The main argument in favor of Coal power plants is that they provide a stable electricity output, while having a low operating cost. However, coal generation would be best described as inflexible rather than stable. These power plants cannot adjust their output according to demand, so they must be complemented with flexible generation from natural gas or hydroelectricity.

On the other hand, a common criticism against Solar and wind power is that we cannot count on them because they depend on sunshine and wind. However, a similar argument applies against coal: its inflexible generation makes it unreliable when grid demand is variable.

While it is true that coal power is cheap, the comparison between coal and renewable sources like solar power is not always fair:

• Many coal power stations are already decades old, which means investors recovered their investment a long time ago.
• If we compare new solar and wind farms with new coal power plants, the business case tends to be better for renewables.
• Also consider that many coal plants delivering cheap electricity are reaching the end of their service life. For instance, the Liddell coal station will be replaced with a mix of renewables, gas power plants and energy storage.
• Some solar and wind power projects have beaten the price of existing coal generation. One example is an energy auction carried out in late 2017 in Colorado, USA.

Why Rooftop Solar Makes Coal Less Attractive

As mentioned above, coal power stations have an inflexible energy output, which means they need a constant power demand to be viable. Traditionally, coal generation has been used to serve the base load, which is the load that is always present in a power network. Natural gas and hydroelectricity have been the preferred generation systems for peaks in demand.

However, now that homes and businesses are deploying solar power, the concept of the “base load” is being challenged:

• Solar generation peaks around noon, and many buildings with photovoltaic systems have surplus production for several hours. As a result, net load on a power network can reach an extremely low value around noon.
• When distributed solar generation capacity is high in a region, the potential load that can be served by coal around noon is low. In a few words, rooftop solar power and coal are in direct competition to supply electricity around noon – neither can be ramped down to make space for the other.

Batteries can be used to balance supply and demand in a power grid, and coal proponents sometimes argue that batteries can compensate the inflexible output of coal power. However, it makes more sense to combine batteries with solar arrays, and get two additional benefits:

• Solar generation is now cheaper than coal generation.
• Photovoltaic systems produce no emissions during operation, while many coal-fired units produce more than 1 kg of carbon dioxide for every kWh generated.

Another advantage of solar power is project scale flexibility, while coal is only viable in large-scale power plants. Solar power can be distributed across many buildings, reducing dependence on the electricity network. On the other hand, coal stations depend on individual power lines that can be knocked out by a system fault or by harsh weather.

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