THE BASIC FACTS ABOUT SOLAR ENERGY
Renewable forms of energy have become increasingly popular due to government incentives and mandates, volatile fuel prices, and environmental concerns. Solar power is probably one of the most prevalent topics being discussed in a variety of media. However, many websites related to this topic just repeat the same general words about it and do not offer practical information that would let people understand the capabilities and limitation of this form of electricity generation. This page provides you with the hard reference information and facts about solar energy. It can help you better understand what you can realistically expect from it and make an informed decision about investing in a home PV system.
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THE SOLAR ENERGY FACTS
Here are the main characteristics of sunlight.
Irradiance (average solar power measured at the top Earth's atmosphere perpendicular to the sun's rays): 1366 watt per square meter (or 1361 according to NASA).
Note that there are more than 10 square feet per square meter. Therefore, if you want to get the numbers per sq. foot, they will be about 10 times less than the respective numbers per sq. meter.
"Standard sun" (peak radiated power that reaches the Earth at the equator at a bright noon per square meter at sea level): 1000 watt/sq.m, which is 1 kW/sq.m.
This value is customary used in the rating of PV systems.
Here and below all the numbers are given for surfaces optimally tilted toward sun according to the latitude. For horizontal surfaces you would get less sunlight: the further from the equator the lower solar energy density.
Insolation (the average amount of equivalent hours of "standard sun" per day): from 4-5 sun-hours typical in Northeast of US to 5-7 hours in Southwest. Insolation is often stated in kW-hr, which is numerically the same since the "standard sun" is 1 kW. For more details see Solar Energy Information.
Total amount of sunlight's radiated energy per day per sq.m at sea level:
(Energy per Day)=1kW×(insolation in hours).
Given an average US insolation of 5 sun-hours, this amount is typically 5 kilowatt-hours/sq.m.
Solar watts averaged over an entire day:
Wattsaverage=(Energy per Day)/24.
For an insolation of 5, the watts averaged over an entire day are 5000W/24=208 W/sq.m. Note that only a small portion of this incidental energy can be converted to electricity due to a not very high efficiency of PV systems.
TYPICAL CAPABILITY OF PHOTOVOLTAIC SYSTEMS
Average efficiency of commercially available PV panels: crystalline silicon (CSi)- 12-17%; thin-film (amorphous silicon and other materials)- 8-12%.
Watts per square meter generated from a PV array is:
PVwatts=(Solar watts)×(PV Efficiency), where the efficiency is stated in decimal.
Particularly, peak wattage from a PV module at a bright noon:
PVwatts-peak=1000W×Efficiency, which is typically 120-170 W/sq.m for CSi and 80-120 W/sq.m for thin-films (TF).
Total energy per day produced in average from a PV module per square meter:
PVday= PVwatts-peak×(Insolationhours).
For an insolation of 5 sun-hours this value would be typically 0.6-0.85 kWh/sq.m for CSi and 0.4-0.6 kWh/sq.m for TF .
PV output averaged over an entire day:
PVwatts-average=PVday/24. This is about 25-35 W/sq.m for CSi and 17-25 W/sq.m for TF.
Total energy generated by PV modules per sq.m during a year:
PVyear=(Total Energy per Day)×365, which would be about 219-310 kWh for CSi and 146-219 kWh for TF. Note that inverters have efficiency 95-97%, so actual electricity generation will up to 5% less.
Expected cost of electricity saved over a year per square meter of PV panels: Saving=PVyear×0.95×(Utility Rate), where 0.95 factor takes into an account inverter efficiency and wiring losses.
At an average US rate of $0.12/kWh, this yields $24-35 for CSi and $17-24 for thin films. So, at best you would save $35 a year per 1 sq.m of the panels. These numbers assume you have a highest efficiency system with a rated power of 170W per sq.m. Given the fact that currently an average PV system installation cost is $8,000 per 1000W, such system would typically cost [before rebates] 170/1000×$8,000=$1,360 per sq.m installed (see Solar system cost calculator). This means, that in our example which is based on typical average conditions, the hypothetical payback time would be 1360/35=39 years. In practice, of course no equipment would work that long. The rebates and credits may cut this time by more than a half, however for the society in general the intial cost of an average home PV system would probably never be recovered. Of course, this is just an example. In the areas with a different insolation and with different installation costs, the payback time may be higher or lower.
QUICK FACTS ABOUT THE 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.
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