The earth is about 200 Billion square miles and converting only 20% of the sun's light 24-7 from 1/4 of its surface is equivalent to the energy gotten from 10 Billion square miles constantly lighted and 100% converted -- not that much; but it represents enough calories of energy to feed six billion people (our current population) about seven million times every single day.
Seventy percent of earth's surface is ocean, however even some of the light striking the ocean can still be converted for our use. Overall, I'll guess that 20% of the sun's energy on average can be harvested from any given spot (only half the ratio we currently get with the latest electric solar panels). To make up for Earth's rotation of day & night, I only calculate using a fourth of the available light, assuming a mere 5% of all sunlight on Earth is converted for human food production. (At least 1/4 of the earth is getting some sunlight at all times. That is probably less than the true fraction, but I am limited to rough calculations.) Parts of the earth get less light and others more. The rest of the sunlight continues to warm, brighten the earth, invigorate the weather, and run machines.
The Wikipedia says "The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet." It also says about half of the total 174 Petawatts strikes the ground. (
Solar energy - Wikipedia, the free encyclopedia)
For simplicity: I will take Wiki's number of 175 petawatts & half it to 87.5 Petawatts that it suggests can reach the ground, then take only 5% of that. I get 4.376 Petawatts. 4.376 Pettawatts all day every 24 hour day converted to Joules is (4.376 x 10^15 Joules/sec) x (86,400 sec/day) or 3.78 x 10^20 Joules of pure energy
every day. A food Calorie = 4.1868 Joules, so we're talking about 9.03 x 10^19 Calories available per day to feed people. Going by the USDA food standard of 2,000 Calories per day per person, that is enough calories to feed 4.51 x 10^16 human adults.
I've heard Astronomers say the Sun has enough mass to continue shining like this for 5 billion more years.