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During utility power outages, a simple grid-tie solar PV system is required to auto-disconnect from the grid for safety. One cannot utilize power from the PV system while disconnected from the grid (or battery backup), because "the excess current needs somewhere to go." Therefore the panels are disconnected from the inverter as well. However, while the sun shines on the panels, they will continue to have a voltage differential but no current (What happens to the extra electricity (amps) created by an off-grid solar (PV) array?), like a disconnected battery. Please correct me if I misunderstand.

If the panels can sit at voltage but no current (like a battery), why does excess current need to be sent somewhere when a small amount of power is utilized (unlike a battery)? What prevents us from using only part of the PV power generated?

Edit:

Thank you for the answers and comments so far. With those in mind, please allow me to rephrase my question:

Why does a solar PV system need to be grid-tied or have a battery backup to be usable? Why can't they just include a 60 Hz wave generator to govern A/C conversion, and use power when the sun is shining?

I understand the Sunny Boy string inverter can do this to a point, but why isn't it a standard feature on most inverters, and what limits it to 2000 watts on the Sunny Boy?

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    \$\begingroup\$ You've been given an explanation which isn't correct. Some grid tie inverters do have a local independent mode where they are not tied to the grid, but this makes it more complex to certify that they meet the grid tie requirements of not backfeeding a downed grid. In the systems I've seen, they won't power your house, rather they give you a single outlet for your freezer and charging your phone. \$\endgroup\$ Sep 10, 2020 at 21:32
  • \$\begingroup\$ I can imagine that if the utility cut the power for maintenance, they would not want power to be fed into the grid. As for a solar panel not connected, the sun's energy falling on it will be converted to heat. This answer might help with the latter. \$\endgroup\$
    – Raffles
    Sep 10, 2020 at 22:35
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    \$\begingroup\$ They don't need to send excess power somewhere. Grid tie inverters ARE required to disconnect when the grid fails - but that's principally to avoid killing the guys trying to repair it! \$\endgroup\$
    – user16324
    Sep 10, 2020 at 23:13

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Why does a solar PV system need to be grid-tied or have a battery backup to be usable? Why can't they just include a 60 Hz wave generator to govern A/C conversion, and use power when the sun is shining?

Because in many parts of the World, it isn't bright and sunny all day, every day.

My rough rule-of-thumb is that on a sunny day, when the sun is high in the sky, my solar panels can generate their rated output. If a light cloud blocks the sun, the output can reduce to a tenth of the peak rating. Heavy grey clouds can reduce that to a hundredth of the peak rating.

It would be difficult to use an inverter in islanded mode if you don't know how much power it can give you at any given moment. Plug in one appliance too many, and the inverter will shut down because the demand is too high. If a cloud drifts across the supply, again the inverter could shut down without warning.

So islanded mode is usually only supported on systems that include a battery.

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First a PV panel generates Voltage like a generator by separating positive and negative charges. It will do so in the moment when it is exposed to light. When the circuit between the different potentials is open-circuited, no current can flow, the PV panel will produce voltage but no power. Its just unused. So e.g. your panel produces 100V, then 100V*0A=0W.

Also most inverters are synchronized to net frequency. They do not produce it on their own. So if there is no power from the grid, the frequency information is missing and the inverter will not convert Power from the PV panel. Most power companys here in germany have much less rules and regulations for this type of inverters, because they are inherent safe (or at least more safe). So for customers these are "cheeper" to operate.

If the panels can sit at voltage but no current (like a battery), why does excess current need to be sent somewhere when a small amount of power is utilized (unlike a battery)? What prevents us from using only part of the PV power generated?

"why does excess current need to be sent somewhere " I never heard of that? Where do you get this information from?

My answer: Well nothing. You may use any fraction of energy you want. This is always the case if you do not consume all the power which can be generated and the power grid also does not need the power. But as soon as you or the grid demands the power generated it will be used. This is why home batteries are an idea in some use cases, as they will store energy when nobody needs it. So you can use it by night or to charge your electric car - even in the dark.

Also if you have a hot-water boiler for warm water and/or heating, you can feed energy into the water up until the temperature limit (in most cases around 90°C), while a heat pump only accomplish about 60°C. So from your PV panel you can increase your heat reservoir by 33%

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