Assuming we consume ~20 TW on average, a metre-squared panel kicks out ~40 W on average, and we halve that to account for batteries and other infra... I reckon we're talking about 1 million square kilometres (people will be along in a sec to check my working, but it's just a Fermi estimate).

Call it 10% of the Sahara.

Bear in mind that if we go all-electric, raw energy consumption falls significantly, many panels will be sited on buildings, solar isn't the only renewable, and solar farms aren't ecological deserts - you can graze animals below them.

Honestly, seems like a good trade to me.