How to Read a Solar Generation Report

Solar generation that drops on hot May afternoons in India is not a fault. Thermal derating is a protective response built into every grid-tied solar inverter. India's peak summer conditions trigger it silently, with no error codes, even in systems that passed their PM Surya Ghar commissioning test at a Performance Ratio of 75% or above. This guide teaches homeowners how to read the generation curve in their monitoring app and identify whether a generation drop in the afternoon is thermal derating, panel soiling, shading, or wiring losses, using data the inverter is already recording.

A homeowner in Jaipur installs a 5 kWp on-grid solar system in January. The DISCOM commissioning test in March returns a Performance Ratio of 78%, and the inspector signs off without a concern. By May, the monitoring app shows the same system generating noticeably less power on clear afternoons than it did during that test. No faults. No alarms. No reason the installer can name.

The system is not broken. In short, the shortfall is thermal derating: the inverter reducing its output when ambient temperatures exceed the threshold for continuous full-power delivery. CPRI engineers, publishing in the Indian Academy of Sciences journal Sådhanā, established that standard grid-tie inverters begin reducing output once external ambient temperature reaches 45°C. On peak May afternoons in Delhi, Jaipur, Nagpur, and Ahmedabad, ambient temperatures routinely reach 44–46°C, placing systems squarely at that threshold. Zenergize SiC-based inverters operate at full rated output across a wider temperature range, so the derating impact during Indian peak summer is meaningfully smaller.

Where do I find my solar generation data in India?

To diagnose why solar generation dropped in the afternoon, hourly data is required, not daily totals. That data is available from four sources, and choosing the right one determines whether the diagnostic steps in this guide are possible.

For homeowners on a Zenergize system, the ZenSense app provides real-time hourly generation, historical daily curves, and anomaly alerts. ZenSense is the most useful tool for the curve-shape analysis described below. For systems from other manufacturers, the installer's monitoring portal provides equivalent data.

Homeowners registered under PM Surya Ghar: Muft Bijli Yojana can access generation summaries through the PM Surya Ghar National Portal, but that portal shows daily totals, not hourly curves. A curve-shape diagnostic requires hourly data. Where a DISCOM smart meter is installed, it shows net export and import figures, not generation breakdown.

If the monitoring app shows only daily totals, contact the installer to confirm whether the data logger transmits hourly records. MNRE's updated testing guidelines for Remote Monitoring Systems (RMS) and dataloggers under PM Surya Ghar, released in March 2026, require compliant systems to record and transmit generation data at sub-hourly intervals.

What does a healthy generation curve actually look like on a clear May day?

A healthy generation curve on a cloudless May day is a smooth bell shape. Output starts rising after sunrise, reaches a clean peak between 11:30am and 1:00pm, and then descends symmetrically toward sunset.

The first answer is a shape, not a number: the curve should be smooth and symmetrical. A 5 kWp system on a clear day might show 4.2–4.5 kW at the noon peak, then taper back down proportionally. Small wobbles are normal. The overall shape should not flatten at the top or show a plateau after 10:30am while the sun is still high.

Monthly totals reveal a counterintuitive pattern. A 3 kW on-grid system in Tamil Nadu generates approximately 419 kWh in March but only 366 kWh in May, despite May having longer daylight hours, a 53 kWh shortfall, a 12.7% reduction in the month with more sunlight. This monthly inversion is the first signal that heat, not irradiance, is the limiting factor by May. The hourly curve is where the explanation becomes visible.

What is the derating signature and how do you spot it in your monitoring data?

The derating signature is a plateau or afternoon shoulder in the generation curve: the output climbs normally in the morning, reaches a threshold around 10:30–11:00am on a clear day, and then levels off or begins declining while the sun is still at high elevation.

A healthy curve is a bell shape with a clean noon peak. A system experiencing thermal derating produces a different shape: the output curve resembles a trapezoid rather than a bell. The morning climb is normal. The afternoon descent begins earlier than the sun's position would justify. The plateau persists until late afternoon when ambient temperatures fall.

CPRI's controlled field testing, published in Sådhanā (2021), documents exactly this behaviour. CPRI engineers tested a 60 kW grid-tie solar inverter under Indian field conditions and found that the inverter delivered full rated power continuously until external ambient temperature reached 45°C. Over a five-hour afternoon test window, the average energy loss from derating was 9.1 kWh, equal to 3.27% of the energy delivered during that window. Critically, the inverter generated no fault code throughout the event. Derating is not a failure mode; it is a thermal protection response.

For a detailed explanation of how summer ambient temperatures turn the inverter into a generation bottleneck, see Zenergize's breakdown of thermal derating costs.

How do I compare my actual output against what my system should be generating?

The benchmark is the Performance Ratio established at commissioning. Under India's PM Surya Ghar Operational Guidelines, Performance Ratio of the plant must be at least 75% at the time of commissioning. That commissioning test runs after installation, typically between October and March, when ambient temperatures are moderate.

A PR of 75% measured at 30–35°C ambient in March is not the same figure the system will achieve on a 44–46°C afternoon in May. The gap between commissioning PR and peak-summer afternoon PR is the exact size of the derating problem for a residential on-grid system. It does not represent a defect and does not constitute a warranty claim.

To run the comparison: pull the hourly generation data for a clear day in March and a clear day in May from the monitoring app. Express each hour's actual output as a percentage of the system's rated output for that irradiance level. In March, the ratio will track close to the commissioning PR across the day. In May, the morning hours will match; the afternoon hours, from approximately 10:30am onward, will diverge downward. The size of that divergence, and the hour at which it begins, is the derating impact expressed in the homeowner's own data.

A system that passed its PM Surya Ghar commissioning inspection at 75% PR may operate at 60–65% PR on peak May afternoons, silently, because the commissioning test was conducted at temperatures well below the derating threshold that CPRI testing established.

Causes that look like derating and how to tell them apart

Thermal derating, soiling, shading, and wiring losses all reduce generation totals, but each leaves a distinct signature in the hourly curve. Identifying the shape of the shortfall determines which cause is active.

Soiling

Panel soiling from dust, bird droppings, or pollution reduces output uniformly across the entire day. The generation curve retains its bell shape but sits proportionally lower from sunrise to sunset. Soiling does not produce afternoon-specific flattening or a plateau shape. If the shortfall is consistent from morning to evening on both hot and cool days, soiling is the more likely explanation. A panel cleaning test resolves the question: if generation returns to expected levels the day after washing, soiling was the cause.

Shading

Shading from trees, aerials, or new construction creates sharp dips at specific hours of the day. Because shade falls on panels at predictable times, the hourly curve shows notches or step-downs at those hours rather than a gradual afternoon plateau. Shading patterns repeat consistently on the same days each week. New shading sources are identifiable by comparing current clear-day curves against historical data from the same period in prior months.

Wiring losses

Wiring losses from corroded connectors, loose terminals, or undersized cabling produce a constant percentage reduction across all hours. Like soiling, wiring losses create a lower-but-proportional curve rather than afternoon-specific flattening. Wiring loss patterns are stable and do not correlate with ambient temperature.

The distinguishing characteristic of thermal derating is temperature correlation: the generation shortfall is specific to afternoon hours and worsens on days when ambient temperatures approach or exceed 45°C. It improves or disappears on overcast days or during morning hours on the same hot days. Neither soiling, shading, nor wiring losses share this temperature-correlated, afternoon-specific pattern.

Panel surface temperatures in Indian peak summer commonly reach 60–70°C, and this affects panel output through the panel's thermal coefficient (approximately 0.38% loss per degree above 25°C for Mono-PERC panels). Panel-level derating reduces output proportionally across all hours of the day, not specifically in the afternoon. The afternoon plateau shape in the hourly curve is the inverter's thermal protection signature, not a panel-level effect.

What should you do once you've identified thermal derating?

Three actions reduce the derating impact once the curve-shape comparison confirms thermal derating as the cause.

Check the inverter's installation environment. Inverters mounted in enclosed rooms, directly under metal roofing, or without adequate clearance from adjacent walls operate in higher ambient temperatures than the outdoor air temperature suggests. Relocating the inverter to a shaded exterior wall with open airflow reduces the number of afternoon hours at which derating conditions are met. Ventilation, not just shade, matters: still hot air around the inverter is as problematic as direct sun.

Use the monitoring data on an ongoing basis. The ZenSense app records hourly generation history and supports generation alerts. Setting a May-to-June alert for afternoon output against the March baseline allows the homeowner to track whether the derating window is widening across the season, which would indicate a worsening installation environment rather than normal seasonal variation in ambient temperatures.

Account for inverter technology at the next replacement. Standard silicon-based inverters begin derating at 45°C external ambient, as the CPRI testing establishes. Zenergize SiC-based inverters are rated to deliver full output at ambient temperatures up to 55°C, which means the derating threshold for these systems sits above the temperatures that peak May afternoons reach in most Indian residential locations. The result: a smaller gap between commissioning PR and peak-summer afternoon PR.

Zenergize inverters are BIS IS 16221 certified and eligible under PM Surya Ghar: Muft Bijli Yojana. The Zenergize single-phase inverter range covers the 3–10 kWp capacity range that suits most residential installations under the scheme, and comes with a 10-year warranty.

Frequently Asked Questions

My solar app shows less power after 11am on hot days. Is something wrong with my system?

Most likely, the system is operating correctly. When external ambient temperatures reach 45°C, standard grid-tied inverters reduce output to protect internal components. This is thermal derating. It produces no fault codes and no alarms. The generation data shows a plateau or flattening in the afternoon curve rather than the clean bell shape visible on cooler days. Output resumes normal levels when temperatures drop in the evening.

How do I compare my actual solar generation against what my system should be producing?

Pull hourly generation data for a clear day in March and a clear day in May from the monitoring app. Express each hour's actual output as a percentage of the design-basis figure for that hour. In March, the ratio will track close to the commissioning Performance Ratio across the day. In May, morning hours will match; afternoon hours will diverge downward from approximately 10:30am onward.

Why does my solar generate more electricity in February than in May when there is more sunlight?

May has longer days and more total irradiance than February, but ambient temperatures in May push inverter output into the derating range for several afternoon hours. A 3 kW on-grid system in Tamil Nadu generates approximately 419 kWh in March but only 366 kWh in May, despite more sunlight hours, because the heat penalty on afternoon output exceeds the benefit of longer days.

What is Performance Ratio and why does it matter for a rooftop solar system?

Performance Ratio (PR) measures how efficiently a solar system converts available irradiance into electricity, accounting for all real-world losses. India's PM Surya Ghar commissioning standard requires a PR of at least 75% at handover. Thermal derating reduces effective PR during afternoon hours in peak summer without breaching any warranty or compliance threshold.

How do I know whether my system needs servicing or whether the generation drop is normal seasonal derating?

Thermal derating is temperature-correlated and afternoon-specific. If the shortfall worsens only during afternoon hours on hot days, and morning generation remains near commissioning levels, the cause is derating rather than a service issue. If the shortfall is uniform across all hours of the day on both hot and cool days, soiling or a wiring fault is more likely and warrants an installer inspection.

What type of inverter handles Indian summer heat with less derating?

Silicon carbide (SiC) power electronics tolerate higher junction temperatures than conventional silicon designs, which shifts the derating onset to a higher ambient temperature threshold. Zenergize SiC-based inverters are rated for full output at ambient temperatures up to 55°C, compared to the 45°C threshold established for standard inverters in CPRI testing.

A generation curve that flattens on a hot May afternoon in India is the inverter doing its job. The diagnostic steps above take a homeowner from "something looks wrong" to a clear identification of the cause, using data already in the monitoring app. For homeowners evaluating inverter options under PM Surya Ghar or wanting a site-specific performance assessment, visit Zenergize's product range.