THE SUN’S ENERGY—DANIELS 241 
There is much more to solar-energy utilization than the develop- 
ment of solar engines. House heating, house cooling, refrigeration, 
cooking, distillation of salt water are all parts of the picture which is 
being suggested now and elaborated throughout this volume. Even in 
the development of solar power for operating machines the long- 
range hope lies not so much in heat engines as in photochemistry and 
photoelectricity. But first let us see how much solar energy we need 
and how much we have. 
ABUNDANCE OF SOLAR ENERGY 
In considering any new development, it is well to consider first 
whether it is theoretically possible, next whether it is technically 
feasible, and finally whether it is economically sound and socially 
useful. The symposium was organized to discuss these questions. 
Answering the first question, the total energy of the sun falling on 
the earth is far more than is needed to do the world’s work and it is an 
ever-continuing source of energy. To carry on the energy-rich civi- 
lization of the United States, this country used, in 1952, 164,000 kilo- 
calories of fuel energy or 190 kilowatt-hours of heat per person per 
day. The per capita consumption of fuel by all the people of the 
world was 24,000 kilocalories per day. The total energy from the 
sun striking the land area of the United States is of the order of 
270 million kilocalories, or 313,000 kilowatt-hours, per person per 
day. The theoretically available supply far exceeds the need, but 
at present there is no direct utilization of it because it is in the form 
of low-temperature heat which is difficult to convert into work, and 
difficult to store and transport. 
These figures are based on an arbitrary assumption that the solar 
radiation amounts to about one small calorie per minute per square 
centimeter. It is more than this in Arizona and much less than this 
in some other parts of the country, particularly in the wintertime. 
This unit is too small to visualize. Earlier, I suggested a new unit of 
solar radiation—the “roof,” which is a “million calories per minute.” 
Tt is the solar radiation received by 100 square meters of flat surface 
at the rate of 1,000 kilocalories per minute, or 60,000 kilocalories per 
hour or 500,000 kilocalories, or 580 kilowatt-hours of heat per 500- 
minute day. This 100-square-meter area is 1,075 square feet, about 
the flat area of the roof of a square house which is 10 meters or 33 feet 
on a side. A “roof-day” of 500,000 kilocalories is approximately 
equivalent to two million B.t.u., which is the amount of heat evolved 
by burning 150 pounds (about a man’s weight) of coal or 15 gallons 
(almost an automobile tankful) of gasoline. If all the solar radiation 
were used with 100 percent efficiency, a “roof-day” of radiation would 
evaporate about 860 liters or 230 gallons of water. 
