What are ‘peak sun hours’?
A peak sun hour doesn't include just any hour when the sun is out in the sky. Instead, it refers to an hour in which your solar panels produce a certain amount of energy.
Each peak sun hour is defined as one hour when the intensity of sunlight (solar irradiance) reaches an average of 1,000 watts of energy per square meter (roughly 10.5 feet).
Summer months and locations farther south will generally see more peak sun hours than wintry times and areas farther north. That's because regions closer to the equator are also closer to the sun.
On the other hand, in latitudes farther north, when the sun is closer to the horizon, the sunlight is filtered through more layers of the atmosphere. In those places, the sunlight isn’t as strong by the time it reaches your solar panels, which results in lower peak sun hours.
Peak sun hours also vary depending on where your solar panels are placed. In other words, the peak sun hours' value for one part of the house might be different than its value for another part of the house. These factors can affect the number of peak sun hours:
Direction: The direction your solar panels are facing affects the intensity of the sunlight, as well as the total amount of sunlight received.
Shading: Even if all the solar panels face the same direction, they might experience sun hours at different times based on when a tree (or other obstruction) casts shade on them.
How do we calculate total peak sun hours?
One peak sun hour = 1000 W/m2 of sunlight. However, when calculating the total amount of peak sun hours received at any location, you don’t just consider hours with 1000 W/m2 of solar radiation. Instead, you need to add the total amount of solar irradiance received by the location. You then express that in terms of the equivalent number of hours with 1000 W/m2.
It may sound complicated, but the concept is actually relatively simple to apply. For example, if a given location receives a total of 4,500 Wh/m2 of solar radiation over the course of a day, then that location gets 4.5 peak sun hours.
Regular sunlight hours refer to any time the sun is shining during the day - the hours between sunrise and sunset. However, it doesn’t tell us anything about the strength/intensity of the sun's radiation during those hours, and thus, is unhelpful when it comes to designing a solar panel system.
That is why the concept of 'peak sun hours' has been developed. It precisely measures the amount of irradiance that will hit the solar panels, thereby allowing us to calculate the expected electricity generation.
We know that during peak sun hours, a solar panel should theoretically produce 1,000 W /m2. But, solar panels do not work at 100% efficiency. All solar panels these days come with a Standard Test Conditions (STC) rating, which is the amount of energy those panels can actually produce during peak sun hours.