What is a “Sun-Hour”?
It can be confusing at first, because a Sun-Hour is NOT merely an hour’s worth of sunlight. A Sun-Hour is a unit used to measure the solar radiation from the sun, but to make it useful, it needs to be defined and consistent.
Think of the term “horsepower”. Does it have anything to do with horses? No! If a car has 100 horsepower, that doesn’t mean it has the same power as 100 horses. A horsepower is “a unit of power equal to 550 foot-pounds per second”. It is simply a defined unit. A horse might produce more or less than this. It depends on the size of the horse.
A “Sun-Hour” is similar in this respect. We observe the sun throughout the day from every part of the earth, but it isn’t all at the same intensity. To make it a useful unit, scientist and engineers have defined it to be something standard. A “Sun-Hour” is “1000 watts of energy shining on 1 square meter of surface for 1 hour”. So you see, at noon in New Mexico, the sun might put out 1200 watts per square meter in 1 hour (1.2 Sun-Hours) vs at noon in Wyoming, with the same blue sky, the sun might only put out 900 watts per square meter in the same hour (0.9 Sun-Hours).
In Denver Colorado, in the month of May, the average Sun-Hours per day for a panel at 35 degrees is 6.40. The sun shines for much longer than 6.4 hours per day, though. The sun, being far less intense in the early morning and later afternoon, may only put out 300 watts per square meter, while at noon it may reach to 1100 watts per square meter. Overall, though, there is 6.4 kilowatt hours (6400 watt hours) per square meter accumulated throughout the day. This is really the only thing we need to know when sizing a solar system. We don’t need to know the hour by hour rating of the sun throughout the day, only the total daily value.
See the PVWatts Calculator For Each Specific Location
So does how does this relate to the size of the solar system? Well in several ways:
- How large is the solar system? Basically, how many square meters worth of solar cells are in the solar system?
- How efficient are the solar cells? High-grade solar panels are only about 17% – 20% efficient. This means that if the sun gives out 1 Sun-Hour, the solar panel will only be able to convert 17% – 20% of that to usable electricity.
