When panels are mounted at a tilt — on the ground or on a flat roof — each row throws a shadow behind it. Pack the rows too close and, on winter mornings and afternoons, the front row's shadow creeps onto the bottom of the row behind. Because shading part of a string drags down the whole string, that costs far more energy than the shaded area alone. Row spacing is how you prevent it.
How the spacing is worked out
Two things set the shadow length: how high the panel stands, and how low the sun gets. The panel's vertical rise is its length times the sine of the tilt. The lowest midday sun of the year happens at the winter solstice, when its angle above the horizon is roughly 90 − latitude − 23.5°. Divide the rise by the tangent of that angle and you get the shadow gap; add the panel's own horizontal base and you have the full row spacing, front of one row to front of the next.
Why winter, and why midday
Shadows are longest when the sun is lowest — the winter solstice — and shortest at solar noon within any day. Designing so rows are clear at winter-solstice noon keeps the array unshaded through the productive middle of every day, year round. Some designers trade a little land for a little acceptable winter shading at the very start and end of the day; that's a judgement call between yield and space.
The trade-off
More tilt captures more energy in winter but needs more space between rows, as the chart shows. On tight sites, lowering the tilt slightly can pack in more rows and more total power even if each row is a touch less efficient. To choose a tilt in the first place, use the panel tilt calculator.
Related tools
Set your tilt angle, check how much a little shading costs, fit the array to your roof area, and estimate output with the energy production calculator.
Frequently asked questions
How do you calculate solar panel row spacing?
Rise = length × sin(tilt); winter sun angle ≈ 90 − latitude − 23.5°; shadow gap = rise ÷ tan(sun angle); row spacing = gap + length × cos(tilt).
Why do solar panel rows need spacing?
Each tilted row shadows the one behind when the sun is low. Too little spacing means winter shading, and shading part of a string cuts the whole string's output.
What time of year is used for row spacing?
The winter solstice at solar noon — the lowest midday sun and longest shadow — so the array stays clear around midday all year.