Do solar panels work in moonlight

It can work, but the output is extremely low. Moonlight intensity is only 1/400,000 of sunlight, and full moon output is only about 0.03% of the rated value.

Due to the weak current, it is usually unable to activate the inverter, so it basically has no practical utility value.

Intensity

How much moonlight

The average geometric albedo of the lunar surface is only 12%, which means that the vast majority of the energy it reflects is lost along the way.

The full moon light intensity received on the Earth's surface usually fluctuates between 0.05 lux and 0.3 lux, while the artificial lighting intensity in a typical office is about 500 lux.

This means that the moonlight intensity is less than 1/1500 of indoor light.

From the perspective of energy conversion, monocrystalline silicon solar cells are most sensitive to the spectral response of wavelengths from 400 nm to 1100 nm.

Although the spectral peak of moonlight is near 600 nm, falling within the high-efficiency induction range, because the photon density is less than 10^14 per m² per second (compared to 10^21 under sunlight), the short-circuit current generated by this extremely low-density photon bombardment is usually only 200 μA to 500 μA.

Even using N-type TOPCon panels with a conversion efficiency as high as 24%, their actual output power under moonlight is often maintained below 0.001 W.

Electrons not moving

The open-circuit voltage of monocrystalline silicon batteries collapses sharply under low light, dropping from the standard 0.7 V to 0.1 V or even lower.

On a 450W panel composed of 72 cells in series, the moonlight induction voltage is often less than 10 V.

Due to the existence of a series resistance of about 0.5 Ohm inside the cell, as well as a shunt resistance that varies with temperature fluctuations, more than 80% of the weak charge disappears through internal recombination before flowing out of the panel.

According to the practical application of the Shockley-Queisser limit, when the light intensity is lower than 10 W/m², the fill factor (FF) of the photovoltaic module will decrease from the normal 0.8 to below 0.3.

This performance attenuation means that in a moonlight environment, the panel not only has low power, but its conversion efficiency will also fall from the nominal 22% to about 1%.

Equipment non-functional

The startup voltage threshold of mainstream string inverters is usually between 80 V and 200 V, while 10 panels in series under moonlight can only provide a maximum of 30 V DC electricity.

This means the MPPT tracker (Maximum Power Point Tracking) cannot be activated at all, and the inverter will remain in a shutdown standby state.

The nighttime standby power consumption of a 5kW inverter is between 2 W and 10 W, while the total output of the rooftop panels at this time may only be 0.01 W.

If the system is forced to start, the electricity consumed by the inverter will be 200 times to 1000 times the electricity produced by the panels.

Under a grid-connected voltage of 220 V, this negligible DC current simply cannot complete the inversion process through the power semiconductor bridge arms, ultimately resulting in 0 W of grid-connected revenue.

Efficiency returns to zero

Line losses during power transmission are calculated based on the square of the current. Although the moonlight current is extremely small, the contact resistance and leakage current in the lines will cause these tiny charges to be exhausted before reaching the controller.

In a 4 mm² DC cable, even with a residual current of only 0.5 A, a significant voltage drop occurs over a 50 m transmission distance. Under moonlight conditions, the current intensity is only at the mA level, and current of this intensity cannot even penetrate the contact resistance formed by certain oxide layers.

In addition, if dust accumulation on the surface of the photovoltaic module reaches 10 grams per square meter, it will block about 5% of the incident light. Given that moonlight is already weak, this obstruction will cause the output to drop directly to zero.

A waste of effort

Currently, the price of commercialized monocrystalline silicon modules is about $0.1 per watt to $0.15, while the cost of specialized ultra-high-sensitivity inverters required to capture moonlight for power generation may be as high as $5,000 or more.

If calculated at two full moons per month, the annual cumulative electricity generated by moonlight is less than 0.01 kWh. At a price of $0.1 per unit of electricity, the annual revenue is only $0.001.

This means it would take 5,000,000 years to recover the cost of a system specifically designed for moonlight.

Even the most advanced thin-film solar technology (CIGS) has a trigger threshold for photoelectric conversion much higher than the radiation level of moonlight.

For 99.99% of photovoltaic users, the physical significance of the moon lies only in providing weak visual light, rather than any form of energy supply.

Better to store electricity

A standard 10kWh Lithium Iron Phosphate (LFP) cell pack can be fully charged under 5 hours of effective sunlight during the day.

The charge-discharge cycle life of this type of cell is usually over 6000 times, and the depth of discharge (DoD) can reach 90%.

This means that even on nights without moonlight or during rainy days, the system can stably provide 9 units of available electricity, enough to support a 1.5-hp air conditioner for 8 hours.

The charge-discharge efficiency (round-trip efficiency) of energy storage systems is generally between 92% and 95%. Compared to the conversion efficiency of less than 1% for moonlight power generation, cell storage is the optimal solution for improving energy utilization by more than 100 times.

Energy Output

Energy density

As a reflector of sunlight, the average geometric albedo of the moon is maintained at around 0.12, representing that 88% of sunlight is absorbed or dissipated on the lunar surface.

Since the moon is about 384,400 km from the Earth, the reflected photons undergo Rayleigh scattering and Mie scattering when passing through the atmosphere. The light intensity reaching the panel surface is usually between 0.05 lux and 0.25 lux.

In contrast, even on a heavily cloudy day, the outdoor light intensity can reach over 1,000 lux.

From the perspective of the electromagnetic power spectrum, the peak power of sunlight at sea level is located near 500 nm, while the moonlight spectrum is similar in shape but the Photon Flux Density has dropped by 6 orders of magnitude.

On a 450W high-efficiency N-type panel with a surface area of 2 m², the moonlight energy received per second is less than 0.006 Joules.

Even without considering any conversion loss, this amount of energy cannot overcome the internal contact potential difference of the semiconductor P-N junction, causing the entire system to remain in a turned-off state in an electrical sense.

Quantitative comparison: The solar constant is approx. 1,361 W/m², reaching the ground is approx. 1,000 W/m², and full moon reaching the ground is approx. 0.003 W/m².

Efficiency returns to zero

The nominal efficiency of photovoltaic modules is usually between 21% and 23%, but this figure drops non-linearly as light intensity weakens.

When irradiance is below 100 W/m², the fill factor of photovoltaic cells drops rapidly;

When the light intensity drops to the moonlight level of 0.003 W/m², the influence of series resistance increases, and the leakage current caused by shunt resistance consumes almost all the generated photogenerated current.

Under this extreme weak light, the conversion efficiency of the cells will fall from 22% to below 1%, or even become negative due to internal resistance energy consumption.

Experimental data shows that the open-circuit voltage (Voc) of monocrystalline silicon cells under moonlight usually collapses from the nominal 45V to between 2V and 8V, and the short-circuit current (Isc) lingers at the μA level.

Technical parameters: Under 0.01 W/m² radiation, the proportion of internal resistance loss in standard monocrystalline silicon wafers will surge from the normal 3% to over 90%.

Insufficient power for load

Taking a standard 18W mobile phone fast charger as an example, the instantaneous input power it requires when working is more than 15,000 times the output of an entire 450W panel under moonlight.

If you tried to charge a smartphone with a cell capacity of 5,000mAh (approx. 19Wh) using moonlight, under an ideal 100% conversion rate, this panel would theoretically need to work continuously under a full moon for more than 6,300 hours, equivalent to about 262 days.

However, since the charging management chip inside the phone has its own standby power consumption of about 0.1 W, the weak current generated by moonlight will be immediately consumed by the control circuit itself. The phone's cell will not only fail to increase but might even lose more power due to the screen waking up to check the charging status.

For inductive load appliances like refrigerators (peak 150 W) or washing machines (peak 500 W), the contribution rate of moonlight power generation is beyond 7 digits after the decimal point, having no engineering practical significance.

Machine non-functional

For mainstream string inverters on the market, the MPPT tracking startup voltage is usually between 80V and 150V. Even with 10 panels connected in series, the total voltage induced by moonlight is often less than 30 V, failing to wake up the inverter's control loop.

Even if forced to start, the switching loss and drive loss generated by the power semiconductor devices (such as IGBT) inside the inverter during switching are between 10W and 30W.

If the panels can only provide 0.01 W of power under moonlight, while the machine operation requires 20 W, then the system is net-consuming energy stored in the grid or cell every second it runs.

This inverse ratio of up to 2000:1 between output power and self-consumption makes turning on the inverter at night a pure waste of resources.

Operating data: The nighttime standby current of a 5kW inverter is about 50mA, which corresponds to a standby power consumption of about 11.5W on the 230V AC side.

Storage is best

Current mainstream Lithium Iron Phosphate (LFP) energy storage cell packs have an energy retention efficiency (efficiency of charging and then discharging) of over 95%.

During the 4.5 hours of peak sunlight in the day, if a 5 kW system generates 22.5 units of electricity and 10 units are stored in the cell, this energy is enough to cover all the nighttime electricity expenses of a household, including running a 1,000 W air conditioner for 8 hours.

In contrast, even if the panels covering your entire roof are receiving moonlight, the energy they produce all night isn't enough to balance the self-consumption of the storage cell's BMS management system for one hour.

The cost of a 10kWh energy storage cell has dropped to between $1,500 and $2,500, and the economic return it provides is hundreds of millions of times higher than trying to capture moonlight.

Conversion

The truth of conversion

The bandgap energy of monocrystalline silicon is fixed at around 1.12 eV, meaning only photons with energy greater than this value can excite electron-hole pairs.

Although the spectrum of moonlight is mainly distributed between 380 nm and 750 nm, and its photon energy indeed falls within the absorbable range of monocrystalline silicon, the problem is that the photon density is too sparse.

Under standard sunlight conditions, about 10^21 photons hit the panel per square meter per second, while in a full moon environment, this number drops to the level of 10^14 to 10^15.

This extremely low photon flux density generates very few carriers inside the P-N junction. Most of the generated electrons recombine with holes before they can move to the electrode, causing the conversion efficiency to experience a cliff-like drop.

Conversion is difficult

Under normal 800 W/m² irradiance, the shunt resistance of the panel is usually as high as several thousand Ohms, while the series resistance is extremely low, ensuring that current can flow smoothly to the load.

But under the extremely low irradiance of moonlight, the photogenerated current is even of the same order of magnitude as the panel's own leakage current. In this case, the consumption of current by the shunt resistance becomes non-negligible.

Experimental measurements show that when the irradiance is lower than 1 W/m², the fill factor of photovoltaic cells will drop sharply from 0.82 to around 0.25.

For a 450W high-efficiency N-type monocrystalline silicon module, its full-moon conversion voltage at 25 degrees Celsius can usually only be maintained between 5 V and 12 V.

Because this voltage is much lower than the turn-on voltage of the internal diodes of the panel, the entire P-N junction cannot be effectively "activated."

From the law of conservation of energy, the total power of moonlight projected on an area of 2 square meters is only 0.006 W. Even converting at a theoretical efficiency of 20%, the output power is only 0.0012 W.

Electrons not moving

Under moonlight conditions, because the photon impact frequency is extremely low, the amount of charge generated per unit time cannot establish a sufficient potential difference across the electrodes.

Under normal circumstances, a PV array composed of 10 panels in series has a rated operating voltage of around 350 V, but on a full moon night, the total voltage of the entire array often fails to even reach 50 V.

Since the startup threshold of the inverter (usually 80V to 150V) is not met, the power semiconductor devices inside the system cannot enter the switching operation mode.

At this point, the entire photovoltaic system behaves more like a giant piece of black glass rather than power generation equipment in terms of electrical characteristics.

We can compare the output performance indicators under different light intensities. At 1,000 W/m², the current is 13 A; under moonlight of 0.003 W/m², the current drops to 0.00004 A.

This microampere-level current intensity cannot even overcome the resistance of the oxide layer at the cable joints.

If a micro-inverter is forcibly connected, the current demand of the inverter's internal chip in standby mode is usually between 10 mA and 50 mA, meaning the current generated by moonlight is less than 1/1000 of the machine's self-consumption.

Efficiency returns to zero

In industrial design, the design goal of panels is to optimize performance in the range of 200 W/m² to 1,200W/m². When the ambient brightness drops to extremely low levels, impurity energy levels in the silicon lattice trap the already few free electrons; this phenomenon is known as the "trap effect."

Under such weak light as moonlight, almost all photogenerated electrons are captured by lattice defects and undergo non-radiative recombination, with kinetic energy directly converting into heat loss rather than electricity.

According to actual tests, when the radiation intensity of monocrystalline silicon modules is below 5 W/m², their measured efficiency will drop linearly to 1% or even lower, meaning the equipment has lost its power generation function.

If considering temperature compensation, since nighttime temperatures are usually lower (assuming 10 degrees Celsius), although lower temperatures theoretically slightly increase the panel voltage, this gain is meaningless in the face of the micro-radiation of moonlight level.

Assuming you own a 10 kW PV system worth $15,000, its instantaneous power generation revenue under full moon is about $0.00000005 per hour.

Even running for 1,000,000 hours (about 114 years) under this ideal state, the total electricity value generated wouldn't even be enough to buy a bottle of the cheapest pure water.

The machine cannot be turned on

A mainstream 5,000 W grid-connected inverter requires about 5 W to 12 W of power to support its internal control circuits, display screen, Wi-Fi monitoring module, and power drive circuits while in standby.

If you want to use the electricity generated by moonlight to run this machine, you would need to install more than 5,000 square meters of panels on the roof simultaneously (about the size of half a football field) just to supply the inverter for "powering on and standing by."

Even then, the machine after startup wouldn't be able to generate any excess electricity to supply the appliances in your home.