What if solar panels were near the equator?

In winter, the Sun stays low in the sky in some places. It sets early in the afternoon.

Near the middle of the Earth, the equator, the Sun is high all year. Where would a solar panel make more electricity?

If you live far from the equator, you know winter days: the Sun rises late, creeps low across the sky and sets early. Near the equator, days are about 12 hours long all year, and the midday Sun is always high.

Solar panels need sunlight. In this lab, a rooftop panel sits in London on 21 December, the shortest day of the year there. What happens to the electricity it makes if you move it to the equator on the same day?

The solar resource at a site depends on how high the Sun climbs, how long it stays up and how much atmosphere its light must cross, all set by latitude and the Sun's declination, plus local weather.

This lab takes a 1.7 m², 20%-efficient panel tilted 30° toward the equator and compares its clear-sky output at 51.5° N (London) and at 0° on day 355, near the December solstice, when the Sun's declination is about −23.4°.

In London in December, the Sun is up for only about 8 hours.

It stays low, close to the rooftops. The panel makes a little electricity.

Press play and watch a December day in London. The Sun is up for only about 7.6 hours, and even at noon it is just 15° above the horizon.

Low sunlight crosses a lot of air and hits the panel at a slant. Our panel peaks at only about 135 watts and makes about 0.64 kilowatt-hours over the whole short day.

On day 355 at 51.5° N, the noon elevation is 90° − 51.5° − 23.4° ≈ 15.1°, and the model's day length is 7.6 h. At that elevation the air mass is close to 4, so the clear-sky beam is weaker, and the 30° panel meets the noon rays 45° from its normal.

The model gives 397 W/m² of direct beam on the panel at noon, 135 W of output, and 0.64 kWh for the day. It counts only the direct beam and assumes a perfectly clear sky.

Now Olly moves the panel to the equator. The Sun is up for 12 hours!

It climbs high in the sky. The panel makes much more electricity.

Set the latitude to 0°. On the same December day, the Sun now shines for 12 hours and climbs to about 67° at noon. The panel peaks at about 313 watts and makes about 2.34 kilowatt-hours, more than three and a half times as much as in London.

Now try June. At the equator, a flat panel makes about 2.01 kilowatt-hours in both June and December. Back in London in June, the days are 16 hours long, and the tilted panel makes about 2.45: more than the flat one at the equator. Far from the equator, the seasons swing much more.

At 0° on day 355, the noon elevation is 66.6° and the day is 12.0 h. The 30° panel meets the noon Sun 6.6° from its normal: 921 W/m² on the panel, 313 W, 2.34 kWh, a ratio of 3.66 to London.

The ratio is seasonal. At the equator a flat panel gives 2.01 kWh on both day 172 and day 355, while London's 30° panel swings from 0.64 kWh to 2.45 kWh. (The model always tilts a panel toward the equator, and at exactly 0° latitude it tilts it south, so a tilted panel there loses out in June: 1.15 kWh at 30°. Use a tilt of 0 at the equator.) Long summer days at high latitude can even beat the equator in the model.

Earth is tilted. In December, the northern half of Earth leans away from the Sun.

So places like London get low Sun and short days. The equator gets high Sun all year.

Earth's axis is tilted. In December, the northern half of the planet leans away from the Sun, so northern places get short days and a low Sun. At the equator, the Sun rises and sets at about the same times all year and climbs high every day.

Low sunlight is weaker for two reasons: it crosses much more air, and it hits the panel at a slant. Our model is simple: it assumes a clear sky every day, counts only direct sunlight, and leaves out clouds and dust, which matter a lot in real places.

Day length and noon elevation follow from latitude φ and declination δ: sin(h_noon) = cos(φ − δ). As elevation falls, the air mass grows (about 1/sin h), so the clear-sky beam weakens, and a fixed panel catches it more obliquely.

The model ignores diffuse skylight, temperature effects and weather. That last omission matters: the EIA lists climate and weather alongside latitude as major factors in insolation, and clouds reduce the sunlight reaching the ground, so latitude alone does not set a place's real solar resource.

Scientists have worked out how much electricity solar panels could make in every country.

Sunny, dry places are best. But panels work in cloudy countries too.

The U.S. Energy Information Administration says places at lower latitudes, closer to the equator, and in dry climates generally get the most sunlight, and that the difference between seasons grows the farther a place is from the equator.

A 2020 World Bank study compared every country. Around 70 countries, many in the Middle East and Africa, have excellent conditions. But averaged over a whole year, the top countries, such as Namibia, could make less than twice as much per panel as the lowest, such as Ireland. Long summer days and clouds narrow the gap our December model shows.

EIA notes that latitude, climate and weather are major factors in insolation, that lower latitudes and arid climates generally receive more, and that seasonal variation increases with distance from the equator.

The World Bank's 2020 report 'Solar Photovoltaic Power Potential by Country' found that 93% of the world's population lives in countries with average daily PV potential of 3.0–5.0 kWh per installed kW, that about 70 countries exceed 4.5, and that high-potential countries tend to have low seasonality. The practical potential of the highest (e.g. Namibia) and lowest (e.g. Ireland) differs by slightly less than a factor of two: far smaller than the model's single-day, clear-sky ratio of 3.66.