WHAT IS SOLAR ENERGY?

Let’s start with a quick technical reference. The Sun is basically a massive ball of hot gases. Just like probably all stars, it consists mainly of hydrogen and helium. High pressure and temperature in the Sun's core cause hydrogen atoms to break up. Their nuclei then combine forming helium nuclei. This process is called nuclear fusion. In this reaction, the resulting atoms have less internal energy than the starting particles. Since energy is conserved, the balance of it is released in the form of heat, photons and other particles. The photons are constantly intercepted, absorbed and re-emitted by surrounding molecules. Eventually they reach the surface and emitted to outer space. Technically speaking, this emission is electromagnetic radiation. It has a dual nature as both particles and waves and can transport energy through empty space.
This is what is casually called solar energy. It can naturally convert directly into three usable forms of energy: electric, thermal, and chemical.

SCIENTIFIC SOLAR ENERGY FACTS.

What I am going to tell you may sound a bit too technical, but this is what you might want to know if you are considering to buy a home PV system and want to know exactly how much electricity it can produce for you. The scientists estimate the amount of power radiated from the Sun is approximately about 63,000 kW per square meter of its surface. The intensity of this emission drops with the squared distance from the Sun as the sphere of its radiation is expanding. An average radiation density at the top Earth's atmosphere is called solar constant. There is some controversy about its exact value. According to National Institute of Standards and Technology (NIST) solar constant is 1366 watt per square meter, and according to NASA it is 1361. If we take into account the fact that only half the Earth is lit at one time and average the total amount of the irradiance intercepted by the Earth over its entire surface area, we'll get approximately 341 watts per square meter. Of this amount the atmosphere and clouds reflect back some 26% and absorb 19%. Overall about 55% of the incoming radiation is reaching our planet's surface. Of course, the sunlight does not fall on the entire planet evenly. Its actual intensity varies by time and latitude. The irradiance is largest near the equator, where on clear day at noon at sea level it is close to 1000W per square meter. The solar industry casually refers to this value as the "standard sun".
It is customary used in rating of photovoltaic energy panels, although outside of equator you may rarely get this amount of sunlight. In any case, the fact is "standard sun" is just potential peak irradiance at noon into a surface that directly faces the sunlight. It is not the actual condition under which your photovoltaic system will be working during entire day. As Sun moves in the sky, the irradiance per square meter obviously drops because the same wattage is now spread over a larger area (see the diagram). If we denote the density of radiant energy into a perpendicular surface by E (watt/m²), then the density on a horizontal surface is E×cosZ, where Z- is zenith angle. Zenith angle by definition is the angle from a vertical line to the Sun's position in the sky. It is a function of latitude, time of year, and time of day. Note that E<1000W/m² and cosZ<1. Since the incident radiation varies due to many factors, a more useful characteristic of sunlight is so-called net insolation (which is abbreviation for INcident SOLar radiATION). It is a measure of the total amount of solar energy over an entire day for a square meter on a given surface. Insolation is usually expressed in kilowatt-hours per square meter or in equivalent "sun-hours" per day. Both measures are numerically the same because one sun-hour is 1 kw-h/m². Since daily insolation varies with weather conditions, geographical locations and time of the year, it is often averaged over a certain period of time, usually a month or a year. Various organizations maintain databases and maps of mean yearly insolation for most locations worldwide. In US this value varies typically from 4-5 hours in Northeast to 5-7 hours in Southwest. If for example, a particular area has yearly average of 5 sun-hours per day, it gets mean energy per day of 5 kW-hours/sq.m. If you spread this amount over a 24-hour period, you would get 5000W/24=208 W/sq.m. Note that only a small portion of this incidental radiation can be converted to electricity due to a not very high energy efficiency of solar panels. Given the fact the best PV panels under optimal conditions have efficiency η<20%, in our example they would produce daily less than 1kWh/sq.m. If you can store this electricity in batteries and use it evenly over an entire day, you would get less than 1000/24=41.6 watt/sq.m (see solar panel calculator).

SUMMARY.

In conclusion, here is a summary of true solar energy facts.

• Peak radiance at noon on a perpendicular surface: 1 kW/sq.m;
• Insolation (average net sunlight energy over a day per square meter): 4 to 7 kWh depending on your location and PV panels angle;
• Total electricity generated by a PV array over a day: E_pv=Insolation×Efficiency/100 kWh/m², where panel's efficiency is from 6% to 20%.
  For example, for the best cells efficiency of 20%, the daily yield would be 800 to 1400 watt-hours per sq.m.
• Power generated by the most efficient panels averaged over a day: P_pv=E_pv/24, which is 33 to 58 watt/m².
  Note that the above numbers relate just to DC output of PV panels. In a complete system there will be additional energy losses of 3 to 10% in the inverter and another 3-5% in the wiring. So, the resulting amount of solar energy will be 6 to 15% lower.
• Electricity generated over a day by a PV system per each kW of nameplate DC power ≈0.8×Insolation, where 0.8 is a factor that accounts for losses in inverter and wires.

Note that 1 sq.ft=0.0929 sq.m. So, if you want the numbers per square foot, as a rule of thumb, they all will be 10 times less than the respective numbers per square meter.

QUICK REFERENCE INFORMATION AND FACTS ABOUT SUN

Diameter: 1,392,000 km (863,040 miles);
Mass: 1,989,100×10²⁴ kg;
Temperature at surface: ~5700 oC;
Average Earth-Sun Distance: 150 million km (93 million miles);
Content by mass: 74% Hydrogen, 25% Helium, 1% other;
Luminosity (total amount of power radiated in all directions): 3.85*10²⁶ watt (~385 billion megawatts);
Radiated power density at Sun's surface: 63,300 kW per square meter.

References and additional information:

US solar energy resource interactive atlas
Earth radiation balance and photovoltaic basics