When Pakistan has 20,000 MW of production capacity, than it means, if all producing units are working to its maximum than Pakistan would have 20,000 MW output at that time. Well, electricity cannot be stored, so it has to get used. If there are consumers who want this amount of electricity all the time (24 hours, 356 days), than total production of electricity would be 175.2 billion KWh of electricity. (Note: 175 billion KWh … that is = 24h x 365 x 20000 MW).
Obviously, Pakistan do not produce that much electricity neither Pakistan consumes 20,000 MW at any moment. Few months ago (during last period of previous govt), Pakistan was producing 15000 MW (peak output), but today, Pakistan is producing 10,000 MW (peak output).
In 2007, Pakistan produced around 100 billion KW of electricity. Normally for Pakistan, electricity production is around 15 percent more than electricity consumption … that extra production is line losses and electricity theft. Now, it does not mean that electricity consumption is actually what Pakistan needs, as occasionally due to load shedding or unavoidability, consumption is lower.
Anyhow, country electricity consumption has its own peak and low. Installed capacity when fully operative should satisfy peak demand, as for low, that should not be problem. I think that Pakistan peak demand can be fulfilled with 20000 MW installed capacity if properly managed, though things would become easier if 2000 MW more capacity is installed, as most of the time, all units are not working and obviously dams cannot produce electricity when there is less water (even though they are part of installed capacity).
How much Pakistan is expected to need by 2016? Well, it all depends on who is in power. If corrupts like NS and Zardari would be in power then most likely there would be no or little growth in economy, and that would mean slow or maybe no growth in electricity demand. Possibly, Pakistan might need lower electricity in future if economy go completely down the drain.
In 1999, Pakistan had huge excess electricity generating capacity and it was expected that Pakistan would not go short of installed capacity for next 15 years. I think that installed capacity in 1999 could be around 17500 MW … with potential to produce around 155 billion KWh of electricity, though Pakistan was producing around 60 billion KWh and was consuming around 54 billion KWh. Probably, maximum demand at any time was no more than 9000 MW and rest of capacity was waste of resources (Pakistan was obliged to buy all generated electricity by IPP at high price, regardless of Pakistan need it or not). Pakistan was in position to export electricity … obviously at loss as excess electricity produced by IPP was mostly generated by fossil fuel.
Result was that Musharraf government did not increase installed capacity substantially (maybe increased around 2500 MW, almost all of them nuclear and hydro), though production of electricity kept increasing, from 60 billion KWh in 1999 to 100 billion KWh in 2007, and consumption of electricity also kept increasing from around 50 billion KWh to around 90 billion KWh. Such rapidly increasing demand due to very fast economical growth during last 8 years caused shortage of electricity during peak hours, and thus now country is thinking to increase electricity installed capacity substantially. It does not mean that peak demand has reached 20000 MW even now, but then since all units are not operative at all time, shortage do happen even if peak hour demand reaches 16000 or 17000 MW.
If 16000 MW would get added to national grid, then theoretically it would be able to cater 36000 MW of peak hour demand, but practically it might be able to cater around 30000 MW, depending on type of generating units. But, it is uncertain if demand is going to increase to level that would justify such increase in electricity generating capacity, as all would depends on how the economy would do in future.
