Trust a solar production estimate only when the tilt, the azimuth, the shade, the system losses, and the weather file are the ones this array will actually have. If any of those is a default from another roof, the kilowatt-hours that follow are someone else's year. This page will not print a sample annual yield. A number borrowed from a neighboring proposal is how people buy the wrong array.
Name the inputs out loud
Tilt and azimuth are the angle and the compass direction of the modules as they will be built, not a stock south-facing latitude tilt. Changing either one changes the shape of the year. How angle and direction move energy is owned by solar panel tilt and orientation. What this page requires is simpler: the estimate has to use your angle, and it has to say so.
Shading is a description of what blocks the sun, by season if the blocker is a tree or a neighboring wall. The phrase "a little shade" is not an input. A vent, a chimney, or a dormer can hit one string at noon and leave the annual total looking fine until you see the month. How shade cuts output, and what module-level gear can and cannot recover, stays in how shade affects PV output.
System losses are the gap between the module nameplate and the AC kilowatt-hours you can use. Temperature, soiling, mismatch, wiring, and inverter conversion all belong in that gap. A model that hides them inside one unexplained factor is unfinished. Ask for the list. Do not borrow a loss percentage from another city's proposal and call it conservative. There is no loss percentage on this page.
A weather file, or the months the installer left
A weather file is a record of irradiance and temperature for a place, over a year or many years. A photograph of a clear noon is not that record. A slogan about sun hours for a whole country is not your roof. Pacific Northwest National Laboratory's operations guide says the installer should leave estimated monthly production, and that if those figures were not left, the system may be modeled with PVWatts. The same guide says insolation can vary by plus or minus 10 percent from average, and to begin troubleshooting when production falls outside the expected range. That is guidance for the sites the guide covers, not a permit rule and not a guaranteed band for your bill: Pacific Northwest National Laboratory.
Ask for the months, not only a yearly total. An annual figure with January missing is how winter surprises people who heat with electricity or who simply looked at July. If the modeler cannot name the weather file's location, treat the estimate as a sketch.
What the months are allowed to do
Monthly production is an input to a panel count and to a payback, and it is a poor substitute for either decision by itself. How not to jump from a bill to a module count is in how many solar panels a home needs. How production enters a payback, including the split between energy you use and energy you export, is in how a solar payback period is calculated.
An estimate is a model of a weather file and a layout. It is a design tool. It does not bind the utility, and it does not survive a tree that grows or a tilt that gets built differently from the drawing. When the array is up, compare later months with the months that were promised, and keep the original file. If nobody can find the file, you cannot tell a dull month from a broken string.
