So 3-5 day outages 2-3 times a year sounds like a right pain the ass, and certainly has me looking at adding a powerwall to our PV system. I had wanted one anyway for backup in the event of an earthquake or other event that might disrupt power for an extended time, but now we're almost guaranteed those outage events, rather than it being a hypothetical.
If you want serious off-grid functionality, you need a beefier inverter. Outback sells such things, but the resulting system is a good deal more complicated.
In particular, how does an AC coupled system handle the case where the battery was fully discharged before sunrise? The inverter needs to turn on when the array starts producing, but the battery can’t charge without the backed up loads being powered, and the backed up loads can’t be powered until there’s enough battery charge to supply the inrush current.
A DC coupled system like the Powerwall 1, RESU, or pretty much any lead-acid system doesn’t have this problem at all.
I'm not trying to raise FUD here and am genuinely curious -- would a powerwall-like system have its own set of issues or dangers in the event of nearby fires/evacuations?
So I imagine scenario 1 is your house doesn't catch fire, in which case it's fine, and you have power even if the grid is down, or scenario 2 where your house does catch fire, and maybe the battery contributes to that, but it would burn down either way.
But keep in mind that the 'red flag days' I'm describing where PG&E would cut power are not days where fires are actually burning in the area - just days where fires are more likely to start. They're shutting down their power lines so that if the wind knocks one down it won't start a fire. This is a response to last years devastating fires in Sonoma county, which were believed to have started by downed power lines in high wind and dry conditions.
They're unsecured as far as i have noticed, so i imagine the troublemakers will be pulling them sooner or later.
Ahh, that makes more sense. I was thinking about situations where there would be fires burning in the immediate vicinity, so perhaps dangers from overheating (explosion?) or firefighters/emergency responders having to clear areas quickly, etc.
Handling DC is much easier than AC here since you don't need to keep the grid in sync with the inverter and you win efficiency due to not needing the inverter either.
I'll probably switch out all my lamps for LED strips, RGBW and controlled by arduino, so I could light up the house purely from solar without having to think about the grid, store energy into a battery and get light overnight too. The efficiency of the LEDs would help reduce power usage too.
I think about 200W should be peak usage if I do everything and charge my laptops and use some tools like my soldering iron plus the wifi router. Should be manageable at 24V (10A cables aren't too expensive). If all else fails, go to 48V.
My understanding is that for anything big like a fridge (even a minifridge) the efficiency and reliability of standard well-optimized 120V AC appliances means it's worth it inverting to AC instead of sticking to DC. Lots of RVs use/used 12V DC systems so things are available, they're just not very good. Inverters are very efficient these days. Plus it's really helpful to have a way to run AC things you already have if you need to.
Though if you only want lights, I do see how DC could make more sense if you can either find or DIY the right electronics to handle the voltage issues.
Most of what I want to power basically uses a power supply to go to DC already including router and wifi, so it's not a problem, I just need to either find the right DCDC converter or solder one myself (I'm confident I can solder something together that should be able to hold 4 or 5 amps, above might need thicker copper plating on the PCB).
What I did was set up a sub-panel with a generator interlock. For now, we're going to use a portable generator in extended outages. Once batteries are a bit cheaper, the batteries will run the sub-panel.
What about heat in a power outage? That's why we have a wood stove. You never know when an ordinary fossil fuel furnace will fail!
Right now we'd have no heat. I'm thinking of putting together a small general purpose battery backup that we can normally charge via the wall but can also charge via the SPS on the solar system we're installing. I'd use it for whatever was highest priority; depending on time of year that could be boiler, sump pump, fans, or unexpected things.
They have some nice ones that can connect to a battery as well.
A house won’t have the same space or weight restrictions, but that doesn’t make them any cheaper. FLA batteries are the least expensive currently available, and I’d ballpark it at around $4000 in batteries alone to get a few hours of air conditioner.
The SPS is rated for 2000W, best-effort. That gets you probably one large window AC. It will run when the grid is down and the sun is shining on your panels, which depending on how your roof is angled and what trees etc there are might or might not be enough?
It will have times when it loses power due to clouds. I believe when that happens the SPS turns itself off and you need to manually re-enable it by flipping a switch in the basement. So might be a lot of hassle.
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.
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.
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  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.
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.
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.
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.
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.
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.
> 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.
At 12 volts, a one-amp lamp draws 12 watts. A 24kwH battery ($5500 last year) will power this lamp (neglecting losses) for 2000 hours. Say, 80 days.
So, used all day, 48 watts will last last 20 days, 96 watts 10 days. Used half-time, you'd get about a week at 200 watts. Two hundred-watt light bulbs for 10 hours a day.
Point being: solar power is great, but for -survival- purposes, our 2000kWh/month habits can't be sustained.
Also, a 100 watt LED "bulb" is ridiculously bright. That's far more light than you'd get out of a tungsten bulb, 200 W of LEDs is [more than] enough to light an entire house. Electric heating and cooling is the real problem when it comes to energy consumption.
Living off of solar and batteries means foregoing things like the toaster and the blender.
My point was that it's important to distinguish that long blackouts with severe inclement weather are extreme events, and on average a solar power system should be fairly stable as long as your usage is kept to essentials. I completely agree with OP that you really do have to cut back on power-hungry luxuries for solar-only to be really viable, but you should be able to at least keep the lights on for quite a while. In terms of holding out during more extreme events, yes... relying on any single energy source leaves you vulnerable in that respect.
- Backfeeding into the grid is unsafe
- Backup generators will throttle to not produce too much electricity for your home, so you can safely use a transfer switch
- Solar can't be throttled, excess power needs to be used, stored somewhere, or sent to the grid
If your inverter is sufficiently fast, and your transfer switch can at least disconnect external power from the internal system within less than half a period, you might be able to keep the internal power stable without any hickup at all.
And it's not like the required control logic for the inverter is too hard to integrate nowadays with microcontrollers having WiFi etc.
I have not found an off-grid mode in its settings yet.
In short, newer inverters can’t be overloaded if there isn’t enough load and too much sun.
He was insisting on installing a cellular modem on my inverter and maintaining remote access and control over it. I did not want this, so I did not allow them to.