Peak sun hours and performance ratio explained
The two inputs that decide a solar estimate, where to find real numbers for your roof, and how each one moves your kWh.
Reading peak sun hours for your location
Peak sun hours are not the hours the sun is up; they are the equivalent hours of full 1000 watts per square meter irradiance packed into a day. A location with 5.5 peak sun hours might have twelve hours of daylight, most of it weaker than full strength. National solar maps and datasets publish annual and monthly averages by region, and the annual average is the safest single number to enter. Because the value drives daily output linearly, moving from 4 to 5 sun hours lifts your estimate by a full 25 percent.
What the performance ratio bundles together
Panels are rated in a lab at 25 degrees Celsius, but a hot roof runs far warmer and loses efficiency as temperature climbs. Add resistance in the wiring, the inverter's conversion loss, soiling from dust and pollen, minor shading and module mismatch, and real arrays land near 0.75 to 0.80 of nameplate. A brand new, cool, unshaded, well ventilated system might reach 0.80 or a touch higher, while a hot climate with some shade can sit closer to 0.70. Adjust the field to reflect your conditions rather than leaving the default in every case.
Turning kWh into a rough bill impact
The tool stops at energy, but the step to money is short. Multiply the yearly kWh by your electricity rate to get a ceiling on savings; a 6,570 kWh year at 20 cents is about 1,314 dollars of avoided grid energy. The real figure is usually lower because you cannot always use solar power the moment it is made. Net metering credits, time of use rates and battery storage all change how much each exported kWh is worth, so treat the multiplication as an upper bound.
Why a professional model still matters
This calculator assumes an unshaded array pointed a sensible direction, which is rarely the full story. A designer uses hourly irradiance, your exact roof pitch and azimuth, and a shading scan of nearby trees and chimneys to model output month by month. That process catches problems a single ratio cannot, such as a dormer that shades half the array every winter afternoon. Use the estimate to sanity check a quote, then rely on the site specific model for the final design and financing.