Why a Typical Home Solar Setup Does Not Work With the Grid Down - And What You Can Do About It

During 2017, I saw a lot of news articles talking about how the Evil Power Companies were being Meanie McMean by not letting people with solar panels use them when the grid was down.  The implication (in many news articles) was that these powerless people with solar panels could use them to power their home while the grid was down, if only the evil power company didn’t require that solar not work if the grid was down.  The picture painted was one of power company executives, twisting their mustaches, cackling in the glow of their coal fired furnaces, going on about how if they can’t deliver power, nobody shall have any power!

This is a companion discussion topic for the original entry at https://www.sevarg.net/2018/05/20/why-typical-home-solar-setup-does-not-work-off-grid/

(Comments from Blogger)

2018-06-07 by Klaus

I have a battery backed solar installation using a DC-coupled LG Chem 10kWh LiIon battery. It’s not a true off-grid system, but should at least be able to keep the fridge and a few essentials running even on cloudy winter days.

I fully agree that this wouldn’t normally make sense financially, but the current WA state production incentives are high enough for a predicted break-even for the overall system in 7-8 years. If those incentives make sense is a different question entirely.

The system would be capable of more grid-friendly operation by using the battery to avoid very sudden changes in production. Currently there’s no incentive for me to do so, but I’d gladly do so if the utility fees were restructured. The production payment is currently a flat $0.21/kWh, combined with net metering at +/-$0.09/kWh. If they’d change this according to the system capabilities, people would be more motivated to not break the grid. For example, maybe a higher incentive if your system guarantees it’ll not exceed a maximum rate of change for production/consumption?

That couldn’t solve the duck curve by itself, but I think we could do better than the current system which promotes unhelpful setups.

2018-06-07 by EugeneGTI

This is the best explanation I have seen in one article. Having grid tied system I observed the cold peak production past panel rating and also attempted to feed wave in a power outage, needless to say, unsuccessfully. Great point on physical inertia and destabilizing effect of out of sync microinverter dropoffs. Too bad v2g never caught on in a meaningful way…

2018-06-08 by Russell Graves

Wait, $0.21/kWh generated (even if you consume it yourself), or $0.21/kWh exported to the grid, plus the net metering?

A set of ramp rate guarantees would certainly help grid stability, as would optimizing for morning/evening export, but, as you note, the incentives weren’t set up that way, and any time anyone proposes changing them, they’re called evil shills of the power company, or something along those lines (by people whose understanding of power grid operatin is “solar good, coal bad, nuclear bad”).

2018-06-08 by Russell Graves

Thanks! I’m tired of poorly written articles by people who don’t understand power systems, and while I’m by no means a grid engineer, I’ve known plenty of them and have picked up bits and pieces around the edges. Plus, I enjoy reading papers on grid stability factors. My reading list is odd, to say the least.

The UL 1741 SA standards really, really will help with the stability issues, but they still won’t let you run a grid that way.

V2G is unlikely to catch on, simply because wearing out a personally owned vehicle battery pack to help out with grid stability isn’t popular. We might see more of it if (I think it’s a firm if, not a when) we see self driving fleets of EVs, and they can make agreements with the power company, but right now, any sort of small scale system that wants to contribute to stability can’t. We don’t have the smaller markets for bidding in frequency response and such like the UK does (admittedly, on a much smaller grid).

I’m working on some other posts looking at solutions for this, but I don’t expect to see any sort of real change in the next decade or longer.

2018-06-08 by Klaus

The fiscal year 2018 production incentive is $0.21/kWh generated even if you consume it yourself. That’s for WA state manufactured panels, it’s $0.16/kWh for out of state ones.

Net metering is separate, so you effectively get $0.30/kWh when consuming your own production. Net metering won’t pay out more than you consume, it’s on a yearly schedule and you have until end of March of the following year to use up credited energy.

I don’t think changing incentives is necessarily that difficult - the current legislation has pre-announced sunset dates (no pun intended) built in, so that’s a good opportunity to tune things for a followup. See Current Solar Incentives in Washington - they decrease on a yearly schedule and systems installed after Jun 2021 get no incentive. The rates are locked in at time of install, and payout is limited to 8 years or 50% of installed cost (whichever comes first) for existing systems.

In my case, the predicted production incentive (50%) + federal tax credit (30%) + net metering savings seem very close to the full system cost after 8 years or so, but that’s a fairly optimal case. I got the highest incentive level and have a large unobstructed south-facing roof with a 10.2kWp system.

This is already the second version of WA state incentives that I’m aware of, this new system replaced an older one with potentially higher but less predictable payouts. If they are careful not to take away already-promised incentives, there’s plenty of room to add new incentives based on more grid-friendly criteria without paying more than the current highest incentive level.

2018-06-09 by Russell Graves

Huh. I knew Washington had good solar incentives, but that’s sort of obscene for rooftop solar. It’s not a very good source of energy to the grid.

2018-08-01 by Black Cat

I previously understood you needed something like batteries to power you when the grid was down. But I still learned a whole lot from this post. Surprising that you don’t like solar panels that work with the grid. I guess balance is really tipped in the favor of the solar panel customer. Makes me want to go out and get some solar panels after I replace my old roof. Thanks for all the info.

2018-08-02 by AustinHook

“Also, “cloud edge” effects (the edge of certain cloud formations can focus more light on your chunk of ground than full sun).” I was told about this by a friend who has been running grid tied solar for a number of years. At the time I was studying DelftX - ET3034x solar course (edx.org online courses). Speculated that it could be additional light kind of refracted from cloud edges, but after he loaned me a 330W panel and micro inverter to play with, I started using my test gear (from my ham station) to measure various parameters, (DC short circuit amps, open circuit voltage, MPP and AC out), I saw the effect for myself, in real time. Fairly late in the course I realized how strong the temperature effect is, not just the winter high MPP, but also the working temperature of the panel when it has been in sunlight for a number of minutes. Didn’t have a thermometer on the panel, but relying on temp vs efficiency curves of typical panels from the course, I guess that the up to 30 degrees C higher temperature of the panel, when in direct sun, could make a significant difference. With my AC wattmeter I could see on several occasions that it took 5 minutes or more to fully decline to the pre-cloud value, after having just ramped up maybe 15% or so beyond that, when it had just come out of the cloud again. So now I attribute the cloud edge effect to the changing temperature of the panel as it cools down during the cloud, becomes briefly more efficient, and then loses that advantage once the panel heats up again. Any one else agree? VE6QBD/K2QBD

2018-08-02 by Maxim Ivanov

A solid write-up, appreciated reading it!

One thing keeps me restless though, that the article sort of assumes that chemical batteries are the only [viable, practical] energy storage technology. I’m not really following the market and the economics though, so I’m just going to plainly ask.

Have you looked into pressurized air tanks as energy storage systems? Perhaps also kinetic flywheels? What’s your opinion on those storage options. They seem to have far longer lifetimes than chemical batteries, and very little maintenance costs overall. Also flywheels – quite literally – operate on rotational inertia, could that be any useful for the net effects on the grid?

Sincerely honest questions; I don’t really know much on the subject. Any response appreciated!

2018-08-02 by Unknown

Thank you for this amazing article, it has dissipated some doubts from me. Here in Europe we operate house electricity at 50Hz, so you go to 60Hz? I imagine some misalignment in electrical clocks and such!

2018-08-02 by StormCchaser

There’s another negative effect of solar (or wind) on grid stability. When they are generating, they have an incentive to sell their energy at any cost. That means they out-compete more reliable generation sources - nuclear and fossil fuel. That reduces the usage factor of those plants, increasing the per-kWh cost of the remaining electricity they generate. As penetration of intermittent power increases, these plants will shut down.

That sounds like a good deal until the day the sun doesn’t shine and the wind doesn’t blow. Where is the power going to come from? The answer: either you get blackouts, or you pay to keep all those other plants available even when they are not producing power. In other words, intermittent power creates this hidden external cost on the grid.

2018-08-02 by StormCchaser

Oh, one other comment. Another negative of residential solar is that the installation and maintenance costs will be higher on a per unit basis, because you don’t have the economy of scale of large industrial utility installations.

2018-08-02 by Russell Graves

It’s not that I don’t like solar panels that are grid tied, it’s that I think the current approach to dealing with them (net metering) is particularly grid-hostile, and I think that keeping the power grid functional and maintained is more important than some people being able to zero out their power bill while still keeping literally all the benefits of the power grid.

Right now, yes, the balance is tipped quite in favor of the solar customers, and I expect that to change going forward, just at a different time for various places.

You can put together a grid-friendly residential solar power system, and it’s a project I’m actively working on (expect details next year on that front - no solar installers will call me back after I explain that I’m doing something very different from the one thing they do), and also do it with backup power capability, but it doesn’t look an awful lot like “normal” rooftop solar. And it will probably never "pay off."

On the other hand, I’m huge, huge fan of solar - in the large, utility scale farms. Those are awesome, they have the trackers to provide peak power in the morning and evening when it’s needed, and they can operate in various ways that are genuinely helpful to the grid. A solar farm, operating somewhat curtailed, can boost output almost instantly, even compared to a natural gas turbine.

2018-08-02 by Russell Graves

Part of the effect is certainly the panel cooling, but if conditions are right, you can get a boost substantially above even cold panel output with a cloud edge. It’s a combination of things, but you can definitely get greater-than-standard illumination as well.

2018-08-02 by Russell Graves

Another LowTechMagazine reader, by chance?

They exist as storage mechanisms, but I haven’t had a chance to mess with them. Round trip efficiency when you’re trying to store electricity in them is poor, though, as you note, they do have quite the theoretical lifespan.

The problem is that useful amounts of storage require very, very large systems, and large, rapidly spinning flywheels are somewhat terrifying. They don’t have too many graceful failure modes - they either work perfectly or blow up catastrophically, if you’re spinning them quickly. I’ve seen the aftermath of car flywheel/clutch explosions, and it’s not something I’d care to keep around my house.

Compressed air storage is something I’ve considered messing with, and I’ll probably do it at some point, but the efficiencies are just awful on it, at least at small scale.

2018-08-02 by Russell Graves

Clocks in the 60Hz nations are simply designed to run with 60Hz electricity. If you were to take them to Europe, they would run slow.

However, I don’t believe very many clocks still use the power grid anymore. Quartz oscillators are so cheap now.

2018-08-02 by Russell Graves

Correct, and you see this in the “So-and-so is selling power for $0.015/kWh! Solar is AWESOME!” news articles. Well, it’s not quite fair to compare an intermittent generation source to a reliable generating source that can put a MWh on the grid any time you need, not just when the weather cooperates. Point that out, and you get called a fossil fuel industry shill more often than not.

Wind is especially bad about doing this with the various per-kWh fees they get regardless of grid status, leading to things like negative electricity prices (as it’s cheaper to pay people to use power than to throttle back base load plants that aren’t designed to throttle, at least for the short term). It’s going to lead to some interesting challenges in keeping grid maintenance funded going forward, and unless we keep enough thermal/nuclear generation capacity to run the grid without renewable inputs, we’re going to have some cold, dark nights at times.

There’s a lot that can be done, especially in the way of opportunistic loads, which is an area I’m doing some research and writing on. For many tasks, applying more of a realtime scheduling algorithm (“I want this done by the morning, I don’t care when…”) can work well, and many other things are most commonly done when people are awake, which does work well with solar (at least, if you point your panels to the morning and evening sun instead of south). But, there are still an awful lot of challenges, and residential solar is not the most helpful approach to grid stability and longevity right now.

2018-08-02 by Russell Graves

Right, but when the homeowner gets to use the grid as a free battery and essentially gets all the benefits of the power grid while paying nearly none of the costs, that doesn’t matter (to the homeowner, at least). It’s a solution that doesn’t matter when you have that one weird guy at the end of a long feeder doing it, but as we’re doing it, it’s not a scaleable solution.

2018-08-02 by Unknown

I don’t understand this sentence: "they’d happily provide 1.8A at 12V, 1.8A at 24V, 1.8A at 40V, 1.8A at 60V"

How exactly are you varying the voltage of the solar panel? Don’t they just provide whatever voltage they provide and the only think you can very is the current?

2018-08-02 by Russell Graves

Solar panels behave as a constant current source, so they will push current at whatever voltage you connect them to.

I could connect my morning panels to a 12V battery right now (they’re operating at 3A/58V), and they’d still push in 3A at 12V.

A PWM controller isn’t a fancy unit - it’s literally just a digital switch that connects the panels to the battery or not.

2018-08-02 by Unknown

Worth noting on batteries: They suffer age related degradation as well as as cycle based degradation

Have you considered using a Flow battery - Wikipedia ? They are not easy to find, but they are available commercially, often targeted at solar power.

2018-08-02 by Russell Graves

I paid about $1400 for my bank of 8x Trojan T105RE flooded lead acid batteries in a 48V configuration.

That’s 225Ah nameplate, so about 11kWh nameplate, with a peak current capability of comfortably over 200A, if I needed it briefly.

Can you find me a commercial flow battery that’s remotely comparable? I couldn’t find anything even close.

2018-08-02 by Unknown

You’ve looked and found nothing? That’s what I was wondering.

When I looked I only found high prices, although it was difficult to properly compare because they last longer.

2018-08-02 by Russell Graves

The few that exist at a residential scale are usually “Contact us for information” and lacking any sort of pricing, which tells me a lot. They also tend to be of the “You write us a large check and we get you a something” style install, which I’m not a huge fan of.

I looked at the salt water batteries, but they’re seriously limited in peak output, and can’t be stored outside (they still freeze).

I’d love to find something better than lead, but I just haven’t found it yet.

2018-08-03 by Unknown

How about getting rid off AC in home installation ? All those gimmstics would just go away … :slight_smile:

2018-08-03 by Mike Mee

https://redflow.com/ is a an Australian-based flow battery producer that seems to be making good progress after overcoming problems with pricing and supply after years of research.

PS: A really GREAT article. Thanks for taking the time to write this up in such detail. I’m on my second rooftop solar installation – one in California and another in Western Australia). It’s only after reading this article that the local WA power company posture of paying $0.03 for generation while separately charging me $0.23 for usage seems justified (vs the net metering arrangement I had in California).

2018-08-05 by Russell Graves

Not really. A DC system still suffers from the limits of panel output. And tends to be significantly more expensive in wiring costs.

2018-08-15 by Unknown

I agree with your general thesis - that grid tied residential solar customers are getting a good (too good) deal. Luckily for me, I am one of those happy solar owners!

However, you made a statement about a residential power bill that is overly general and not true across the whole US. You said “your power bill consists of two separate items: Energy charges, and demand charges”. Here in Massachusetts in National Grid territory, our bill consists of a flat fee per month plus a per kwH charge. No demand charge. And the flat fee is ridiculously low.

2018-08-15 by Russell Graves

No, I made a statement about commercial/industrial power rate schedules.

"…but for any sort of serious load (industrial/commercial use), your power bill consists of two separate items: Energy charges, and demand charges."

Residential and small commercial bills are just the per-kWh fee that combines the two.

2018-10-31 by DIY Random Hackery

Actually, 50/60 Hz iron core transformers and motors are so optimized for their 50/60 Hz frequency that many will not operate on the wrong frequency, unless you’re drawing a fraction of its rated power.

2018-10-31 by DIY Random Hackery

Keep an eye on Hawaii, they are the canary wrt the US electric grid. They’re isolated and energy costs are ridiculously high (typical $0.33/kwh). They’re also further south so it’s been incredibly affordable (like 3-5 yrs payback) to install residential solar. One of the unexpected problems they’ve come across is that the $0.33/kwh includes subsidies for low income households. As more and more “rich” people keep shrinking their electric bills, the funds available to subsidies has been drying up. That creates a vicious cycle where the cost of electricity keeps going up, and more people move to residential, grid-tied solar installs reducing the cash flow into HELCO. Note that a 0 kwh/mo bill is about $20 in Hawaii. They’ve slowly increasing that fixed part of the bill. About the only good thing about Hawaii is that the point at which residential MPPT generation becomes harmful is far higher than anywhere else, because they high cooling requirements. Peak generation generally coincides with this cooling demand. Also for residential installations, rooftop solar will reduce cooling requirements by keeping the sun off the building.
I completely agree this can’t go on; eventually the electric company will reach a tipping point where they’re losing money. Electric cars and extra low tariffs associated with them are an even worse proposition. “fixed” electric prices (while nice and reliable) are not sustainable unless they are allowed to change it. The various US utility commissions have not shown much action on this subject.
Another good read is Germany; “Recent Facts about Photovoltaics in Germany” by Fraunhofer (a large research institute) July 2018 (so quite recent)

2019-03-31 by John Carter

My idea of a useful solar backup system is one provides limited solar backup for short grid outages (less than a day), with the option of lower power for a longer term outage. I consider my backup system a “Wait until daylight” design. Power goes off, I power up the 2000 watt pure sine wave inverter and operate the transfer switches for fridge, kitchen outlets, a few LED lights and the furnace (if needed). Depending on the season, the 540AH battery bank can provide 10-20 hours of limited power. If an outage is longer than 8 hours, I can get out a generator to power more items or to recharge the battery bank if there’s not enough sun - if a storm caused the power outage, there might not be useful sun the next day.
The only way my system can “pay for itself” is in the convenience of having immediately available silent backup power. While I have a generator and use it when needed, during the last outage of note (12 hours with the temp at 30F and 7" or snow) I never heard another generator running. Even the smallest inverter generators can be heard at a distance so I’ll hazard a guess that there’s no one else with a gen within a half mile of me. That’s not a big deal when the power is only off 12 hours but what if an outage ran 12 days? A few people might see the solar panels out back but there’s nothing for anyone to hear. Paranoid? Not really. When containers of kerosene and gasoline have “walked off”, I’d rather not run a generator and advertise having power. You might be thinking “Who has a 12 day outage”? Wouldn’t be the first time in this area as people in the city were without power for more than two weeks after a particularly bad ice storm. Unincorporated areas generally need even longer to get their power back so I try to have options.