Microinverters vs String Inverter: Shading Mismatch Loss

Shadow (cloud / tree / chimney)

Position along array 50%

Width (panels covered) 3

Shade depth (irradiance loss) 75%

String Layout

Panels per string 10

Live Output

Microinverters / optimizers (independent MPPT per panel)

0 W

Single string inverter (one MPPT, panels in series)

0 W

Mismatch loss from series wiring

How it works

Each panel's own maximum-power point follows real PV behaviour: short-circuit current scales almost linearly with irradiance G, while open-circuit voltage falls off logarithmically (it barely changes in light shade, then drops fast in deep shade):

I_mpp(G) ≈ I_stc · G
V_mpp(G) ≈ V_stc · (1 + β·ln(G)),  β ≈ 0.05
P_panel  = I_mpp · V_mpp

Microinverters / DC optimizers give every panel its own MPP tracker, so the array total is simply Σ P_panel — a shaded panel only loses its own power, nothing more.

A single string inverter puts every panel in that row in series on one MPP tracker, so only one current can flow through the whole string. A panel's bypass diode kicks in once its own MPP current falls below ~30% of full sun (G < 0.3), shorting it out of the voltage sum entirely — but the remaining panels are still forced to share the reduced current set by the weakest surviving panel:

I_string = min( I_mpp,i )  over non-bypassed panels i
V_string = Σ V_mpp,i        over non-bypassed panels i
P_string = I_string · V_string

Because one dim panel drags every panel behind it in the string down to its own current, a shadow covering even 2–3 of 10 panels can cost a string inverter far more than the shaded panels' own share of the array — the gap between the two live readouts above is exactly that mismatch loss. This is the real-world reason rooftop installers add microinverters or per-panel optimizers on roofs with partial shading (chimneys, vent pipes, trees, neighbouring buildings).

Full sun Shaded Bypassed (diode shorted)

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