At the distance we are from the Sun, it beams down about 1kW per square metre onto a surface at right angles to its rays. The Earth is 12,700 km in diameter, so it receives about 1.25 x 1014kW of solar power.
Compare this with humankind’s global electricity consumption of about 2.5 x 109kW, or humankind’s global consumption of all forms of energy which is about 1.7 x 1010kW.
That is, the Sun is sending us about 7,000 times as much energy as we’re using, or about 50,000 times as much as our electricity consumption.
Of course we can’t cover the whole planet with solar panels, solar panels are only 10-15% efficient, most parts of the world spend part of their time under clouds, and 71% of the Earth’s surface is sea.
Nonetheless, the proportion of the Earth’s land surface that would have to be covered with solar panels to generate all our electricity is quite small; even if we changed to using electricity instead of other forms of energy it still would still be small. In fact, it would be comparable to the area of the roofs of our buildings.
Intermittency is an issue, of course, but not an insurmountable problem – see Wind & Sun: Intermittency. Utilizing other renewable energy sources, even intermittent ones, in addition to solar power, helps to minimize the issue. The really big one is wind.
The wind is driven by the heat of the Sun, so it’s indirectly solar power anyway. However, the sun’s heat anywhere contributes to the power of the wind. Even the heat released when the sun shines on solar panels contributes, so the solar panels make no difference to the availability of wind power.
Quantifying the energy that could be extracted from the wind is very much more difficult than quantifying that available from the Sun (difficult enough itself). However, some idea of the size of the resource can be guessed by looking at the output of existing wind farms and the areas covered by them. The UK currently generates about 6% of its electricity from onshore wind farms covering about 0.3% of the country. To generate all the UK’s electricity from onshore wind farms would require around 5% of the UK to be covered by them – not forgetting that within a windfarm, the turbines and associated infrastructure only occupy about 5% of the land; most can still be used for other purposes (generally agriculture). That is, only 0.25% of the UK would actually be taken up. Of course the scenery would be affected over a larger area. If we generated only half our electricity with onshore wind farms and half with solar panels, correspondingly less land would be occupied.
The UK currently gets about 3% of its electricity from offshore wind farms, which I’ve not taken into account at all.
The UK is one of the more densely populated areas of the world – but it’s also one of the windier areas. Nontheless, it is clear that the size of the wind resource is, like the size of the solar resource, many times as great as humankind’s electricity consumption.
(N.B. This page only deals with the size of the resource, not the cost of the infrastructure to access this resource. I deal with that elsewhere.)