The capacity of power generation equipment is normally rated in kilowatts, or megawatts when dealing with large buildings and utility-scale projects. However, you cannot compare different generation technologies based only on wattage, since their operating conditions differ. In fact, a comparison based on wattage can lead to incorrect conclusions.
Solar photovoltaic arrays and wind turbines have a rated wattage that is based on predetermined test conditions, but their actual output changes depending on sunshine and the wind. On the other hand, fossil fuel power plants can sustain a constant output, but they require paid inputs such as coal and natural gas.
A common criticism against solar panels and wind turbines is that their power output cannot be sustained. However, this statement overlooks two important facts:
Also consider the rapidly declining costs of renewable generation. In many parts of the world, solar panels and wind turbines can now beat the kilowatt-hour price of fossil fuels.
If you have a coal-fired power plant producing 100 MW permanently, the yearly electricity output is 876,000 MWh. On the other hand, a photovoltaic array with a rated output of 100 MW (AC) has an output that is determined by sunshine:
At a glance this seems like a disadvantage, since you need over four watts of solar generation to displace one watt of coal power. However, if you compare the kilowatt-hour price, a new solar array will generally beat a new coal-fired power station.
The capacity factor of an electricity generation system describes the fraction of its maximum output achieved over a specified time period:
With solar power, you can expect a capacity factor above 20% with a good site. In the case of wind power, a suitable site will likely result in a capacity factor above 40%. You would require permanent sunshine to get a 100% capacity factor with a solar power system, but that is only possible in outer space.
Fossil fuel power stations can approach 100% capacity factor if their fuel input is always available, but this is not necessarily an advantage. You must also consider kWh costs with each technology, and the possibility of using batteries to compensate for the intermittent output of renewables.
One megawatt of solar power does not replace one megawatt of coal-fired generation. However, when you compare the kWh cost, solar power is often the best option. Also consider that photovoltaic arrays have no emissions from power generation, and they can become a dispatchable electricity source when enhanced with batteries.