A solar panel keeps working on a cloudy day because its cells respond to light, not to blue sky. Cloud scatters sunlight rather than switching it off, so the panel receives a weaker, more even glow from the whole sky and produces less current in proportion. Under thin high cloud a system can still deliver a large share of its clear-day output; under thick, dark overcast it commonly falls to somewhere around 10-25% of rated output. What changes inside the cell, the inverter and the sky explains why cloudy-day output varies so much.

Direct, diffuse and reflected light

The sunlight reaching a panel is measured as irradiance, in watts per square metre (W/m²). It arrives by three routes:

• Direct (beam) light travels in a straight line from the sun's disc and casts sharp shadows. On a clear day around noon it supplies most of the energy, and total irradiance on a well-angled panel can approach 1,000 W/m².
• Diffuse light has been scattered by air molecules, water droplets, dust or smoke, and arrives from every part of the sky. Under full overcast it is almost all the panel receives.
• Reflected light bounces off the ground, walls or nearby roofs. It is usually a small contribution for rooftop panels.

Clouds change the mix. A thin veil of high cloud removes some direct light but scatters much of it forward, so irradiance stays fairly high. Thick, low, rain-bearing cloud reflects far more light back to space, and what gets through is entirely diffuse: often only a few hundred W/m², and sometimes below 100 W/m² under very dark skies.

What the cells do when the light dims

Inside a silicon cell, each photon with enough energy can free an electron, and the cell's built-in electric field pushes those electrons out as current (the basics are covered in how solar power works). Fewer photons means fewer freed electrons, so current falls roughly in proportion to irradiance. If irradiance drops to a fifth, current drops to about a fifth.

Voltage behaves differently. A cell's voltage depends on the logarithm of the light level, so it falls only modestly as the sky darkens. That is why a string of panels can show a normal-looking voltage on a grey morning while delivering very little power: power is voltage multiplied by current, and it is the current that has dropped away.

Quality monocrystalline cells handle low light well. Many datasheets include a low-irradiance rating, commonly showing efficiency at 200 W/m² within a few per cent of the full-sun value. Cooler cells under cloud help slightly too, because silicon produces a little more voltage when it is cool. Diffuse skylight is also slightly richer in blue wavelengths than direct sun, but silicon responds across the visible and near-infrared spectrum, so the colour shift matters far less than the drop in intensity.

Why the inverter matters on grey days

The inverter converts the panels' DC into AC, and it has its own low-light limits:

• Start-up thresholds. An inverter needs a minimum string voltage and enough power to run its own electronics before it starts exporting. Early and late on a dark day it may simply wait.
• Part-load efficiency. Conversion efficiency is typically highest across the middle of the inverter's power range and lower at very small loads, so the first few hundred watts are converted less efficiently than midday power.
• Maximum power point tracking (MPPT). As irradiance flickers, the tracker keeps searching for the best combination of voltage and current. Fast-moving broken cloud makes that search harder, although modern trackers respond many times a second.

Realistic output under different skies

Output depends on season, roof angle, shading and the equipment, so the figures below are indicative ranges only. As a reference point, a 6.6 kW system in Sydney produces roughly 24-27 kWh a day on average across the year.

These percentages describe instantaneous power. A day's energy also depends on how long the cloud lasts, so a grey morning that clears by lunchtime costs far less than a week of winter drizzle.

The cloud-edge effect

On days with scattered cumulus, monitoring apps sometimes show power briefly exceeding clear-sky midday output. This is the cloud-edge, or cloud-enhancement, effect. When the sun is uncovered but bright cloud sits close beside it, the panel receives full direct light plus extra light scattered off the sunlit sides of the cloud. For seconds to minutes, irradiance can rise noticeably above clear-sky levels.

Well-designed systems cope with these spikes. The inverter limits its output to its rated AC capacity, and cables and protection devices are selected with margins for higher-than-standard irradiance. A short flat top on the power graph during broken cloud is normal behaviour, not a fault.

Orientation and shading under cloud

Because diffuse light arrives from the whole sky dome rather than one direction, panel orientation matters less on overcast days. East- and west-facing arrays, which give up some output to north-facing panels in clear weather, lose relatively less under full cloud. Shading also softens: a chimney that casts a hard shadow in direct sun blocks only a slice of the sky under overcast conditions. The catch is that total light is much lower, so there is less energy to gain or lose either way.

Reading your monitoring after a grey week

Low production during a cloudy spell is expected, and a single poor day says little about a system's condition. More useful checks include:

• Compare clear days with clear days, ideally in the same month of a previous year, rather than a cloudy week with a sunny one.
• Look at the shape of the daily curve: a smooth arc on a clear day suggests normal operation, while a curve that is notched or flat at zero on a sunny day deserves attention.
• Check whether every string is affected equally, which points to weather, or only one, which points to shading or a fault.
• Expect a battery to fill less during a run of cloudy days, so evening grid imports can rise temporarily.

Many long-standing beliefs about solar and cloud do not survive a look at the physics; solar myth-busting works through the most common ones.

Next steps

Cloudy-day output is one reason system size, orientation and battery capacity should be designed around real local conditions rather than best-case sunshine. To see how a system would perform on your roof through the seasons, request a free assessment from Blue Energy Solar. If you already have solar and suspect it is underperforming, the energy market lists a System Performance Audit that compares 12 months of generation with modelled output, from $390 for a residential system; prices are indicative and confirmed after a site assessment.

Frequently asked questions

Do solar panels produce anything under moonlight or streetlights?

Technically a panel generates a tiny current under any light, but moonlight is so faint that the output is negligible, and streetlights are weaker still at roof level. The voltage may register on a meter, yet there is far too little current to start an inverter. For practical purposes, solar generation ends at dusk, which is why evening and overnight use relies on a battery or the grid.

Will a home battery still charge on a cloudy day?

It can, but more slowly. The battery only charges from solar that is left over after the home's own loads are met, and on an overcast day that surplus may be small or absent. A battery may therefore finish the day partly charged. Some systems can be set to top up from the grid in cheaper tariff periods, which is a setting worth reviewing before winter.

Does rain affect solar output differently from ordinary cloud?

Rain itself does little to the panel; the reduction comes from the thick cloud that produces it, so output during rain is usually at the low end of overcast levels. Water on the glass barely changes how much light passes through. Heavy rain can even help afterwards by rinsing loose dust off tilted panels, so production on the next clear day may be slightly higher.