I'd also like to understand the reason for this restriction on whole house power when the grid is down but the sun is shining. Does anyone know why that might be the case?
Because of the way these systems are designed, they do not work well when the grid is not present. If grid power is down, and your solar system feeds power into your home electrical system, you will also be 'backfeeding' power into the grid. Since you likely cannot possibly generate enough power to satisfy your entire neighborhood's power usage, this will in effect overload your system. There is a second safety issue of feeding voltage into a grid system that the power workers are expecting to be dead. Electrical code requires these grid-tie systems to cease feeding power into the grid if the grid power is removed.
Most of these systems do this by just shutting down -- unlike backup generators with auto-transfer switches -- they usually do not have an automated method to disconnect the grid power while still powering your house.
If you were to provide some sort of disconnect, due to the nature of these systems, they would try to 'push' all available power into your house, because they're designed to force all available solar power into the electrical system. The grid can absorb this, but generally your home cannot.
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. Likely due to cost and/or owner education.
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.
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.
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.
If you have natural gas piped to your home then a natural gas generator is a solid option. Expensive, though.
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.
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.
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.
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.
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...
> Islanding can impact utility asset integrity. For example, these conditions can interfere with manual or automatic reclosing, or loop feed automatic switching on the radial distribution system. Utility assets incorporated into or reconnecting to an island with abnormal voltage or frequency conditions may result in extensive equipment damage, for example, a line recloser damaged by reclosing out of phase or lightning arrestor damaged due to abnormally high voltages.
> Public safety risk may increase on delta connected systems. For example, a downed wire can remain energized after a device opens to isolate a fault. Depending on the location of the DG installation and the impedance of the downed wire, there may not be sufficient fault current or voltage deviation to trip a generator offline resulting in power being provided to the downed wire.