My solar panels produce less than expected: 7 causes and diagnosis

You calculated 5 kWh/day and only get 3.5. Before assuming something's broken, consider the gap between theoretical and real is typically 20-30%. More than that, real problem.

1. You overestimated peak sun hours

People use 5 PSH when their real zone is 3.5 PSH in winter. Verify with PVGIS (Europe) or PVWatts (USA) using exact coordinates. Monthly difference: January produces 30% of July in northern US.

2. Partial shading at peak hours

A single shaded cell (sq inch) can drop a whole panel to 30%. Bypass diodes help but don't eliminate. Check at 1 PM if shadows: chimney, antenna, nearby building, tree.

3. Accumulated dirt

Dust, pollen, bird droppings: 10-25% loss after 6+ months unwashed. Simple water rinse: recovers 90% of loss. Do it spring and fall.

4. Heat reducing efficiency

Panel at 140 °F produces 12-15% less than at 77 °F (nameplate). In hot summer, midday production isn't proportional to irradiance. Physics, no failure. You design for annual average, not theoretical max.

5. Voltage drop in long cable

Undersized cable between panels and inverter loses 3-7% in transit. Measure voltage at panels vs inverter: gap >5%, bad cable sizing. Upgrade to larger gauge.

6. One dead or degraded panel

A panel fails and nobody notices. With microinverters or optimizers, app shows it. No panel-level monitoring: measure individual current with DC clamp meter.

7. Wrong MPPT config

Wired panels in series when controller expected parallel (or vice versa), you produce 50% less. Check MPPT config and array data (Vmp, Imp).

Shading explained in shading and solar production.