The average daily sun hours (solar insolation) in units of kWh/m2 per day is sometimes referred to as "peak sun hours" too. The term "peak sun hours" refers to the solar insolation which a particular location would receive if the sun were shining at its maximum value for a certain number of hours. Since the peak solar radiation is 1 kW/m2, the number of peak sun hours is numerically identical to the average daily solar insolation. For example, Baltimore receives 4.7 kWh/m2 per day can be said to have received 4.7 hours of sun per day at 1 kW/m2. Being able to calculate the peak sun hours is useful because PV modules are often rated at an input rating of 1kW/m2.
The daily amount of solar radiation striking any location on earth varies from sunrise to sunset due to clouds, the sun’s position in the sky, and what’s mixed into the atmosphere. Maximum solar radiation occurs at solar noon—the time when the sun is highest in the sky, compared to the rest of the day. Sunlight in the morning and evening does not deliver as much energy to the earth’s surface as it does at midday because at low angles more atmosphere filters the sunlight. Besides day-to-day differences, there are also seasonal effects.
In midsummer, due to the sun’s higher position in the sky, an hour of sunshine packs more energy than the same hour of sunshine in the winter.
A peak sun-hour is roughly the amount of solar energy striking a 1-square-meter area perpendicular to the sun’s location over a 1-hour period straddling solar noon in the summertime. So we can compare apples to apples, the amount of power is standardized at 1,000 watts (1 kilowatt) hitting that 1-square meter surface. By adding up the various amounts of solar irradiation over the course of a day, and counting them as units equivalent to 1 solar-noon midsummer hour (1,000 watts per square meter for 1 hour), we get a useful comparison number—the peak sun-hour.