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Home Solar Resiliency (jefftk.com)
51 points by luu on Aug 1, 2018 | hide | past | favorite | 56 comments



This is something I've been thinking about recently due to a potential new policy from PG&E (Northern California electric utility) in which they would preemptively shut down electricity to high fire risk areas when red flag conditions are present. I live in the Oakland hills which would be affected by that plan. PG&E says they expect such an outage would occur 2-3 times per year in October/November, and that because they have to do a lot of safety checks before restoring power, that we should expect those outages to last 3-5 days.

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.


Having tried and failed to work with Tesla for solar, I strongly suggest you skip the powerwall. Use LG Chem’s RESU instead. It’s slightly cheaper, and they are willing to let third parties install it. It pairs with SolarEdge’s StorEdge.

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.


SolarCity installed the PV system, so I figured it'd be easier for them to continue with the upgrade. But it sounds like things are kind of a mess over in Tesla/SC land these days, so who knows...


Another factor is that the Powerwall 2 DC coupled model got canceled. And I don’t see how an AC coupled battery will give as reliable of a backup system without complicated integration with the inverter and without extra automatic switches that can isolate the battery from the loads.

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.


> has me looking at adding a powerwall to our PV system

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?


Well, I'm not sure what happens to it if your house catches fire - but if your house burns down I don't see how the battery could have made it worse.

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.


One big reason for shutdown rules with home PV systems is to keep firefighters safe. If your house is on fire, the firefighters really don't want to be contending with high voltage electricity as well.


Problem is that, as long the sun shines, a voltage will be generated. Depending on how they are linked together this makes it more or less dangerous.


I've been noticing ground level external emergency shutoff switches for roof based solar around my city, so i imagine its something thats been making its way into fire/electrical/building codes recently.

They're unsecured as far as i have noticed, so i imagine the troublemakers will be pulling them sooner or later.


Stored energy is stored energy. Batteries may burn, explode, spark (ignition source), electrocute, and/or be a source of noxious fumes or chemicals.


> 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.

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.


If downed power lines are that big of a risk I would think that the correct solution would be to bury them instead of shutting down power.


Out of curiosity, what are your reasons for not using a generator?


I've been looking into a bit of DIY solar lately. Not for powering the house but for lights and other low-voltage (9 to 24V) tools, probably even charging my laptop if I find a good adapter and a voltage regulator that can handle a few amps.

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.


I looked into this some and found some of the most useful advice was for people going to Burning Man. For example: https://www.theplayalabs.com/shopping-list-solar

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.


I'll probably not hookup fridges or anything large like that. My laptop would be fine, it uses a switching power supply to 19V (though it charges of 12 to 24V just fine).

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).


I just built my house, and it has a lot of solar panels. We planned to generate enough to offset heating with electric heat pumps and charging two cars.

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!


> What about heat in a power outage?

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.


Curious about SPS + A/C. Due to a medical condition in my household, having A/C even during an extended power outage would make life a lot easier.


Highly recommend Outback or SolarEdge inverters, but based on your AC load, you are going to need a sizable battery. Tesla’s Powerwall isn’t the only solution (LG Chem has one as well), and as long as the battery is only ever charged from solar (not the utility, it’s an inverted setting), it’ll qualify for the 30% federal tax credit along with the rest of your rooftop solar install. Depending on your state and utility, there may be even more incentives available beyond the 30% federal tax credit.


+1 to SolarEdge. I have an HD Wave and it seems to be a really nice inverter.

They have some nice ones that can connect to a battery as well.


Air conditioning is incredibly power-hungry when powered from an inverter. My solar experience is with an RV, but even with that lower-powered AC unit, you can realistically expect only 30-60 minutes with the batteries an RV can carry.

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.


An alternative to this is to have a generator that works with either propane or car-gas.


Are those lead batteries or lithium? A lithium battery pack is about 10 times lighter, and 2-3 times smaller, than a lead battery pack that can store the same energy.


> Curious about SPS + A/C

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.


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?


Most solar systems are grid-tie, in that they synchronize their output to your local electric grid, and feed any excess power into that grid. They do this by increasing their voltage output until all power generated by the panels is consumed, either by your house or by the attached grid.

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.


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.


> 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.


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.


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.


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


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?


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.


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.


Look up "anti-islanding". If the grid is down, you can do whatever the heck you want with your own wiring, but back-feeding power into the grid is a very firm no-no.

> 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.


a blackout lasting more than a week...

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.


I agree that high energy consumption is a problem, but this analysis is overly pessimistic because you're not factoring in the fact that the system is recharging during the day. As long as the average load is lower than your average power draw you can sustain it indefinitely. The battery capacity only needs to be there to compensate for your worst case scenario of bad weather/reduced light exposure. So maybe not surviving a nuclear apocalypse kind of scenario, but otherwise workable.

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.


There is also the assumption that you use all lamps/electric devices for 24 hours every single day. But humans tend to sleep and the sun is shining during the day so you end up using your lamps for only 8 hours or even less per day.


In a lot of places blackouts or grid failures tend to coincide with bad weather that doesn't supply much solar charging. The bad weather is quite often windy though so a small wind turbine to diversify the generation sources is helpful.

Living off of solar and batteries means foregoing things like the toaster and the blender.


True, which is why your battery needs to be large enough to supply power during your worst-case scenario for bad weather (or have back up systems like wind/thermal like you mention). You also definitely want a plan for cutting consumption to only essentials during such a blackout. A toaster / blender really doesn't take that much energy; sure they're 500 W or so, but you don't typically operate them for long periods of time. The majority of energy usage in most homes is devoted to heating and cooling, which are high power and run for long periods of time.

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.


Here's a video of a family that lives off grid you might find interesting https://www.youtube.com/watch?v=v8Pe_u_4q5M


Most other commentators here have hit the nail on the head, in summary:

- 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


Actually, solar cells are perfectly fine if you don't connect the outputs. Your inverter has to handle the float/no-load voltage (20~50% higher than at peak power), but that's it. Now, the inverter in this case has to be throttled by the secondary, instead of just blasting as much amps as it can with the solar power. You might want to have a nice-ish clock reference for the phase of your micro-grid, if you care about mains-phase-locked clocks keeping their accuracy. You'd want to count cycles and compare against those in the main grid, as well as adjust the frequency (much stronger than the large grid, as there won't be anything that initiates an emergency shutdown at the odd frequencies you might need to catch up/slow down to the main grid phase) so that the cycle count matches up when you sync back 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.


Newer inverters can throttle solar, as there are some markets where no export is permitted to the grid (the inverter optimizes the MPPT to effectively shut certain solar modules down).

https://www.solaredge.com/us/solutions/feed-in-limitation-an...


I have such an inverter(SolarEdge HD Wave)[1], but you have to specify a specific power level to throttle to -- and it requires some additional supporting hardware to measure what it is feeding the grid with, and still requires at least a sync signal from the grid.

I have not found an off-grid mode in its settings yet.

[1] https://www.solaredge.com/us/products/pv-inverter/single-pha...


It’s not user friendly (export limit setup), as it should be set by your solar installer at system provisioning. There’s only a few markets in the world it applies to (Arizona, Hawaii, parts of Australia).

In short, newer inverters can’t be overloaded if there isn’t enough load and too much sun.


I kicked the solar installer out after he did the physical work, I did the rest of the provisioning and configuration after the inspections cleared and the meter was swapped.

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.


Here they required installing a data logger which is really a Raspberry Pi running a web server + a USB to RS-485 adaptor. It was required for warranty beyond the legal minimum because they want to monitor the environment temperature in the room where the inverter is installed. However it is not on the public network and does not call home.


Backfeeding isn't really a problem practically unless you're (dangerously) hacking a system together yourself without knowing what you're doing. Any inverter/charger designed to work in this kind of system either has a transfer switch built in, or interfaces to an external one that stops backfeeding when the grid is down.


Does anyone have a good tutorial or a site that sells a off-grid small kit (600Wish with 1-2kWh battery) with battery backup? I want to get one for my house if its affordable enough as an insurance on top of a generator.




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