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Low Irradiance Performance In Solar Panels: Why It Matters For Cloudy Climates

Low irradiance performance describes how well a solar panel produces electricity when sunlight intensity is below the 1,000 W/m2 used for Standard Test Conditions (STC). Since panels spend most of their operating hours at irradiance levels of 200-800 W/m2, low-light performance directly affects annual energy production. Good panels maintain 95-98% of their expected relative efficiency at 200 W/m2, while poor panels can drop to 88-92%.

Why STC does not tell the whole story

Standard Test Conditions rate every panel at 1,000 W/m2, but this irradiance level only occurs around solar noon on a clear day. During the morning ramp-up, late afternoon decline, and any cloud cover, the irradiance hitting your panels is substantially lower.

A typical sunny day in a location like Phoenix might see 1,000 W/m2 for 4-5 hours. But the total production day lasts 10-12 hours, meaning the panels operate at reduced irradiance for more than half the day. In cloudier locations like Seattle, Portland, or across northern Europe, the panels may spend 70-80% of annual production hours below 600 W/m2.

If two panels both produce 400W at STC but one maintains 97% relative efficiency at 200 W/m2 while the other drops to 91%, the first panel produces about 78W at 200 W/m2 irradiance while the second produces only 73W. This 5W difference at low light, applied across hundreds of low-irradiance hours per year, compounds into a meaningful annual energy gap.

How low irradiance is measured

IEC 61853-1 defines a performance matrix that measures panel output at multiple irradiance levels and cell temperatures:

Irradiance (W/m2)What it representsRelative efficiency (good panel)
1,000Full sun (STC baseline)100% (reference)
800Light haze or early/late sun99-100%
600Thin overcast or morning/evening98-99%
400Heavy overcast or deep morning/evening97-98%
200Dense clouds, dawn, dusk95-98%
100Very heavy overcast, twilight88-95%

The relative efficiency is the actual power divided by the power you would expect from simple linear scaling. At 200 W/m2, a 400W panel should produce 80W if performance scaled perfectly. A panel with 97% relative efficiency at 200 W/m2 produces 77.6W.

What causes low-light losses

At low irradiance, the photocurrent generated by the cell is small. Two loss mechanisms become proportionally more significant:

Shunt resistance (Rsh). Every solar cell has small parasitic current paths that bypass the p-n junction. These shunt currents are roughly constant regardless of irradiance. At full sun, shunt losses might be 0.1% of the photocurrent. At 200 W/m2, the photocurrent is 5x smaller but the shunt current is unchanged, so shunt losses become 0.5% of the photocurrent. Cells with manufacturing defects or micro-cracks have lower Rsh and worse low-light performance.

Recombination at the junction. At low carrier concentrations, recombination mechanisms that are negligible at full sun can become significant. Well-passivated cells (HJT, TOPCon) have lower recombination rates, maintaining their fill factor better at reduced irradiance.

Performance by cell technology

Cell TechnologyRelative Efficiency at 200 W/m2Low-Light Rating
HJT (n-type)96-98%Excellent
TOPCon (n-type)95-98%Excellent
Mono-PERC (p-type, premium)94-97%Good
Mono-PERC (p-type, budget)91-94%Average
Polycrystalline89-93%Below average
Thin-film CdTe93-96%Good (excellent diffuse light response)

N-type cells (HJT and TOPCon) generally outperform p-type PERC cells at low irradiance because they have inherently higher shunt resistance and better surface passivation. The amorphous silicon layers in HJT cells provide particularly effective passivation that maintains performance across the full irradiance range.

Where low irradiance performance matters most

Cloudy climates. In the Pacific Northwest, UK, Germany, Scandinavia, and similar locations, the annual energy advantage of a good low-light panel can reach 3-5% compared to a panel with mediocre low-irradiance performance.

East/west-facing roofs. Panels that face east or west spend more time at lower irradiance than south-facing panels. Good low-light performance partially compensates for the sub-optimal orientation.

Morning and evening production. If you have time-of-use electricity rates where morning or evening power is valuable, panels that perform well at 200-400 W/m2 produce more during these rate periods.

Sunny climates with afternoon clouds. Even in locations like Florida or tropical regions where morning sun is strong, afternoon thunderstorms regularly reduce irradiance. Good low-light performance helps during these periods.

Related terms

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Frequently Asked Questions

What does low irradiance performance mean for solar panels?
Low irradiance performance measures how efficiently a solar panel converts sunlight into electricity when the light intensity is below the 1,000 W/m2 standard test condition. It is typically tested at 200 W/m2 and reported as a percentage of the power that would be expected based on linear scaling from STC. A panel with 97% relative efficiency at 200 W/m2 produces 97% of its expected output at that light level.
How much of the day do solar panels operate at low irradiance?
In most locations, panels operate below 1,000 W/m2 for the majority of daylight hours. Only around solar noon on a clear day does irradiance reach or exceed 1,000 W/m2. During morning and afternoon hours, and on all cloudy or overcast days, irradiance is typically 100-600 W/m2. In northern climates like Seattle or London, panels may never reach 1,000 W/m2 during winter months.
Which solar panel technology performs best in low light?
HJT and TOPCon panels generally perform best at low irradiance, maintaining 96-98% relative efficiency at 200 W/m2. This is because n-type cells have higher shunt resistance than p-type PERC cells, which reduces parasitic leakage currents that become proportionally more significant at low light levels. Thin-film CdTe panels also perform well in diffuse light.
Does low irradiance performance affect annual energy production?
Yes, especially in cloudy climates. A panel with 97% relative efficiency at 200 W/m2 versus one with 92% can produce 2-4% more annual energy in locations like the Pacific Northwest, Germany, or the UK where much of the year's irradiance arrives as diffuse light. In sunny climates like Arizona, the difference is under 1%.
How is low irradiance performance measured?
Per IEC 61853-1, the panel is tested at multiple irradiance levels (200, 400, 600, 800, and 1,000 W/m2) and temperatures. At each point, an I-V curve is measured and the maximum power recorded. The relative efficiency at each irradiance level is compared to the STC rating to produce a performance matrix.
Why do some panels lose efficiency in low light?
The main cause is low shunt resistance (Rsh). At low irradiance, the photocurrent is small. If the cell has shunt paths from manufacturing defects or micro-cracks, these parasitic currents become a larger fraction of the total current, reducing fill factor and power output. Cells with high shunt resistance (above 1,000 ohm-cm2) maintain performance better.
Do panel datasheets show low irradiance performance?
Some do. Premium manufacturers list power output or relative efficiency at 200 W/m2 on the datasheet, often showing a value like 97% or 98%. IEC 61853 energy rating data, when available, provides a full performance matrix at multiple irradiance and temperature combinations. If the datasheet does not list low-irradiance data, check the manufacturer's technical documents or third-party test reports from PVEL or TUV.

Sources

Marko Visic
Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.