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I would like to know what would happen to the excess energy produced by my off grid solar panels. The scenario is that I have enough panels to run my appliances and fully charged my battery at same time and still have some energy left from the panels, I would like to know what would happen to that extra energy.

The system is off grid

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    \$\begingroup\$ The power output of a solar panel can be altered by shifting the solar panel voltage. What happens, when the batteries are full, and the load is satisfied, is that the charge controller shifts the solar panel voltage to a higher value, and thereby harvests only as much power from the panel as it needs. So, you see, there is no "extra energy." \$\endgroup\$
    – user57037
    Commented Jun 30, 2017 at 6:43
  • \$\begingroup\$ @mkeith: there is extra energy. The sun still delivers the same power to the panel. So where does it go if the panel is not able to deliver it as electrical energy? --> see my answer \$\endgroup\$
    – Curd
    Commented Jun 30, 2017 at 9:54
  • \$\begingroup\$ @Curd, I have always assumed that a solar panel delivering power to a load would be slightly cooler than a panel which is not converting incident light to electricity. It seems like a direct consequence of conservation of energy. But I don't think the OP was asking about this. And I have never seen any discussion or proof that this really happens. \$\endgroup\$
    – user57037
    Commented Jun 30, 2017 at 16:02

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Does a solar panel produce full power all the time?

Short answer : No

First, let's take a look at a solar panel's characteristics, which are always some variation of this graph :

Image source : http://www.pveducation.org

Image source : http://www.pveducation.org

You can see current and voltage and current are linked through the red curve. In practice, it means that if you do not pull current, your solar panel will quietly sit at Voc (open-circuit voltage) and pulling current from the panels lowers the voltage, but increases the total power output, until you reach the maximum power point (maximum of the blue curve), from where pulling more current will reduce the voltage so much that te total power available decreases.

Which means, as @mkeith pointed out, that you can adapt the solar panel's energy production at will.

Now assuming your power conversion works correctly, it should perform both these tasks :

  1. If the battery is fully charged, provide exactly the power needed for your appliances
  2. If the battery is not fully charged, provide the appliances power + charging power for the battery, while taking its maximum charge current into account.

What happens to the sun's energy when not turned into electricity?

Short answer : It is turned into heat

As shown above, a solar cell does not produce its full power all the time, but still receives energy from the sun. Conservation of energy tells us that it should go somewhere. Basic logic tells us that there are only two ways : Light and heat. This page from pveducation tells us that it is simply turned into heat, which will then be dissipated via convection, conduction or radiation.

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    \$\begingroup\$ @Sclrx Do the panels themselves just get hot as if they were in the sun? More profoundly, does drawing power from the panels actually cool them? \$\endgroup\$
    – user52386
    Commented Jun 30, 2017 at 16:22
  • \$\begingroup\$ @Scirx: "Short answer" : It is turned into heat". Really? Surely, the part of the light spectrum that produces photovoltiac energy is different from that which heats a body. If that's the case, why should blocking the production of photovoltiac energy produce heat? Instead, that part of the spectrum should just "bounce off" ... much like it does if I place my hand in the sun. \$\endgroup\$
    – RudyF
    Commented Jan 13, 2019 at 9:37
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If you withdraw less energy from the solar panel than it is able to produce there are only two things that are physically possible to happen:

  • The solar panel absorbs less light (i.e. it becomes more reflective; it gets brighter). I have, however, never heard of such an effect.
  • The solar panel heats up more and radiates excess energy as heat radiation
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    \$\begingroup\$ The implication being that a heavily drawn upon panel actually cools whilst in direct sunlight? A Peltier effect? \$\endgroup\$
    – Paul Uszak
    Commented Jun 30, 2017 at 11:46
  • \$\begingroup\$ It's not actively cooling. It's just getting less hot. I.e. it doesn't work in the dark when the panel is not illuminated (of course, because you can't get energy from a solar panel in the dark). \$\endgroup\$
    – Curd
    Commented Jun 30, 2017 at 11:52
  • \$\begingroup\$ @PaulUszak: "heats up more" is not really the opposite of "cools down". It heats up, and in that case, heats up more. \$\endgroup\$
    – PlasmaHH
    Commented Jun 30, 2017 at 11:53
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    \$\begingroup\$ @Curd: As temperature increases, I would expect the amount of heat lost to convection and radiation would likewise increase. If a large load is applied to a formerly-unloaded panel, I would expect that the combination of heat loss and power consumption would remove energy faster than the sun re-applies it, thus lowering the temperature compared with leaving the panel unloaded. Should that not be called "cooling"? \$\endgroup\$
    – supercat
    Commented Jun 30, 2017 at 14:51
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    \$\begingroup\$ @mkeith: yes, theoretically that would be an option too. But considering that the panel is at least 1 or more mm thick and absorption coefficient is normally in the order of 1/1µm (just a guess) there had to be a huge change (many orders of magnitude). \$\endgroup\$
    – Curd
    Commented Jun 30, 2017 at 16:27
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If your load (battery charging plus household) requires less energy than what your solar panels are able to produce, the excess potential energy simply goes to waste.

There are no harmful side effects from excess potential solar energy. The panels will not overheat, no wires will melt, etc. It is simply lost energy that you cannot recover. If this happens frequently, adding additional battery capacity will harness this energy as long as your charge controller is capable of handling the additional charging current. But if you do not ultimately consume this extra stored energy, there is no justification for this added capacity and expense.

As you may imagine, it is hard to strike the perfect balance between energy production, storage, and consumption. It often comes down to a question of economics.

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