One heating degree–day is the amount of heat required to keep a structure at 65ºF when the outside temperature remains one degree below the 65ºF threshold for
24 hours. One heating degree–day is also the amount of heat required to keep that structure at 65ºF when the temperature
remains 24ºF below that 65º threshold for 1 hour.

One cooling degree–day is the amount of cooling required to keep a structure at 65ºF when the outside temperature remains one degree above the 65ºF threshold for
24 hours. One cooling degree–day is also the amount of cooling required to keep that structure at 65ºF when the temperature
remains 24ºF above that 65º threshold for 1 hour.

What we call "current solar radiation" is technically known as Global Solar Radiation, a measure of the intensity of the sun’s radiation reaching a horizontal surface. This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky. The solar radiation reading gives a measure of the amount of solar radiation hitting the solar radiation sensor at any given time, expressed in Watts per square meter (W/m2).

Solar Energy - The amount of accumulated solar radiation energy over a period of time is measured in Langleys.

1 Langley  = 11.622 Watt-hours per square meter  = 3.687 BTUs per square foot  = 41.84 kilojoules per square meter

Energy from the sun reaches the earth as visible, infrared, and ultraviolet (UV) rays.
Exposure to UV rays can cause numerous health problems, such as sun burn, skin cancer, skin aging, and cataracts, and can suppress the immune system.

MED stands for Minimum Erythemal Dose, defined as the amount of sunlight exposure necessary to induce a barely
perceptible redness of the skin within 24 hours after sun exposure. In other words, exposure to 1 MED will result in a reddening of the skin.
Because different skin types burn at different rates, 1 MED for persons with very dark skin is different from 1 MED for persons with very light skin.