Every December, the same message arrives on our WhatsApp from solar owners across North India: “My panels were producing so well in summer. Why has generation suddenly dropped? Is something wrong?” Most of the time, nothing is wrong at all. Winter and fog change how much sunlight reaches your roof, and once you understand why, those low numbers in the monitoring app start to make sense.
The short answer: solar panels keep generating in winter and even in fog. Cold weather actually improves their efficiency. Output dips in December and January because days are shorter, the sun sits lower in the sky, and fog, smog and dust cut the light reaching your panels. The dip is deeper in fog-prone North India, and a solar system should be judged on a full year, not one grey week.
First, a myth worth clearing up: cold is not the problem
Many people assume solar panels work best when it’s hot. It feels intuitive. Summer means strong sun, strong sun means more electricity, so heat must be good. In reality, heat is one of a solar panel’s enemies.
A panel’s rated power, the “540 W” printed on its label, is measured under standard test conditions: sunlight of 1,000 watts per square metre and a cell temperature of 25 °C. For every degree the cells run hotter than that, output falls slightly. This is called the temperature coefficient, and you’ll find it on every panel datasheet. For today’s mono PERC panels it is typically around −0.34 to −0.40% per °C; for newer N-type TOPCon panels it is about −0.29 to −0.32% per °C.
On a summer rooftop in Rajasthan or Delhi, panel cells easily run 20–30 °C hotter than the air around them. On a cool winter day, they stay close to their ideal temperature. Here’s what that means for a single 540 W panel receiving the same amount of sunlight:
| Condition | Cell temperature (assumed) | 540 W PERC panel | 540 W TOPCon panel |
|---|---|---|---|
| Hot summer afternoon | 65 °C | ≈ 464 W | ≈ 475 W |
| Mild winter afternoon | 40 °C | ≈ 512 W | ≈ 516 W |
| Cold, clear, breezy morning | 20 °C | ≈ 549 W | ≈ 548 W |
Illustrative calculation at 1,000 W/m² using −0.35%/°C (PERC) and −0.30%/°C (TOPCon). Real cell temperatures depend on wind, mounting and sunlight level.
So hour for hour, a clear winter day is excellent solar weather. If your total units fall in winter, the reason lies somewhere else: in the amount of light, not the temperature.
So why does winter generation drop?
The winter dip is a combination of astronomy, weather and what’s happening on your roof. Let’s take them one at a time.
The days are simply shorter
Around the winter solstice in late December, Jaipur gets roughly 10.5 hours between sunrise and sunset. In June it gets about 13.7 hours. That’s more than three hours less daylight, and since the first and last hour of any day produce very little anyway, the loss of useful generating time is significant.
The sun sits much lower in the sky
This is the factor most people underestimate. At midday in June, the sun over Jaipur stands almost directly overhead, about 86° above the horizon. At midday in late December it reaches only about 40°.
A low sun hurts in two ways. Its light travels a longer path through the atmosphere, so more of it is scattered and absorbed before it arrives. And the same beam of light spreads over a larger area of ground, delivering less energy per square metre of panel.
Shadows get much longer
Here’s a detail that catches many homeowners out. At 9 AM (solar time) in late December, the sun in Jaipur is only about 23° above the horizon. At that angle, a one-metre-high parapet wall or water tank casts a shadow roughly 2.3 metres long. Something that never touched your panels in May can quietly shade a corner of the array every winter morning and afternoon, and because panels are wired in strings, shading even part of one panel can pull down the output of others.
Fog, smog and dust
Then comes the weather. Fog, smog and dust are the reasons North India’s winter dip is so much deeper than South India’s, and they deserve a closer look.
What fog actually does to a solar panel
Fog is, quite literally, a cloud sitting at ground level: tiny water droplets suspended in the air, thick enough to cut visibility below a kilometre. India’s winter fog is mostly radiation fog, which forms on long, clear, still nights when the ground loses heat quickly, along with fog carried in by moist air. It’s concentrated over the plains of North India from December to February.
To understand fog’s effect, it helps to know that sunlight reaches your panels in two forms. Direct light comes in a straight line from the sun and carries most of the energy on a clear day. Diffuse light has been scattered by the atmosphere, clouds or fog and arrives from the whole sky. Fog strips away most of the direct light, leaving your panels to run on diffuse light alone. They still work, but at a fraction of their usual power.
A typical foggy day follows a familiar pattern. In the early morning, the inverter may switch on later than usual, because it needs a minimum voltage and power from the panels before it connects to the grid. Generation stays low through the grey hours. On many days, radiation fog thins and lifts by late morning as the ground warms, and output can climb quickly once it clears. On the worst days, during cold waves, fog or low cloud lingers until evening, and several such days in a row produce the lowest totals of the year.
How much generation does fog cost?
It depends entirely on how thick the fog is, how long it lasts, and how much pollution is mixed into it. A light mist that burns off by 10 AM costs part of the morning. A fog that persists all day can cut a day’s generation by well over half.
The clearest large-scale picture comes from satellite-based solar data. Solargis reported that in January 2024, when some parts of northern India saw persistent fog or smog for up to 20 days, solar irradiance across parts of the region came in 30–50% below the long-term average. The same month set records for both the lowest average monthly temperature and the lowest irradiance. The impact showed up on the national grid too, with Mint reporting that India’s solar generation dipped by more than 3% that January amid dense fog.
Worth keeping in proportion: even in the foggiest cities, dense fog affects a limited number of days in a limited part of the year. February to June and October to November usually bring strong generation that more than makes up for it.
Smog and dust: the losses you can partly control
Fog gets the headlines, but air pollution quietly takes its own share. Winter traps pollutants close to the ground, and the particles affect solar output twice over: once while floating in the air, dimming the sunlight, and again after they settle on the panel glass, physically blocking it.
A widely cited 2017 study by researchers from Duke University and IIT Gandhinagar, published in Environmental Science & Technology Letters, estimated that dust and particulate pollution cut solar energy production in parts of India by roughly 17–25%, even with monthly cleaning. About half of that loss came from deposits on the panel surface. The researchers also noticed something every solar owner should know: in northern India, uncleaned panels could lose around half their output within three to four weeks, and production roughly doubled once the panels were cleaned.
Winter makes this worse in a sneaky way. From October to February there’s very little rain to rinse the glass, and morning dew can bind dust and soot into a thin, muddy film. If your output stays stubbornly low on a clear afternoon after the fog has lifted, dirty glass is often the culprit. You can’t clean the sky, but you can clean your panels, and winter is when that effort pays back most clearly.
How winter looks across different parts of India
The winter dip isn’t the same everywhere. Fog and smog are concentrated in the north and across the Indo-Gangetic plains, while western and southern India usually stay clearer. Here’s a rough guide. For your exact location, the free Global Solar Atlas shows long-term sunlight data.
| Region | Typical winter conditions | Size of winter dip |
|---|---|---|
| Punjab, Haryana, Delhi NCR, western UP | Frequent dense fog and smog in Dec–Jan, cold waves | The largest in the country in bad years |
| Eastern UP and Bihar | Fog and haze across the Gangetic plain | Large |
| North Rajasthan | Occasional dense fog spells, dry dust | Moderate, larger during fog spells |
| Central and south Rajasthan, Gujarat | Mostly clear, dry winters with some haze and dust | Mainly from shorter days and lower sun |
| Karnataka, Telangana, Tamil Nadu | Mild winters with relatively little seasonal change | Small |
| Hill states and Ladakh | Very clear, cold air; snow on some days | Snow-covered panels produce almost nothing until cleared |
A qualitative guide based on typical winter weather patterns; actual conditions vary year to year.
What about Jaipur and the rest of Rajasthan?
Rajasthan has one of the strongest solar resources in India, and most of the state, Jaipur included, enjoys largely clear, dry winters. Jaipur’s annual average global horizontal irradiance is roughly 5.2 kWh per square metre per day according to Global Solar Atlas data and published studies, peaking before the monsoon and dipping during the monsoon and mid-winter.
For a Jaipur rooftop, the winter dip mostly comes from the shorter, lower-sun days and from dust. Dense fog is less common here than in Punjab or UP, though the IMD regularly reports dense fog over north Rajasthan during cold spells, and hazy days happen across the state. Two things work in Rajasthan homeowners’ favour. Winter electricity use usually drops sharply once air conditioners and coolers are switched off, so a smaller winter generation still covers much of a smaller winter bill. And the powerful February-to-June season does the heavy lifting for your yearly savings.
Reading your monitoring app without panicking
The most common winter mistake is comparing December’s numbers with May’s. That comparison will always look alarming, and it tells you nothing about whether your system is healthy. The useful comparison is your system against itself in similar conditions: this December against last December, a clear day against a clear day.
A few patterns are completely normal in winter. Daily units that roughly match the same month last year are fine. A very poor day that happened to be foggy or smoggy is fine. An inverter that wakes up late on a foggy morning is fine. A gradual decline over a couple of weeks that recovers after cleaning is just dust.
Other patterns deserve a call to your installer. Watch out for very low output on a genuinely clear, sunny day, or generation well below last year’s same month in similar weather. A drop that doesn’t recover after a proper cleaning is another warning sign. So are error codes on the inverter, or one string reading zero or far below the other all day long. If you notice one string dipping only in the early morning or late afternoon, that’s usually winter shade rather than a fault, but it’s worth checking which structure is causing it.
A simple habit helps enormously: download your monthly generation figures every year. Within two years you’ll have your own baseline, and you’ll be able to tell at a glance whether your system is behaving normally.
Does winter fog hurt your solar savings?
For grid-connected homes and businesses, the honest answer is: a little, for a few weeks, and far less than it feels on a grey January morning. Rooftop solar is sized and evaluated on a full year’s generation, and a sensible design already expects weaker winter and monsoon months.
Net metering is what smooths out the seasons. A bidirectional meter records the units you draw from the grid and the units your panels send back. In sunny months, surplus units are credited against your consumption under your state’s rules; in weaker months, you draw the shortfall from the grid. In Rajasthan, these rules are set by the Rajasthan Electricity Regulatory Commission (RERC) and applied by the three distribution companies: JVVNL in the Jaipur zone, AVVNL in the Ajmer zone and JdVVNL in the Jodhpur zone.
One caution here. How surplus credits carry forward, and what happens to unused credits at the end of the financial year, has changed through several RERC amendments. Before you build winter shortfalls into your financial calculations, confirm the current settlement terms with your DISCOM or on the RERC website. (Information last checked: 20 October 2026.)
Winter does deserve extra thought in a few situations. Homes that run geysers, room heaters or heat pumps may use more power in winter, not less, and that should be factored into sizing. Shops, offices and factories with steady daytime loads all year will import more from the grid in December and January, so their financial projections should use month-by-month generation rather than an annual average divided by twelve. And for anyone relying on batteries, consecutive foggy days matter much more, which brings us to the next point.
Off-grid and hybrid systems during long fog spells
A battery stores solar energy; it can’t create it. During a run of foggy days, your panels may not produce enough to fully recharge the battery, and an off-grid or hybrid system can run short if it wasn’t planned with winter in mind.
The fix is mostly about honest planning. Size the battery for your essential winter loads (lights, fans, refrigerator, router, perhaps a pump) rather than for the whole house. Decide how many low-sun days you want the system to ride through, knowing that every extra day of autonomy adds cost. Keep a backup charging source: a hybrid inverter can top up from the grid, and remote sites may use a generator on the darkest days. Off-grid sites with high winter needs can benefit from a little extra panel capacity so batteries refill faster on short days. And on foggy days, shift heavy jobs like laundry and water pumping into the clearest afternoon hours.
Design choices that protect winter output
You can’t control the weather, but design decides how much of the available winter sunlight you actually capture. If you’re planning a new system, these are the questions worth asking your installer.
Was shading checked for the winter sun? A summer site visit can miss the long December shadows completely. A common design rule is to keep the array shade-free from roughly 9 AM to 3 PM on the winter solstice, and this affects where panels go, how far apart rows sit and how high the structure needs to be.
Is the tilt sensible for the whole year? In North India, south-facing panels tilted close to the local latitude (around 25–28° for Jaipur and Delhi) give strong year-round output. A steeper tilt would help in winter but cost more units in summer, so fixed systems are usually optimised for the full year rather than one season.
Were the strings sized for cold mornings? Panel voltage rises in the cold. A string that looks fine on paper at 25 °C can exceed the inverter’s maximum input voltage on a frosty January morning, so a good designer calculates string length for the coldest expected temperature as well as the hottest.
Does the inverter suit dim conditions? An inverter with a wide operating voltage range and a modest start-up voltage begins harvesting earlier on dim, foggy mornings. Two or more MPPT inputs help when parts of the roof face different directions or see different winter shade.
Can the panels be cleaned safely? Panels that are hard to reach don’t get cleaned. A walkway, a water point near the array and a structure that allows safe access make regular winter cleaning realistic.
As for panel technology, any good-quality modern monocrystalline panel works well in winter. Better temperature coefficients mainly pay off in summer heat; for fog, clean glass and an unshaded array matter far more. Choosing ALMM-listed panels from reputable manufacturers is the sensible baseline.
A simple winter care routine
Most avoidable winter losses can be recovered with a short, regular routine between October and February:
- Clean every one to two weeks in dusty or smoggy areas, and more often after dust storms, nearby construction or a run of foggy, polluted days.
- Clean early in the morning, when panels are cool, dew has already softened the dirt, and you lose the least generation.
- Use soft water and a soft brush or sponge. Skip detergents, scrubbing pads, metal scrapers and pressure washers, which can damage the anti-reflective coating and seals.
- Stay safe. Winter dew makes roofs and glass slippery. Never stand on panels, and leave high or awkward roofs to a trained team.
- Look for new shade around 9–10 AM and 3–4 PM, and trim branches that have grown since summer.
- Check your app weekly and compare with the same period last year.
- Keep up the annual inspection of connectors, earthing, surge protection and the inverter, ideally before the high-generation season starts.
If you’d rather not climb onto the roof every fortnight, a solar maintenance and AMC plan can take care of cleaning and performance checks for you.
The bottom line
Winter doesn’t break solar panels, and fog doesn’t switch them off. Cold weather makes each hour of sunshine more productive; it’s the shorter days, the low sun, the fog, the haze and the dust that bring December’s numbers down. In fog-prone parts of North India the dip can be steep in a bad January, while in sunnier places like most of Rajasthan it is far gentler.
What matters is that your system is designed for the low winter sun, kept clean through the dry months, and judged on what it delivers across a whole year. Do that, and a few grey weeks become just one part of a very sunny story.
Frequently asked questions
Do solar panels work in winter?
Yes. Solar panels run on light, not heat, so they keep generating through winter. Cold temperatures actually help panel efficiency. Total daily units are lower mainly because days are shorter, the sun is lower in the sky, and fog, haze or dust reduce the sunlight reaching the panels.
How much does solar output drop on foggy days?
It depends on how dense the fog is and how long it lasts. A light morning mist that clears by 10 or 11 AM may cost only a modest share of the day’s units, while a thick fog that persists all day can cut generation sharply. Solargis reported that in January 2024, a month of severe fog and smog, solar irradiance in parts of northern India was 30 to 50 percent below the long-term average.
Do solar panels generate any electricity in fog?
Yes, usually a little. Fog scatters sunlight rather than blocking all of it, so panels keep working on diffuse light, just at a much lower power level than on a clear day. The inverter may also start later in the morning because it needs a minimum voltage and power before it switches on.
Are solar panels more efficient in cold weather?
Yes. Panel output falls as cell temperature rises above 25 degrees C and rises slightly when cells are cooler. With a typical temperature coefficient of about minus 0.30 to minus 0.40 percent per degree C, a panel can lose 10 to 15 percent of its rated power on a hot summer afternoon but very little on a cool, clear winter day.
Why is my solar generation low in December and January?
Some dip is normal. December and January have the shortest days of the year, a lower sun angle, longer shadows from nearby structures, and in many regions fog, smog or haze. Dust also builds up because there is little rain to wash the panels. Compare your output with the same month last year, not with April or May, to judge whether the system is healthy.
Should I clean my solar panels more often in winter?
In dusty or smoggy areas, yes. Without rain, dust and pollution particles settle on the glass, and morning dew can turn them into a thin muddy film. Cleaning every one to two weeks with soft water and a soft brush or sponge, early in the morning, usually works well. Avoid detergents, abrasive pads and high-pressure jets.
Is rooftop solar worth it in foggy cities like Delhi or Lucknow?
For most grid-connected homes and businesses, yes. Solar is sized and evaluated on annual generation, and the strong output from February to June and after the monsoon usually outweighs the weaker fog weeks. What matters is setting realistic winter expectations and designing the system properly.
Does winter affect my solar savings under net metering?
Your winter bills may rise slightly if generation falls below consumption, but net metering lets surplus units exported in sunnier months be credited against your bills under your state’s rules. In Rajasthan these rules are set by the RERC and applied by JVVNL, AVVNL and JdVVNL. Settlement terms change from time to time, so confirm the current carry-forward and year-end rules with your DISCOM.
Should I change my solar panel tilt for winter?
For most fixed rooftop systems, no. A tilt close to your latitude gives good year-round output; in Jaipur that is roughly 25 to 27 degrees. A steeper tilt would help winter output but reduce summer output, and adjustable structures add cost and maintenance. Seasonal tilt changes only make sense in specific cases such as some off-grid systems with high winter loads.
Will a battery help during foggy days?
A battery helps you use stored solar energy at night or during power cuts, but it cannot create energy the panels did not produce. During several days of continuous fog, an off-grid or hybrid system may not fully recharge its batteries. Plan for this by sizing the battery for your essential loads, keeping a grid or generator backup, and reducing non-essential use on dark days.
Keep reading
- How much does solar panel installation cost for a home?
- Estimate your savings with the solar ROI calculator
- PM Surya Ghar Yojana: rooftop solar subsidy explained
- Rooftop solar for homes
- Rooftop solar installation across Rajasthan
Wondering whether your own system’s winter numbers are normal, or planning a new installation that’s designed for every season? Talk to our engineers on WhatsApp or call +91 74148 13813 (Mon–Sat, 9 AM–7 PM), or request a free site survey.