GW station, a country's 60 GW fleet. Engineers and licensing documents work one level down, because the thing that is built, fuelled, refuelled and tripped is an individual reactor unit — and units are rated in megawatts. Moving between the two is the first sanity check on any nuclear number you read.

Conversion factor: 1 GW = 1,000 MW, so multiply gigawatts by a thousand. A station advertised at 3.26 GW is 3,260 MW, which resolves neatly into two EPR units of about 1,630 MW each — the sort of check that catches a misplaced decimal in a story.

Which Rating Are You Actually Reading?

Reactor units carry the megawatt rating

Design names encode it directly — VVER-1200, AP1000, BWRX-300 — and the licence, the turbine and the grid connection all follow that per-unit figure.

Sites are summed into gigawatts

Once several reactors share a site, the total crosses into gigawatt territory, and that is the figure quoted against national capacity or a competitor technology.

Thermal ratings are a different number entirely

A reactor's MWt figure is the heat the core produces; the MWe on the grid connection is roughly a third of it, so never mix the two in the same table.

Small modular designs restart the arithmetic

A plant built from 77 MW modules needs about thirteen of them to reach a gigawatt, which changes how a site total is presented and phased.

Checking a Station Announcement Unit by Unit

Whether you are fact-checking a press release or laying out a plant capacity table, the sequence is the same.

Put the announced station total in the left field

Enter 3.26, 5.6, 1.2 — whatever the headline says. Megawatts appear as you type, and a decimal comma is accepted, so a figure copied from a European source as 3,26 is read correctly.

Divide by the number of reactors on the site

5,600 MW across four units is 1,400 MW each, which matches an APR-1400. If the division gives an odd figure, the total probably mixes gross and net ratings, or counts a unit that is not yet operating.

Build a site total from unit ratings instead

Working the other way, press the swap button (↔) for MW → GW: six 77 MW modules come to 462 MW, or 0.462 GW, which is how a small modular project is usually compared with a conventional station.

Take the clean figure into your copy or spreadsheet

The copy button returns the number on its own — no unit, no separators — so it drops straight into a capacity column. Ctrl + C inside a field behaves the same way.

Gross and net are not interchangeable: a unit's gross rating includes the power its own pumps, fans and cooling systems consume, typically 3–5 % of output. Published station totals sometimes add gross figures while national statistics use net, which is why two credible sources can disagree by a hundred megawatts.

Reactor Designs and Their Per-Unit Output

Approximate net electrical ratings for reactor designs in service or under construction, with a plausible station configuration for each.

Reactor design	Type	Per unit (MW)	Station total (GW)
EPR	PWR, 2 units	1,630 MW	3.26 GW
APR-1400	PWR, 4 units	1,400 MW	5.6 GW
AP1000	PWR, 2 units	1,117 MW	2.234 GW
VVER-1200	PWR, 2 units	1,100 MW	2.2 GW
CANDU 6	PHWR, 4 units	700 MW	2.8 GW
Rolls-Royce SMR	Compact PWR, 3 units	470 MW	1.41 GW
BWRX-300	Small BWR, 4 units	300 MW	1.2 GW
NuScale module	Modular PWR, 6 modules	77 MW	0.462 GW

Read down the table and the reason for two units becomes obvious: the largest single reactor is a shade over 1.6 GW, so any site described in whole gigawatts must have more than one machine behind it. The modular designs at the bottom invert the relationship — it would take nineteen BWRX-300 units to reach the 5,600 MW that four APR-1400 machines deliver on one site.

Handy When You Are Auditing Plant Capacity Figures

Station and unit ratings tracked together

Type in either box and the other follows, so you can run a list of reactor ratings past a fixed station total without resetting the page.

Reverse to add modules into a site figure

The swap button runs MW → GW, the direction you need when a phased build adds one unit at a time and the site total keeps moving.

Reach kW and W for the station's own loads

The searchable dropdowns include the smaller SI steps, useful when house load or auxiliary equipment is quoted in kilowatts beside a megawatt turbine.

Decimals kept where the ratings need them

Results carry up to eight decimal places, so a 1,117 MW unit lands as 1.117 GW rather than being rounded into a figure that no longer adds up.

Reactor Rating Questions Journalists and Engineers Ask

Why is a station described in GW when its reactors are rated in MW?

Because the two figures answer different questions. A reactor's megawatt rating is an engineering fact about one machine — its turbine, its generator, its grid connection. The gigawatt figure is a site or portfolio total, meant to be compared against national demand or a rival project. A 3,260 MW site is two reactors to an engineer and 3.26 GW to a policy paper.

What is the difference between gross and net output on a reactor unit?

Gross is what the generator terminals produce; net is what leaves the site after the plant's own auxiliaries — reactor coolant pumps, feedwater pumps, cooling towers, ventilation — have taken their share. The gap is usually 3–5 %, so a unit quoted at 1,650 MW gross is often near 1,600 MW net. Grid statistics almost always mean net.

What do MWt and MWe mean on a reactor datasheet?

MWt is thermal power — the heat released in the core. MWe is electrical power delivered by the turbine-generator. A large pressurised water reactor running about 4,500 MWt produces roughly 1,600 MWe, a thermal efficiency near 33 %. Only the MWe figure belongs in a capacity total, and only that figure should be converted to gigawatts.

If a station is 5.6 GW, how much electricity does it actually deliver in a year?

Multiply by the hours in a year and then by the capacity factor — the fraction of full output achieved once refuelling outages and maintenance are counted. Modern nuclear fleets run at 80–92 %. At 90 %, 5.6 GW of capacity yields roughly 44 terawatt-hours a year. The rating alone tells you nothing about output without that factor.

How many small modular reactors make up one gigawatt?

It depends entirely on the design, and the range is wide. At 77 MW a module needs about thirteen units to pass 1,000 MW; at 300 MW it takes four; at 470 MW just over two. That is why the term covers everything from a 0.077 GW module to a machine a third the size of an EPR.

Gigawatts to Megawatts