What if solar panels received less sunlight?

Some houses have solar panels on the roof. The panels use sunshine to make electricity.

What happens on a cloudy day, when the clouds hide the Sun?

Solar panels are dark, flat panels that turn sunlight straight into electricity. You may have seen them on roofs, over parking lots or in big fields.

But sunshine is not the same every day. Clouds, dust and pollution can block some of it. In this lab, a single rooftop panel faces the Sun on a long summer day. What happens to the electricity it makes when clouds let through only a quarter of the direct sunshine?

A photovoltaic (PV) panel converts a fixed share of the light that falls on it into electrical power. That share, its efficiency, is set by the cell technology; the light itself depends on the Sun's position, the panel's angle and what the atmosphere lets through.

This lab holds everything else constant (latitude 40°, 21 June, a 30° tilt facing the equator, a 1.7 m² panel at 20% efficiency) and cuts the direct sunshine from 100% to 25%, a cloudy sky in the model's terms.

On a sunny day, the panel soaks up lots of sunshine.

In the morning it makes a little electricity. At noon, when the Sun is high, it makes the most.

Press play and watch the day go by. In the early morning the Sun is low and the panel makes very little. As the Sun climbs, the power rises, peaking at about 310 watts around noon, then falls again toward evening.

Over the whole sunny day, about 15 hours of daylight, the panel makes about 2.3 kilowatt-hours of electricity. That's about enough energy to run a 100-watt light for 23 hours.

Under a clear sky the model's beam irradiance follows an air-mass formula, so it is weak near sunrise and sunset and strongest at solar noon. The 30° tilt brings the rays within about 13° of straight-on at midday in June. Peak power is 310 W; integrated over 14.8 hours of daylight, the panel yields 2.32 kWh.

That is 20% of the direct sunlight it intercepts, the model's fixed efficiency.

Now clouds cover the sky. Only a little sunshine gets through to the panel.

The panel makes much less electricity. If the clouds let through a quarter of the sun, it makes about a quarter of the power.

Choose the cloudy sky: only a quarter of the direct sunshine gets through. The panel's power drops to about 77 watts at noon, a quarter of the sunny-day value. Over the day it makes only about 0.58 kilowatt-hours.

In the model, the power always follows the sunshine exactly: half the sunshine gives half the power. To make as much electricity on this cloudy day as one panel makes on a sunny day, you would need four panels.

At 25% sunshine the model gives 77.5 W at noon and 0.581 kWh for the day, both exactly 25% of the clear-sky values, because P = beam × cos θ × A × η is linear in the beam. Four panels (6.8 m²) at 25% sunshine reproduce the single panel's clear-day 2.32 kWh.

For comparison, the same clear-sky panel on 21 December gives 208 W at noon and 1.19 kWh over 9.2 hours of daylight, about half the June energy even under a clear sky.

The panel needs light to make electricity. Less light means less electricity.

Clouds are like a blanket over the Sun. They stop some of the light from reaching the panel.

A solar panel turns a share of the light that hits it into electricity. The panel in our model turns one fifth of it into electricity. If less light arrives, there is less to turn into electricity, so the power drops.

Our model is simple in one important way: it only counts the direct beam of sunlight. In the real sky, some light is also scattered by air and clouds and arrives from the whole sky. Real flat panels can use that scattered light too, so on cloudy days they can make more than our model says.

At a fixed efficiency, electrical power is proportional to the light the panel receives. The model computes the clear-sky beam, scales it by the sunshine percentage, and multiplies by the cosine of the angle of incidence.

Model limits: diffuse skylight and ground-reflected light are left out, so cloudy-day and low-sun output are underestimated; efficiency is fixed, although PVEducation notes that silicon cells lose about 0.4–0.5% of their power for each degree Celsius they warm up; losses in wiring and inverters are not included; and the clear-sky formula averages over altitude, dust and humidity.

Real solar panels make the most electricity on clear, sunny days around noon.

They can also use light from all over the sky.

The U.S. Energy Information Administration explains that sunshine is strongest at solar noon on clear, cloudless days, and that clouds, dust, volcanic ash and pollution all reduce the sunlight reaching the ground.

It also explains that flat solar panels can use both direct sunlight and scattered light from the sky. Our model counts only the direct beam, so real panels can make more than it shows when clouds roll in. Panels have also improved: the best ones today turn almost a quarter of sunlight into electricity.

The EIA notes that solar radiation is most intense at solar noon on clear, cloudless days; that latitude, climate and weather are major factors in insolation; that clouds, dust, volcanic ash and pollution reduce it; and that flat-plate PV collectors can use global radiation, diffuse plus direct, while concentrating systems need direct radiation and suit arid regions with few cloudy days.

On efficiency, the EIA reports that commercial PV panels averaged under 10% in the mid-1980s, about 15% by 2015, and are now approaching 25% for state-of-the-art modules.