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Another issue I just remembered.

Even if the system is designed disconnect the grid and operate in off-grid mode, without attached batteries, things will be very dicey. You will suffer constant brownouts and blackouts as clouds and weather obscure direct sunlight. I have a ~10kw system installed that only generates about 3kw when the sun is behind a cloud. Your lights will be constantly flickering and things powering up and down all day.

With properly designed system including the right inverter, panels, and batteries, you could go nearly completely off-grid. The cost would be several multiples of the cost of a normal grid-tie system, possibly even an order of magnitude more if you want to be able to have continuous power with all amenities(cooking, A/C, hot water, etc...) through a multi-day sun blocking event, such as a hurricane or blizzard.

> They do have models that can function off-grid, and even models with attached batteries to handle surge loads and night-time operation, but these models are not as widely installed.

Talking to solar salespeople, they say it's not code to run your house off-grid directly on the panels, without batteries. I'm not sure why; maybe they think handling brownouts is too much of a problem?

We're planning to install an SMA inverter [1] that has a "Secure Power Supply" outlet in the basement that gives best-effort power during a grid outage. This seems to be the only option on the market that gives you some power when the grid is out without needing the very large (quoted at $10k for the smallest size) expense of a battery system.

[1] https://www.sma-america.com/newsroom/current-news/news-detai...

Would a generator be able to supplant the grid? In effect have an auto-transfer but to the diesel/natural gas/propane generator? The generator would be idling except when power needs exceeded solar panels' capacity.

Might feel like a step backwards in terms of environmental impact, but presumable the fuel used would be minimal while the sun was shining or batteries not empty.

This is something I struggled to understand when my system was installed.

With the normal grid-tie systems, all of the energy produced by the panels must be consumed. If you are producing more solar energy than you are consuming, while running on generator, the inverter will still attempt to force that energy into your home. The generator will take the brunt of it and will likely be damaged or destroyed.

They do have systems that can take inputs from Solar, Generator, Grid, and Battery and use the best available option. But they are significantly more expensive than the typical grid-tie systems.

At this point, it's probably cheaper or simpler just to use a bigger generator or a battery.

Unless you undersize your solar so it can never put out more power than your house uses, you need something to absorb or dissipate excess energy - either a battery and/or a STATCOM or similar device.

> Would a generator be able to supplant the grid?

Yes.

Our neighbor across the street depends on a respirator. We live in a rural Bay area location that has grid dropouts several times a year. Some last a few seconds, some more than a day.

His setup: A transfer switch operates ~ 3 seconds after grid failure, followed ~ 2 seconds later by autostart of a propane generator. His house stays on generator power until the grid comes back up for at least 5 continuous minutes. Then the transfer switch flips back to the grid and the generator shuts off.

Why propane? Because gasoline slowly degrades (polymerization) and needs to be drained, disposed of, and replaced with new (typically every year or so). Propane doesn't degrade over time.

> Why propane? Because gasoline slowly degrades (polymerization) and needs to be drained, disposed of, and replaced with new (typically every year or so). Propane doesn't degrade over time.

If you have natural gas piped to your home then a natural gas generator is a solid option. Expensive, though.

A good bit cheaper than propane, though. The only people I know who use propane do so because they don't have NG pipes to their house.

Why not just include a cheap resistance heating element mounted somewhere safe so worst case your solar system has some load to take the power?

You would have to constantly adjust the resistance of the element to sink the proper amount of power.

Some inverters allow you to over-size the DC(solar) size of the system and indeed dissipate power in excess of its capability as heat.

There are inverter systems that can do nearly anything you want, but they are not typically installed.

That’s a pure waste of equipment. A maximum power point tracker can simply move away from the maximum power point to reduce production. Essentially every solar installation has an MPPT. The only insteresting bit is figuring out when and how much power to avoid producing, and it’s exactly the same problem as figuring out when to dump power into a dump load.

A case where a dump load is important is small scale hydroelectric power. If you try to draw less than full power from the turbine, the turbine will start spinning faster and faster. So you can use (slow and complicated) valves or mechanical brakes, or you can use a dump load.

An interesting case I don’t fully understand is how an off-grid (islanded) SolarEdge system handles a full battery. If I understand correctly, the SolarEdge modules independently track the maximum power point and produce an output voltage of P/I where I is the current drawn by the inverter. So, if the inverter draws less current, the string voltage will increase and the same power will be produced. I think SolarEdge targets 450V for the string, so, even if the modules have a maximum output voltage, there’s not a lot of head room before the string hits 600V and risks exceeding the wire insulation rating. My guess is that the inverter will turn the array off entirely when the battery is too full. Or perhaps the modules have a smallish current limit, and the inverter can reduce its effective input impedance if it needs to curtail production.

The SolarEdge system I have targets 380v.

At power up, it runs the string voltage up to 500v to test for isolation(short-to-ground) issues. It has a mode that can temporarily run the string voltage up to 1000v to meet testing requirements in some jurisdictions.

Each panel has a "Power optimizer" installed -- basically just a DC-DC converter that has active communication with the inverter. It should be able to control the power production of the entire system by limiting the current supplied by each module, though I don't know for sure that this is how it does it.