Solar panels on cloudy vs. clear days – Renewable Journal for 1/4/2015

My solar panels face east and west instead of south, giving me an opportunity to see how cloudy days compare to clear days for electricity production.

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My home is a tri-level, and given how it’s oriented in my neighborhood, its multilevel roof faces due north, south, east, and west. After reviewing the rooflines and nearby trees, SolarCity’s engineers concluded that the best roofs to put the panels on were the east and west facing roofs, rather than the south facing roof. After watching how the solar panels generate electricity for several months now, I’ve noticed something interesting. My panels generate electricity more equally on days with high, hazy, light clouds as compared to days of direct sunlight. I found this fascinating, because it’s essentially the same effect as something scientists have observed with respect to plants.

Imagine for a moment you’re sitting under your favorite tree on a clear, sunny day. You look down at the ground and you see well defined shadows from all the tree’s leaves. Now, imagine you’re sitting under that same tree on a day when there are hazy, light clouds across the sky. When you look down at the ground, you don’t really see individual shadows, but rather it’s somewhat darker under the tree than it is out in the open. Years ago scientists hypothesized that plants might photosynthesize better on hazy days with diffuse light than they do under heavy cloud or even in bright sun, and after a bunch of tests, scientists found that the plants they studied did, in fact, photosynthesize better on hazy days.1, 2

The reason for this is pretty simple. In direct sun, the well defined shadows of your favorite tree not only shade the ground where you’re sitting, but they also shade other leaves. While the leaves in full sun photosynthesize great, the partially or fully shaded leaves photosynthesize poorly or not at all. In diffuse sun, though, sunlight is scattered by the clouds and so it comes from all different directions. This means that a lot more leaves get indirect sunlight and so more photosynthesis occurs in the tree than would happen in direct sun. Of course, too much cloud and the entire tree is essentially shaded, so there is some amount of cloudiness that’s ideal for your favorite tree, and for plants in general.

The same thing happens with my solar panels. On clear days, the east facing solar panels wake at sunrise and generate electricity throughout the morning and mid-afternoon. There’s an optimal angle that minimizes reflection off the panel’s surface (it’s been happening between 10 and 11 in the morning), and the panels generate less electricity throughout the afternoon until they finally turn off. The west panels are similar, but they wake up later in the morning, generate the maximum electricity at around 1 or 2 in the afternoon, and stay on until sunset.

On hazy days, though, the west-facing solar panels wake up earlier and generate more electricity in the morning than they do on clear days. Similarly, the east-facing solar panels go back to sleep later and generate more electricity in the afternoon than they do on clear days.

I don’t have enough data yet to say what the optimal amount of cloud cover is, but it was still cool to see how a scientific hypothesis related to plants has been replicated with my solar panels.

Recent articles on how diffuse light affects plants

Variations in the influence of diffuse light on gross primary productivity in temperate ecosystems (Cheng et al 2014)

Influence of clouds and diffuse radiation on ecosystem-atmosphere CO2 and CO18O exchanges (Still et al 2009)

1 reply »

  1. As a photographer, I like those hazy days more, too. The diffuse light, at least for the kind of shooting I like to do, cuts down on contrast issues and provides more even lighting.