Megawatt-hours is simply megawatts multiplied by hours, if the power of the generator stays constant at full power. To get megawatt-hours from megawatts, you need to know the duration. Usually in financial reporting, megawatt-hours means megawatt-hours per year actually, but it could mean megawatt-hours per quarter for example. And when comparing different months, megawatt-hours per month is a common unit.
An example: if you have 500 megawatts of generators, for a year it would be 24*365*500 = 4380000 MWh which is so large number that you should switch to GWh: 4380 GWh.
It’s also possible that the generators aren’t running at full power all the time. This is taken into account by capacity factor. For example a peakload natural gas generator could have 15% capacity factor so in that case it would be 0.15 * 4380 GWh = 657 GWh.
For solar power, capacity factor varies based on average cloudiness and latitude. In sourthern Finland for example, averaged over one year, common capacity factors are 11-12%. But that doesn’t tell the whole story as solar in Finland generates lots of energy during summer and practically no energy during winter.
So 500 MW of solar in Finland would be 24*365*500*0.115 = 503700 MWh.
Bit older onshore wind power is 30% capacity factor when choosing a good site (a bad site can ruin production), today capacity factors are exceeding 35% for the newest turbines and good sites, and good offshore wind power site with newest turbines is expected to have 45-50% capacity factor.
Older non-pumped hydro has usually 50% capacity factor, although it could be really anything depending on how many turbines are installed. But older hydro was intended not for peaking power but rather load-following power and for that 50% capacity factor is ideal. Newer peakload hydro installations that are intended to produce power only during shortages should ideally have 15% capacity factor, i.e. with same storage pool size and same rainfall they would have more generators than older hydro installations.
