Why a pergola is its own sizing problem, not a small roof
A freestanding structure with its own engineering needs
A solar pergola is an independent structure, typically aluminum or steel framed, sitting on its own footings rather than tying into a house roof. That independence is also its main advantage: panel orientation and tilt can be optimized freely, unlike a roof mount that is stuck with whatever pitch and direction the existing roof provides. It also means the frame carries its own wind and snow loads, which the wind uplift rating calculator helps check. If you are still choosing between a pergola and a plain ground array, the ground mount vs roof mount space calculator compares the space and output sides.
How many panels actually fit
Panels are rigid rectangles, so a pergola holds only as many as tile its frame. With panels of about 21 square feet, roughly 3.6 by 5.9 feet, a 10 by 10 foot pergola fits 2 panels, a 12 by 16 foot one fits 8, a 16 by 20 foot one fits 12, and a 20 by 24 foot one fits 20. Dividing the pergola's area by the panel area would suggest 4, 9, 15, and 22, which is why this calculator places whole panels in the better of two orientations. The example values on this page, a 20 by 16 foot pergola, give 12 panels and 4.80 kW at 400 watts.
Why tilt changes the structural math here more than on a roof
A tilted array catches wind differently from a flat one, and the relationship is not a simple one. ICC's guide to the ASCE 7-16 changes notes that the pressure coefficients for solar panels are not linearly related to tilt angle over the full range, and the later ASCE 7-22 added separate provisions for ground-mounted fixed-tilt systems. A pergola's structural design needs to account for the specific tilt angle chosen, not treat the frame and the tilt decision as separate problems, and the tilt angle optimizer shows the energy side of that choice for your latitude.
Why distance from the house is a real, if usually small, factor
Every foot of wire between the pergola and the home's electrical panel or inverter adds a small amount of resistance loss, which the voltage drop calculator models for your actual conductor. This calculator assumes about 1% per 100 feet with properly sized wire, so a 40 foot run costs about 0.4%. At typical backyard distances the loss is minor, but it grows the farther the pergola sits from the house, which is worth accounting for on a larger property.
What retrofitting an existing pergola actually requires
Adding panels to a pergola built for shade alone is not a simple bolt-on job. Many existing structures are not engineered for the point loads panels and mounting hardware add, so a structural assessment is a required first step, and the array weight load calculator shows the weight a frame has to carry. Codes also treat detached structures differently from houses: under Section 605.11 of the International Fire Code, detached, nonhabitable structures are among the exceptions to roof access requirements, so a pergola array typically doesn't need the ridge setbacks and pathways a house roof does, though your code official decides.