Solar panels, production, shade and indoors - a Herculean task for one blog!

Posted on 13 Sep 2018 by

There's quite a bit to the question of running solar from artificial lights. Feel free to read all of this, but if you've hatched a cunning plan to use indoor lighting to power panels to run your entire house then the answer is "not possible".

On the assumption that you are still reading, let's start with a few variables:

  • Yesterday, on an average day in September in Darwin, the sun delivers about 7.5kwh of energy per square metre
  • On the same day in Hobart, the sun delivers about 4kwh of energy per square metre
  • In Sydney, same day, the figure was about 5.8kwh

Which is mostly down to how much atmosphere the sun is coming through to get there (ie the angle that it is coming in) and how long the sun is in play between dawn and dusk.

  • A good quality solar panel is around 20% efficient (so 1kwh of sun is converted into 200wh of usable power and the rest goes off as heat, reflected light etc)
  • Old panels might be around 11% efficient and the very best panels manage about 22.5%

Let's talk PANELS - So a really good panel will harvest about 20% of what hits it assuming all else is in order and the panel is clean and facing the right way. Then, taking Sydney as an example, panels would be harvesting about 1.16kw per sq metre yesterday, and a typical 270w panel is about 1.65sqm in size, so would have harvested about 1.9kwh of power in theory... the 19 panels on my roof should have harvested about 36kwh of power...

...but they didn't, due to a combination of dirt, occasional shade, angle and numerous system innefficiencies.

In fact they harvested 24kwh, or about 2/3rds of what they might have done in a perfect labroratory scenario.

The next thing is that a 270w monocrystaline panel generates about the same as a 270w polycrystaline panel - 270w. The mono is slightly more efficient so does it out of a slightly smaller area at a slightly higher panel price but each has merits in certain conditions, so don't be put off by the polycrystaline name - 270w is 270w, and the quality of the panel (and the likelihood that the manufacturer will still be around 20 years hence) is far more important. 

The next topic is SHADE - which makes a huge difference to the production from a solar panel, and I mean HUGE. I haven't got the room, here, to go into percentages, but one panel in full sun will produce many times what half a dozen panels are producing in broken sun or partial shade, This leads us to two important points:

  • Panels must be installed where shade isn't going to be an issue, and they must be facing the sun (different angles for different locations - 43 degrees in Darwin, 34 degrees in Sydney, 13 degrees in Hobart for example)
  • If panels are not subject to any shading, you might want to wire them in series for high voltage/low current (as long as the regulator can handle the voltage), to take advantage of the lower losses through cabling, reduced cable sizes etc BUT...
  • ...if there is a risk of partial shading (boom on a yacht for example) then wire the panels in parallel because that will work better (as long as the regulator can handle the current)

Why's that? Well, if a panel has some shade then it is a bit like bending a hosepipe - the flow is reduced. So if you have 10 panels in series and one is shaded, the production from all of them is squeezing through the solar equivalent of a kinked hose whereas, in parallel, each panel is working independently of the others.

Now, what about INDOORS - ARTIFICIAL LIGHT? Well, things get a bit complicated due to light and heat, types of radiation etc but, for simplicity, we know that the sun is producing, let's say, 1kwh per sqm, so how much is your 60w light globe producing per sqm? Here's the problem - the globe is sending light in all directions, and heat, there's heat going all over the place too. So if you were able to take a square metre of solar panel and wrap it into a sphere around the globe, you might be able to receive all 60w that is being produced, and then you remember the 20% efficiciency thing, so the panel is only capturing 12w, but your ordinary flat panel a few metres from the globe, well that's getting a small portion of the whole 60w that is going to light (and heat) the room, so you need a very big panel to grab as much as possible, hoping for more than 1 60w globe to be switched on, and then you might just grab enough power to run a very small strip of LEDs, with a lot of experimentation needed to make it work.