Solar heat number

http://hypertextbook.com/facts/1998/ManicaPiputbundit.shtml

Bibliographic Entry	Result (w/surrounding text)	Standardized Result
Tipler, Paul A. College Physics. New York: Worth, 1987: 316.	"The average energy flux at this distance on a surface perpendicular to the sun's rays is about 1353 W/m2	1.353 kW/m2
"The Sun." Encyclopedia Britannica. vol. 27. Chicago: Encyclopedia Britannica, 1998: 456.	"… it's radiative output, called the solar constant, is 137 ergs/m2/sec, or 1.98 cal/cm2/min"	1.38 kW/m2
Cowen, R. Science News. 152 (27 September 1997): 197.	"… the sun's output had climbed from 1367.0 to 1367.5 watts per square meter"	1.367–1.3675 kW/m2
Brooks, William O. and George R. Tracy. Modern Physical Science.New York: Holt, 1957: 566.	"We get energy from the sun at the rate of five million horsepower per square mile."	1.44 kW/m2
Rosner, Robert. MacMillan Encyclopedia of Physics.vol. 4. New York: Simon & Schuster, 1996: 1545.	"The most obvious solar effect on the earth is radiation, roughly 1.4 kW/m2 (the so called solar constant)"	1.4 kW/m2

1 joule = 1 watt of power applied for 1 second.
You will need 4,180 Joules of energy to raise 1 kg of water (approx 1 liter depending on temp)  by 1 degree C.


1 liter spread out across 1 square meter would be a layer of water 1mm thick.

Let's assume 0 evaporation and 100% solar absorption.

It would take about 3 seconds to raise the water 1 deg C.

But you fighting other forces:
* Evaporation (easy enough to solve with a sealed container)
* Emissivity  (re-radiating the energy back out)
* Reflectivity (reflecting the IR radiation back before absorbing)
* Solar Absorption (Amount of IR converted to head from solar radiation) this is seldom perfect
* Convection (heat radiating in to the air)

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For most materials absorption and emissivity of IR is usually the same for any given frequency.

http://en.wikipedia.org/wiki/Emissivity
http://www.efunda.com/formulae/heat_transfer/radiation/overview_rad.cfm

http://www.infrared-thermography.com/material-1.htm
Paint: Krylon, flat white #1502  @ 3m wavelenght  = 0.992 emissivity

So weirdly enough this one specific "Visibly" white paint has one of the highest emissivities, and would absorb or reflect a lot of heat.

Paint: 3M, black velvet coating 9560 series optical black @ 3m to 10m wavelenght   = 1.00 emissivity
These are near perfect in emissivity

Water's emissivity is around  0.95 to 0.963, but it evaporates from the photos of light striking even more then from the temperature.
 (Read about pan evaporation rates.)

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Selective Coatings

http://www.virginiasolartechnology.com/crystal_selective_coatings.html
With the solar absorption ranging from 0.94 to 0.96 and the thermal emissivity ranging from 0.04 to 0.09,

The basic challenge in the use of solar energy is to transform solar energy radiation into useful heat. Solar /Thermafin Manufacturing has set a benchmark for efficiency and quality with the introduction of Crystal Clear™ solar selective coating.

Crystal Clear™selective coatings increases solar collector efficiency due to its unique optical properties. It has the ability to strongly absorb short-wave light while simultaneously reflecting long-wave light. These intrinsic properties of Crystal Clear™ enable the collector to reach optimum temperature in a minimal amount of time and retain the absorbed heat longer.

http://www.solec.org/
http://www.solchrome.com/flatplate.htm   (India)