A terawatt hour is a unit of energy. A terawatt, or a gigawatt, is a unit of power. To estimate how many TWh can be generated annually from 200 GW of solar capacity, we need two more numbers:

- The number of hours in a year.

- The percentage of nameplate capacity that the system can be expected to produce on average, e.g. the capacity factor.

There are 8760 hours in a year. A solar PV system in a sunny region may have an annualized capacity factor of 25%. (Installations in really sunny regions may achieve upward of 30%, and Saudi Arabia certainly has a lot of sun, but dust offsets some of the benefits of sunny deserts.)

200 * 8760 * 0.25 = 438000 gigawatt hours of electricity generated per year, e.g. 438 TWh, or about 2% of present annual global electricity generation of 21,000 TWh.

Watts measure power, Watt hours measure energy. A 200 GW power plant could output a maximum of ~1,700 TWh in a year which is about 8% of 21,000 TWh.