What happens when you are off-grid and producing more power than your house uses?

[MattSl]

I apologize, this is I'm sure a newbie question and one that probably has an easy question. My cursory searches came up blank, probably because I'm using newbie terms.

Say I'm off-grid, power outage. My house only uses 4000w, and I'm producing 10000w, batteries are full, etc. What happens to the rest of the power? Go to a heat sink or something?

[SunEagle]

I believe your inverter will throttle back and just not send any power besides what is used. Some people find other "loads" to use their excess power on but if not then don't worry. Your system should be ok.

[MattSl]

Thanks for the response!

So the excess power just dissipates in the panels or in the inverter? I guess it is the 'bleeding out' of the excess power process that I'm interested to understand.

[SunEagle]

Thanks for the response!

So the excess power just dissipates in the panels or in the inverter? I guess it is the 'bleeding out' of the excess power process that I'm interested to understand.

Pretty much. Without a load to send the power to the panels and inverter just sit there.

The power is just lost like a fire hose watering plants. That is the main reason not to over panel unless you have Net Metering.

[MattSl]

Excellent, thanks for the answer!

My utility does offer net metering, but I was curious about this in a power loss situation. Appreciate the insight!

[foo1bar]

Say I'm off-grid, power outage. My house only uses 4000w, and I'm producing 10000w, batteries are full, etc. What happens to the rest of the power? Go to a heat sink or something?

Some will be lost as resistance in the wires, conversion from DC to AC, etc.

But mostly you just won't have that situation.

You might have where the panels *could* produce 10kW, the load from the house and batteries is only 4kW, and 0.1kW in losses from wire resistance, conversion, etc.
But instead of the panels producing 10kW, the charge controller will pick an operating point where they produce about 4.1kW.

Panels have a current/voltage curve when they are exposed to a given amount of light.
At one end of that curve is maximum current - ie. "short circuit current" - "Isc".
That's 0 volts, but maximum current going through the cables. And 0W is being produced by the panel.
At the other end of the curve is maximum voltage - "open circuit voltage" - "Voc"
And that's the most voltage you get - but it's 0 amps. And therefore 0W is being produced by the panel.
The charge controller will pick an operating point somewhere in between to operate at.
And if it doesn't need the maximum possible power from the panels, it can use a point farther away from max-power and get a lesser amount of power out of the panels.

Or the system may have a "dump load" - where it can send excess power. And the system owner may have something like a hot water heater there - so the water gets heated by any excess power not needed for other things. But in that case, I'd say that dump load is part of the house - and therefore the house is still using all the watts produced by the system. (ignoring the 5-10% in wiring losses, etc. etc)

[sdold]

The "excess power" doesn't exist, so there is nothing to dissipate or "bleed over" to some other load. If your inverter is allowing a certain amount of current to flow from the panels to the load, that's all the power there is. If you have a 10 kW system, that just means it's capable of that, not that it is always generating that. Think of a battery, it doesn't produce power unless you connect a load to it. It doesn't sit in a drawer producing power. Same with solar panels.

[peakbagger]

Off Grid folks usually have opportunity loads identified that they turn on when the batteries are charged. Some may have electric resistance heater in a hot water tank to heat some water up.

[robbyg]

The "excess power" doesn't exist, so there is nothing to dissipate or "bleed over" to some other load. If your inverter is allowing a certain amount of current to flow from the panels to the load, that's all the power there is. If you have a 10 kW system, that just means it's capable of that, not that it is always generating that. Think of a battery, it doesn't produce power unless you connect a load to it. It doesn't sit in a drawer producing power. Same with solar panels.

The OP's question was one I had after reading some post about having to dump excess power into hot water heaters or your inverter will overheat and die. As an EE who never really looked at Solar Panels before, I thought what the hell, am I missing something! I had to do some reading to realize all the interactions solar panels have with the Inverters. Yep they behave just like Batteries without storage. If you don't load them they just sit there, if you do load them they produce the current applicable to the resistance of the load until you max out the available solar energy or the panels max specs.

[Murby]

In the winter months, best and easiest place to dump excess energy is into electric baseboard heaters..

In the summer, turn on fans or set the air conditioner a bit lower.