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Part VII: Solar ElectricityChapter 11. Grid-Tied and Off-Grid PV

Shading and PV: Why One Leaf Matters

Why a small shadow can cut a string harder than the shaded area suggests, and what to do about it.

Difficulty
Intermediate
Cost
Free
published

Cells in a module are in series, and modules in a simple string are in series too. Series means one current. A leaf does not have to cover the glass to matter. It has to cover enough of one cell that the cell cannot pass the current the rest of the string wants. The shaded area can be small and the loss can be a section of the module, or the whole series group, depending on how dark the shade is and whether a bypass diode conducts.

This page explains the mechanism and stops short of a wiring plan. Layout choices are the inverter comparison. Where the shadow falls through the year is the shade map. Project notes in the links at the bottom stay on those sites.

Difficulty

Level
Intermediate
Time
An hour on the roof plane from the ground, with the sun in mind
Skills
Seeing a vent or a branch as a series problem

Cost estimate

Free

Moving a planned module out of a shadow is free. Module-level electronics cost more than a single string inverter and less than a string that is dark every afternoon. Trimming a tree you do not own is not a tool in this budget.

Materials

ItemQtyNotes
The roof sketch from the site survey1Vents, chimneys, dormers, and the trees that are still growing.
A winter and a summer look at the same plane2A June noon visit misses the shadow that arrives in December.

What the leaf does

A bypass diode, usually several per module, can short around a shaded substring so the rest of the string keeps a current. That saves the string from being dragged all the way down, and it drops the voltage of the section it skipped. It is a damage limit and a partial save. It is not a promise that one shaded corner costs only its own area.

A lighter shadow may never turn the diode on. Then the shaded cell sets the current, and every cell in series with it lives with that lower current. That is the case people mean by "one leaf." Hard shade that trips the diodes costs you a substring or a module of voltage. Soft shade that does not trip them costs you current through the series group.

Dirt, pollen, and a ridge of snow are shade. A vent that clips one corner for an hour is shade. A tree that will be taller in ten years is shade you are designing now.

Shade on a series stringThree modules in series, the middle one partly dark, and a second row where each module stands alone.Series string. One current.Module electronics keep the neighbors working.A bypass diode skips the shaded section. The shade still costs power.
A series string shares one current. Module-level electronics keep the unshaded modules at work.

What to do about it

Put modules where the sun is, even if that array is smaller than the roof. A clear smaller array beats a large string that includes a daily shadow. If the shadow is unavoidable and you still want those modules, module-level electronics are the layout that confines the loss. Read the comparison before you treat the word optimizer as a spell.

Do not string a module through a chimney shadow because it fills a gap in the drawing. Do not plan on washing the array every week as the design. Do wash it when it is actually dirty, from a method that does not put you on a wet roof. The ladder page is the constraint.

A neighbor's tree is the neighbor's. A wire that you might add to "fix" a shaded module is not on this page. Diodes already in the module junction box are the manufacturer's. You do not open that box to improve them.

Composite: the vent on the corner

Picture an invented south roof. One plumbing vent shadows the lower corner of the eastern module for part of the morning, and a young tree clips the western module in late afternoon. In the composite the modules are one series string because the roof looked like one rectangle. The morning loss and the afternoon loss are not the same hour, but each of them limits the string while it lasts. The household notices a low daily total and blames the weather.

The lesson is to drop the two clipped modules or to give each module its own electronics. Drawing the roof as one rectangle was the expensive mistake. Nobody in this composite is a real owner, and the vent is not a house you can drive to.

  1. Mark hard shade and soft shade

    A chimney is hard. A bare branch can be soft and still enough to limit current. Draw both on the plane.

  2. Keep a series string on clear glass

    If a module is shaded every day, it does not belong in a plain string with the clear modules.

  3. Choose module-level electronics when the shade stays

    Or delete the shaded modules. Both are designs. Hoping the diode will make the shade free is not.

  4. Recheck after the trees grow

    A survey from the year you installed is not a survey from year ten. Leaves are seasonal. Snow is seasonal.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.