What is a major disadvantage of using solar power?

When you need power, you cannot produce electricity if there is no sunshine.

Another disadvantage is due to reflectance from mirrors or panels, some birds might die.

One more example can be given from landscape feature. Suppose originally this land was used for agriculture. Now, you installed solar energy power plant. You decreased area used as agricultural land. Now, there is less production (due to decrease in agricultural land) and higher price for a specific product will be valid. This is a dilemma of whether agricultural lands should be protected to make sure that agricultural activity will be taken place or you are "100%" sure that energy is the most important issue.

One major disadvantage of using solar power is its dependence on sunlight, making it less reliable during periods of cloud cover, nighttime, or inclement weather.