Why are my solar panels producing less power than expected?

Most low-production months come down to season, weather, heat, shade, dirt or snow, not a broken system. Before you call anyone, make sure you are comparing the right numbers: actual production from your inverter or app, over a long enough stretch, against the estimate in your contract. A sudden drop to zero on a sunny day, a fault light or an alert from the monitoring system is different, and is a call to the installer or a licensed electrician.

Start with the right numbers

The utility bill is the wrong place to measure production. Dominion Energy puts it plainly: "Your electric bill does not show all of the energy your system produced. The energy you produce that is consumed by your home does not pass through the Dominion Energy meter."^[1] Read total production from the inverter display or the monitoring app the installer set up.

Then compare like with like. PVWatts, the federal estimating tool many installers start from, warns that its predictions include many inherent assumptions and uncertainties,^[2] and its expected range is based on 30 years of actual weather data at your location, to show how much output can vary from year to year.^[2] A single cloudy month can sit well below an annual average without anything being wrong. Ask your installer for the month-by-month estimate behind the yearly figure in your contract, and compare a full season or a full year.

Season, weather and heat

The U.S. Department of Energy lists the usual suspects: "Solar energy production can be affected by season, time of day, clouds, dust, haze, or obstructions like shadows, rain, snow, and dirt."^[3] In Virginia, short winter days and a low sun angle cut output, and a run of overcast weeks does the same in any season.

Heat matters too. "Solar cells generally work best at low temperatures."^[4] The department explains that higher temperatures bring a slight increase in current but a much larger decrease in voltage,^[4] so panels can make less per hour of sun on a very hot afternoon than on a cool, bright spring day.

Shade, including shade that grows

Shade often explains a system that falls further behind its estimate over the years. JinkoSolar's installation manual notes that even though bypass diodes are built into its panels, shadows can still affect their performance.^[7] How much depends on the inverter. On a string inverter, a set of panels shares one inverter, so shade or a problem on any one panel reduces the whole string's output. With microinverters on each panel, shade or damage on one panel does not affect the others.^[5]

Trees grow, and a tree that cleared the array at installation can shade it a few years later. The Department of Energy notes that trees that cast heavy shade on a roof can make rooftop panels a poor fit.^[6] Watch the array from the ground at different times of day, and note when shadows cross it.

Dirt, pollen and snow

"A buildup of dust or dirt on the module front face will result in a decreased energy output."^[7] In Virginia that is often spring pollen, bird droppings and fallen leaves. Canadian Solar's manual says panels tilted at least 15 degrees generally may not need cleaning because rain has a self-cleaning effect,^[8] so check whether a few rainy days bring output back before paying for anything. Our answer on whether solar panels need cleaning covers when it is worth it and who should do it.

Snow is on the Department of Energy's list for the same reason: expect little or no output while it covers the panels.^[3]

The system has shut off

If the grid goes out, a standard grid-tied system is forced offline.^[1] The Department of Energy explains that household inverters ride through small voltage or frequency disturbances but disconnect themselves from the grid and shut down when a disruption lasts a long time or is larger than normal.^[5] Production should resume once utility power is back. If it stays at zero on a sunny day, something needs a technician: an inverter fault, a tripped circuit, a failed microinverter or a communications problem with the monitoring system.

Damage you can see from the ground

JinkoSolar's manual lists what an inspection looks for, including broken glass, sharp objects resting on the panels, shading from obstacles or debris, corrosion along the cells' busbars, and a burnt backsheet.^[7] After a hailstorm or high winds, look from the ground or a window for cracked glass, a panel out of line, or wires hanging below the array.

Do not go up to look closer. Canadian Solar's manual warns: "The voltage and current present in an array during daylight hours are sufficient to cause a lethal electrical shock."^[8] It adds that electrical or mechanical inspections and maintenance should be performed by qualified personnel only.^[8]

Age

Panels slowly lose output as they age, which the panel warranty sheet should describe. The Department of Energy reports that solar industry professionals now expect panels to operate for 25 to 35 years, up from around 20 years in 2007.^[9] A system that has lost a large share of its output in its first few years has a problem worth raising under the warranty.

When to call a licensed pro

Call the installer first if the system is under its workmanship warranty, and send screenshots of the monitoring data. Call the same day for zero output on a clear day, an inverter fault light, a burning smell, scorch marks or visible damage. The Board for Contractors' alternative energy system (AES) specialty covers installing, repairing and improving solar generation systems from the customer's meter, but not the electrical work,^[10] so inverter, breaker and wiring faults need a licensed electrician as well. If you need a new installer, check their licence on DPOR's record.

For background, read how net metering works in Virginia. Browse solar installers on Loclis, find electricians in Blacksburg, or read more electrical answers.

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