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This one has been on my mind for a while now... I recently installed rooftop solar panels with micro-inverters. While I was informed I would be using the solar power first, and any remaining needs would come from the grid, as well as sending any excess to the grid, I didn't really understand it. Now that I'm using the system, I can clearly see (by watching the meter) that I truly do use the solar power first. This raises a plethera of questions, some of which are:

  • If I have two sources of power which are phase synced, why aren't they consumed at equal rates?
  • In my mind, the utility power source is monsterous compared to my solar panels, how is it they 'overpower' the grid source to actually push power onto the grid?
  • What would happen if solar generation exceeded power consumption on the entire grid?

However, as the title indicates, right now I'm mostly curious on how I'm consuming my solar power before consuming any grid power... Must be magic ;)

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  • \$\begingroup\$ 4 years on: The accepted answer is not fully correct. You say " ... If I have two sources of power which are phase synced, ..." BUT there is no reason to believe that the two sources ARE in phase synchronism. By changing the phase angle of the generated Vout the inverter can make what it supplies 90 degrees reactive leading or lagging or anything in between. The inverter can be made to not deliver power to the grid, to deliver power up to the point where it's load becomes great enough to drop effective Vout lower than some target level, or to accept power. \$\endgroup\$
    – Russell McMahon ♦
    Aug 17, 2020 at 9:41

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Basically, all the power is merged, and you are billed for the difference between the output from your solar panels and the power used in your house.

Simplified, the power grid's transmission lines are inductors, so when the voltage rises on one end, a current needs to get going through the line. If you simultaneously raise the voltage on your end, that current can be smaller, so the network sees your area as needing less power (the local substation serves multiple houses that at least for now will still have a net draw). When solar becomes widespread enough that you and your neighbours produce more than you consume, then indeed the current may flow in the other direction.

When more solar power is produced, then this means less load on the other generators. The power companies will then turn off some water turbines (because these can react quickly) and leave the big power plants online. If more power is produced than consumed (regardless of the source) they start pumping water uphill, which they can then run through generators at times of high demand (basically, using a lake as a huge capacitor).

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  • \$\begingroup\$ So do I understand correctly that the inverters simply slightly exceed the grid voltage in order to push the current to the grid? \$\endgroup\$
    – Cam
    Jul 21, 2016 at 22:10
  • \$\begingroup\$ +1 for rational answer that covers the basics very well. \$\endgroup\$
    – user105652
    Jul 21, 2016 at 22:12
  • \$\begingroup\$ @Cam, that's not necessary even -- just by reducing the voltage difference they reduce the currents flowing. \$\endgroup\$
    – Simon Richter
    Jul 21, 2016 at 22:49

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