Intermittent Output
Unfortunately we cannot (yet) control the weather. There are times when cloud cover obscures the sun (if you live in England, you'll realise this is most of the time) and the efficiency of solar panels drops. Also, quite clearly, solar panels will be completely useless at night.
Expense
While comparatively cheap to run, solar panels (especially solar PV) are expensive to manufacture. While the price continues to fall due to research into semiconductor technology, the cost per Watt is still much higher than for the other alternative energy sources.
In fact, this is one of the main disadvantages of solar energy, and it is slowing progress. Once the cost falls in line with that of oil and gas, I would expect to see a substantial increase in installed solar power.
Energy Density
Solar panels, at least at current levels of efficiency, need to cover a large area to produce a reasonable amount of energy. In fact, the bigger the better. However, as efficiency increases, land use requirements will drop (or energy output will increase with the same amount of land).
Not only that, but energy density increases with increasing latitude due to the curvature of the Earth. A solar panel in Canada will generate nowhere near as much energy as one the same size in Mexico.
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