Splitting an array across more than one roof face
Why area-proportional splitting is a simple starting point
Assigning panels in proportion to each plane's usable area spreads the target evenly across the roof, so no face is filled harder than another. It is a convenient starting point, not an energy optimum: if the planes face different directions, panels on the best-oriented plane produce more, so filling that plane first usually yields more energy for the same panel count, a trade-off the south-facing vs east-west array calculator quantifies. Installers also adjust for known shading and for a household's usage timing, since an east-west split favors morning and evening generation.
Why each series string needs a single orientation
Panels in a series string pass the same current, so the string runs at the pace of its weakest panel. Mixing orientations inside one string forces the sunnier panels to throttle to match the weaker ones at any given moment. NLR's test of a partially shaded grid-tied string shows how hard that weakest-link behavior can bite: in one configuration a shadow cut power by more than 30 times its own size. The usual fix is one orientation per string, with strings from different planes on separate MPPT inputs, which most residential string inverters provide in pairs or more. Manufacturer testing of true east-west arrays has found small losses when parallel strings share one input, but separate inputs are the safer default. Each input also has its own current limit, so check the inverter datasheet when one plane carries most of the panels.
When a small roof plane needs microinverters instead
A string needs enough panels in series to reach the inverter's minimum operating voltage, often somewhere around 4 to 6 panels depending on the specific inverter model. A roof plane too small to support a dedicated string on its own is a common case where microinverters make more practical sense, since each panel operates independently with no minimum string length to worry about, a tradeoff the microinverter vs string inverter output tool lays out directly.
Shade on one plane is a separate question from the split
Splitting by area says nothing about shade. On a string inverter, a shaded plane drags down its own string while the others keep working, but shade is also where module-level electronics earn their cost. NREL's shading testbed report found a microinverter system produced 3.7% more energy under light shading, 7.8% under moderate shading, and 12.3% under heavy shading than a string inverter on identical 8 kW arrays. If one plane has known shade, the partial roof shading sizing adjustment tool shows how much to adjust before finalizing the split.
Check that each plane can hold its share
This tool caps each plane at what fits at your panel footprint, about 21 square feet per panel by default, and says plainly when the target needs more panels than the planes can hold. Confirm each plane's usable area first with the usable roof area calculator, since setbacks and obstructions differ face to face. Structure is a separate check, and the roof truss load capacity calculator shows whether a plane can carry its assigned panel count.