On the way through the atmosphere the properties of the irradiation are slightly changed; e.g. the UV light is absorbed by the ozone layer, some parts of the IR are absorbed by water vapour and carbon dioxide. Depending on the latitude of the observer, the irradiation density is still lower due to the longer path of sunlight through the atmosphere, e.g. to below 1000 W/m2 in Central Europe.
The total irradiated energy per year, including seasonal changes, times of overcast sky, and night time, amounts to about 1000 kWh/m2*year in Central Europe (corresponding to 2 to 3 hours of ideal sunshine each day). The map below illustrates the worldwide distribution of the irradiation density.
The average electricity consumption of a 4-person family is about 4000 kWh, thus, solar energy has the potential to supply the average amount of energy, even in the moderate climate of Central Europe.
Two serious drawbacks arise: The storage and the required area. Daily and seasonal availability of solar energy is often contrary to the customers needs and not every family has access to 40 m2 for mounting their solar modules. Nevertheless, storage solutions are beeing developed and if existing buildings were equipped with solar systems a considerable amount of energy could be produced even without consumption of additional area.
T is the temperature (about 5800 K, see next section), r the radius of the sun (6.9.108 m) and σ the Boltzmann constant (5.67.10-8 W/m2K4) and ε is the emissivity of the surface (assumed to be one). The power is created by nuclear fusion processes inside the core. Due to Einsteins famous law E = mc2 about million tons of matter are converted to energy every single second!
The solar energy irradiated to the Earth is 5.1024 Joule per year. This is 10 000 times the present worldwide yearly energy consumption which is estimated to 5.1020 Joule (as of 2004). In the literature the energy consuption is frequently given in terms of Quads. The Quad is 1015 (one quadrillion) British Thermal Units (BTUs), where a BTU is1055 Joule (the BTU is defined as the amount of energy which increases the temperature of one pound of water by one degree Fahrenheit). Therefore, the annual global consumption is 500 Quads.