What Are Peak Sun Hours?
When you look up at the sky, you might think your house gets 12 hours of sunlight a day. But solar engineers measure sunlight a little differently. A peak sun hour isn't just any time the sun is up. It's a specific measurement of sunlight strength. One peak sun hour equals one hour where the sunlight provides 1000 watts of energy per square meter.
Early in the morning or late in the afternoon, the sun sits low in the sky. The sunlight is weaker. It might take three hours of morning light to equal just one peak sun hour. In the middle of the day, when the sun is directly overhead, the light is very strong. During that time, one hour on the clock equals one peak sun hour.
Most places in the United States get between 3 to 6 peak sun hours a day. States like Arizona and Nevada get numbers on the high end. States like Washington and Maine get numbers on the low end. This daily average changes depending on the season, but contractors use an annual average to build your system.
Why This Number Matters to You
This measurement tells you exactly how much electricity a solar system can make on your roof. If you know your daily peak sun hours, you know how many panels you need to buy to power your home.
Let's say you use 30 kilowatt hours of electricity a day. If you live in a sunny state that gets 6 peak sun hours, you need a smaller, cheaper solar setup to make that power. If you live in a cloudy state that only gets 3 peak sun hours, you need twice as many panels to make the exact same amount of electricity. This directly changes your budget when you shop for Solar Panels.
A typical residential solar system costs 15,000 to 30,000 dollars before tax credits, though ranges vary based on your location and energy needs. If your home gets very few peak sun hours, your costs will sit on the higher end of that scale because you have to buy more equipment.
Where You Will See This Term
You'll run into this term the moment you start asking for solar quotes. Sales reps use satellite images of your house to calculate your specific number. They put this number on your official proposal.
You'll also hear this term when you talk about payback periods. A payback period is how long it takes for your energy savings to pay off the cost of the system. Homes with high peak sun hours pay off their systems much faster. If you're comparing quotes from different companies, always check the peak sun hours they used in their math. If one company uses a much higher number than the others, they might be exaggerating how much money you'll save.
If you plan to install the panels yourself, you'll need to calculate this number on your own. You can find free online calculators that use your zip code to give you an accurate average. Be sure to compare this project against other home upgrades by reading about Hiring Contractors & What Things Cost.
Things That Lower Your Hours
Your zip code isn't the only thing that matters. Two houses on the exact same street can have very different peak sun hours. This happens because every house sits on its lot differently.
- Roof direction: In the United States, south facing roofs get the most direct sunlight. If your main roof faces north, you'll get significantly fewer peak sun hours.
- Roof angle: The slope of your roof changes how the sun hits your panels. If your roof is very steep or completely flat, you might lose some efficiency. You can learn more about roof slopes in our section on Roofing.
- Tree shade: A giant oak tree in your front yard will block the sun. Even a chimney casting a shadow can drop your daily average.
