What if a solar panel lay flat?

Look at solar panels on roofs. Most of them lean, like they are looking up at the sky.

Why do they lean? What if a panel just lay flat?

Solar panels turn sunlight into electricity. On roofs and in fields, most of them are not flat: they lean at an angle, tipped toward the part of the sky where the Sun is at midday.

Why bother? A flat panel faces straight up and still sees the whole sky. In this lab, a rooftop-sized panel sits in London on 21 March, tilted at 40°. What happens to the electricity it makes in a day if it lies flat instead?

A photovoltaic module's output depends on the irradiance reaching its surface, which depends on the angle between the incoming rays and the module. Fixed modules are therefore set at a tilt and orientation chosen for their site.

This lab compares a 1.7 m², 20%-efficient panel at 51.5° N on day 80 (around the March equinox), tilted 40° toward the equator, with the same panel lying flat. It then explores how the comparison changes with the season and with latitude.

The panel leans toward the Sun. Sunlight hits it almost straight on.

It makes lots of electricity in the middle of the day.

Press play and watch the March day go by. In London, the Sun rises at about 6 a.m. sun time, climbs to about 38° above the horizon at noon, and sets about 12 hours after it rose.

Our panel is tilted at 40°, so at noon the rays hit it only about 12° away from straight-on. It peaks at about 275 watts and makes about 1.86 kilowatt-hours over the whole day.

On day 80 the declination is close to zero, so the noon solar elevation is about 90° − 51.5° = 38.1°. A module tilted 40° toward the equator then has an incidence angle of 11.9°, and the model's noon power is 275 W from 808 W/m² of direct beam on the panel.

Integrating over the 11.9 hours of daylight, the model gives 1.86 kWh. It counts only the direct beam, uses a clear-sky air-mass formula, and holds efficiency fixed at 20%.

Now Olly lays the panel flat. The sunlight hits it at a slant.

The panel makes less electricity all day long.

Set the tilt to 0°. Now the noon rays arrive about 52° away from straight-on. The panel peaks at only about 173 watts and makes about 1.17 kilowatt-hours in the day, more than a third less.

Try 21 December. The tilted panel makes about 0.75 kilowatt-hours, the flat one only about 0.21. But try 21 June: the Sun is high, and the flat panel makes about 2.46, a little more than the tilted one's 2.32. Lying flat hurts most when the Sun is low.

At tilt 0, θ at noon equals the solar zenith angle, 51.9°; cos 51.9° ≈ 0.62, against cos 11.9° ≈ 0.98, so noon power falls to 173 W and daily energy to 1.17 kWh (63% of the tilted value).

Season matters: on day 355 the flat panel makes 0.21 kWh versus 0.75 kWh at 40° (28%), and the best tilt is 77° (0.94 kWh). On day 172 the flat panel's 2.46 kWh beats the 40° panel's 2.32; the best tilt is 15° (2.54 kWh). At the equator on day 80, flat (or within a degree of it) is best: 2.25 kWh, against 1.74 kWh at 40°.

Shine a flashlight straight at a wall. You get a small, bright spot.

Tilt the flashlight and the spot spreads out and gets dimmer. Slanting sunlight spreads out the same way.

Imagine a beam of sunlight as a bundle of energy. When it hits a panel straight on, all of it lands on a small patch. When it hits at a slant, the same bundle spreads across a bigger patch, so each part of the panel gets less.

Far from the equator, the Sun stays fairly low, especially in winter, so a flat panel catches its light at a steep slant. Tilting the panel toward the Sun turns it to meet the light more squarely. Our model only counts the direct beam of sunlight and keeps the panel facing the equator all day.

A beam of cross-section S spreads over S ÷ cos θ of panel, so irradiance falls as cos θ. For a fixed module, θ changes through the day and year; PVEducation notes that the power on a fixed module is therefore less than the incident sunlight, and that the tilt angle has a major impact on the radiation received.

The model leaves out diffuse skylight and light reflected from the ground. It also ignores temperature effects on efficiency and the losses in wiring and inverters, so its kilowatt-hours are clear-sky direct-beam figures, not a forecast for a real roof.

Most solar panels face the part of the sky where the Sun is at midday.

Some big panels even turn to follow the Sun across the sky.

PVEducation, a solar-cell teaching site, says the vast majority of solar panels face toward the equator. For the most energy over a whole year from a panel that does not move, it explains, the best tilt equals the place's latitude. A steeper tilt favors winter, and a flatter one catches more of the summer light.

The U.S. Energy Information Administration adds that flat solar panels can use scattered light from the whole sky as well as direct sunlight, so real panels make more than our model shows on hazy days. Panels on tracking systems that keep facing the Sun collect even more.

PVEducation states that, for a fixed tilt, maximum annual power is obtained when the tilt equals the site's latitude, with steeper tilts optimized for large winter loads and lower tilts using more summer light, and that the vast majority of modules face the equator.

The EIA notes that flat-plate PV collectors use global radiation, diffuse plus direct, while concentrating systems need the direct beam, and that collectors with tracking systems receive higher daily and annual insolation than fixed ones. Because the model counts only the direct beam, its figures for hazy or cloudy days are lower than real panels give.