I wanted to put this in one thread so others that research this topic can easily see this. I am debating to install solar hot water (solar thermal) panels.

The two 4x8 AET panel system is approximately $5000 after rebates, etc.

The 4x10 panel does 1000BTU/SF/day so lets figure 800BTUs per SF per day for the smaller 4x8 panel, for just hot water purposes (not whole house radiant).

To get total BTUs captured, we have:

4x8 = 32 SF

32x800 = 32000 BTUs per day per panel

32000 x2 panels = 64000 BTUs/Day

It takes 1,100 BTUs to raise 1 gal of water from 40 to 170 degrees. (8.34BTU/gal/degree)

64000/1100 = approx 60 gallons of hot water per day. That's nearly free hot water for life of the system assuming no loss and 800 BTUs is correct.

64k *365 = 23.36M BTU per year

Nicor Gas - a therm is $0.40

taxes/delivery/etc = $0.80 (assume)

1 therm = 100067 BTU

23360000 / 100067 = 233 * $0.80 = $187 per year

$5000 initial cost at a savings of $187/yr is a ROI of 27 years

NG at current price = 27 yr ROI

NG x2 = 13.5 yrs

NG x3 = 9 yrs

This is if I paid for it cash.

If I finance with a new home:

4.5% 30 yr fixed payment for $5000 = $26/mo

$26/mo = $312/yr

I am paying $312/yr to finance a system that saves me $187/yr in natural gas cost.

Add maintenance and any problems like leaks, etc...this gets even worse.

If NG doubles:

NG x2 = $374 - 312 = $60/yr savings NG x3 = $561 - 312 = $250 annual savings

So NG has to double to eek out some savings. So basically for now, the solar hot water system doesn't make financial sense at all.

I am puzzled about the statement, "It takes 1,100 BTUs to raise 1 gal of water from 40 to 170 degrees. (8.34BTU/gal/degree)" That upper temperature (170 F) seems high.

My domestic hot water is delivered at the faucet at about 115 F or 120 F. So at the tankless water heater, the output is probably set at 125 F. Seems like 170 F would be dangerous in that it would scald folks.

The efficiency of a solar heated hot water system drops off dramatically at higher tank temperatures. Mine is set for a maximum of about 155 F to protect the humidifier that has a maximum inlet temp of 160 F and is fed directly from the solar tank. The outlet of the solar storage tank is mixed with cold water with a mixing value to drop its temp to probably about 120 F or so.

How many people do you need to supply hot water for? I am wondering about a smaller, less-expensive, system.

the tank is typically set at 190 per the solar guy. The solar company I am working with wants to lower it to 170 to preserve the life of the tank. This will still allow a somewhat hot shower in the AM after the water cools through the night as there is no sun producing heat. These are the solar company's temperature recommendations, not mine. I should add I am located in Chicago. The 4x8 system *is* the smaller system. I guess I can do one 4x8 panel instead of two but at that point with plumbing costs etc, the installation doesn't seem worth it. I really want to do it but it does not seem economical unless someone can point to an error in my justification. Another issue is the state of IL is now doing a rebate "lottery" rather than a guarantee rebate. So now, it is not guaranteed that the system will cost $5k. It might be $8k if I only get the federal which makes all this a moot point.

Although I would not use the same method you did to come up with the heat provided, you did come up with a number that is quite reasonable. In our market gas is sold by the m3 so I won't bother getting into all the calculations but suffice it to say that a 2 panel system will do 50-60% easily for a mildly conserving family of 4 in our climate (forget the rainshower shower heads). The average family is using, in Toronto, about 25% of the total gas used for DHW and that is about $300 so your $187/yr is reasonable.

One thing we noted from 100 systems that we monitored is that SLIGHT variations in piping or in usage changes the ROI a lot so your installer cannot be a fly by nighter type. In the end, you will put in the solar system because you want to give the middle finger to the gas company (or some other reason) but not the the financial return. Offsetting electric water heaters is a different story.

S'up, your calculation looks pretty solid to me.

1. Implicit in your calculation is that you would have a NG fired HW heater as backup in the solar scenario, so the solar only displaces fuel cost. IOW, the equipment cost/depreciation on the NG tank is the same in both the solar and non-solar cases. Okay. If you tried to go with a cheaper electric backup on the solar (to get one tank, no flue, etc) it would make your payback **worse** relative to the NG scenario due to the higher cost of elec BTUs.

2. You are calculating savings by multiplying yearly solar harvested BTUs times a cost savings per BTU. However, you might instead figure out how many DHW BTUs you will actually use, and then multiply that by an expected solar fraction appropriate for your location (50-70%). IOW, even a small system might generate more BTUs that you need in the summer, and those BTUs are NOT saving you money. If your expected daily harvest number is an annual average, and not much smaller than your usage, this seasonal variation in supply will significantly hurt your payback also.

Well we all take showers in the AM, when there's been no sun for the night so we wouldn't benefit much from SHW due to this aspect as well. I would have to heat the water a bit to get it hot to the shower in the AM after it cooled to 100deg overnight.

Also there is an error in my math. If NG doubles or triples, the delivery fees will probably stay the same which means it is not a straight /2 or /3 of the ROI value. It would be less. so the payback at 2x NG cost would be higher than I calculated as 13.5years. It might be +15 years still.

At this point I can't make it work. Rather than give the middle finger to the gas company I'd rather save my money and not spend the $ on SHW. Now considering the pump, piping, heat/freeze aspect of this system on the roof, any issue and those costs to repair really hurt the numbers further. I'd rather put up another couple PV panels instead.

Right, when you take the time to do a proper ROI analysis, the truth often reveals itself.

Posted By MikeSolar on 10 Sep 2013 10:24 PM
...snip...

In the end, you will put in the solar system because you want to give the middle finger to the gas company (or some other reason) but not the the financial return. Offsetting electric water heaters is a different story.

In my case, the justification for solar hot water is not giving the middle finger to the gas company or anybody else, but rather being able to stand in the shower for an extra couple of minutes after a hard day of kayaking or yard work knowing that I am just taking advantage of the sunshine rather than burning up an irreplaceable fuel. It does feel really good.

Lee this is not to offend anyone with a system. Just putting up a full analysis. Here are some additional details after bouncing this off my solar guy to see what he says:


He says: "A displacement factor for 70% AFUE (typical of conventional water heaters) is 1.42. That means 100 BTUs of solar is worth 142 BTUs or 42% more. I assumed in my previous e-mail to you that a high efficiency boiler would deliver hot water to a storage tank at 80% annual efficiency for a 1.25 displacement factor. Let's split the difference at 1.33%."



$187 per year that number is now worth $248 in actual NG cost savings. But we lose some efficiency with the panel (heating the pipes, pump, etc). Lets stick with 1.25% making it $230/year savings on NG using solar hot water



He says (good point): You must remember that to pay the gas company $248 you have to earn $380 (I'm assuming your net take home pay is 65 cents on the dollar after state and federal taxes, not fully counting social security and Medicare). Solar energy cost savings are tax free, because you pay your gas bill with after tax income.



My net income is not 65% it is a little higher so lets take $230 and use 75% = $305/year savings



He then says: Now let's talk about financing. Most of what is paid in the early years of a 30 year mortgage is interest (97% in the first year) and the government subsidizes that interest with an income deduction valued (again I'm assuming a 35% tax bracket…your's could be higher or lower) which means that the $25 in interest paid monthly in the first year of the solar loan will reduce the actual cost of the $26 payment for solar by $9.



So we get 25% back of the $312/yr payments. So now we are going to be at $312*.25 = $78 so the real cost per year is $312-$78 = $234/yr

So I am paying $234/yr to save $305/yr on NG...a net of approximately $70/year

Now if NG doubles in price (assuming delivery/taxes stay the same) the price of NG delivered is $1.20 not 0.80 = So the cost goes from $305 to $457.

Now I am paying $234 to save $457 which is a +$200/yr net positive to the homeowner if natural gas prices double.

Of course one repair bill of $1000 kills the numbers for 5 years. Comments?

Just to toss in another option; I have seen fairly lucid discussions in another forum regarding the use of solar PV and heat pump water heaters as a better alternative to solar thermal. part of that analysis considers that all of the energy generated in summer (assuming net metering is available) is captured. It is then available for reuse in the winter. The hot water is also available in the morning as well. PV is much cheaper than it used to be, heat pumps have a COP much higher than electric or gas, and you don't have a backup tank cost to calculate. I am not sure of your location; the heat pump water is often located in the garage and pulls heat out of the air both summer/winter. If you are in a cold climate, it may pay to have windows in the garage to add passive heat in winter. With net metering, you can burn up the summer credits in the winter.

Sorry, i don't have the figures available to offer the financial analysis. The article was written about MA costs and climate; PV makes some sense there. I'm in CA with warmer winter, much more sun; it's a no-brainer here. Only if I can do a DIY project for under $2,000 does it make much sense. Here is the site to read the original article and math; http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead

I'll never trust the math using equations like "1 therm = 100067 BTU ", given that the very definition of a therm is 100,000 BTU. Where does the 67 come from (You've used that number before, and it's hard to make a typo that wild twice!)

If the delivered price of natural gas triples resistance electricity would be just as cheap as gas.

But even at the current residential price of grid electricity and (post subsidy) $2/watt PV and heat pumps are BY FAR the better energy deal if net metered at retail, and a better use of your roof space. Mini-splits would be bumping on a COP of 3 for an average in your climate for better than 10,000BTU/kwh. At 12 cent/kwh that's more expensive than 80cent/therm gas in a condensing burner, but cheaper than $1.20 /therm gas in the same burner. And if the price of gas doubled or tripled it would only mean that grid operators accellerate the installation of wind power- electricity prices won't track lock-step with gas, since the lifecyle per kwh cost of wind is nearly on par with combined cycle gas even at the CURRENT contract price of gas.

Payback on net-metered PV is also in after-tax dollars. Zeroing out the electric bill isn't any less or more valuable than zeroing out the gas bill. Doing the analysis only against a 0.70 EF standalone tank heater is something of a straw-man. You only have so many square meters of roof to work with- and maximizing the value of that utilized area needs to be looked at fairly carefully.

Nowhere in the analysis to I see the all-in cost of operation of the solar thermal including pumping power for both collection and heat dumping. That number is certainly not zero, nor is the standby loss at 170F-190F storage temps.

Man, watts and kwh are so much easier. As for ROI, I turn off my gas in late April and don't turn it on again till early October. My solar does the job and the gas company is still confused, haha.

I'm going to play around with a DIY solar system, mostly as an exercise, and to see if I can save a few bucks. My system will be nowhere near $5K though- maybe $1K, tops. We're in SC, so good solar properties, and our only choice for DHW is electric, so I think I can make it pay.

There are a number of ng fired gen. plants going in around here.
Given the current glut of ng on the market, I would expect many more to be added and or old ones converted.
ng is a reasonably clean source both at production and at generation so these plant can go in almost anywhere and much closer to end use. There is no line lose in a gas lines. There is a fairly substantial loss on wind farm transmission and on the whole grid.
Given the glut, there is not much change of ng doubling let along tripling. The "experts" predict at least 10 years of low gas prices.
As more ng power plants come on line, (partially to stabilize the grid because of alternate sources) I would expect that North America power prices will be lock step with ng.

So lets try to do the math without saying "if"

Posted By FBBP on 12 Sep 2013 09:41 AM
There are a number of ng fired gen. plants going in around here.
Given the current glut of ng on the market, I would expect many more to be added and or old ones converted.
ng is a reasonably clean source both at production and at generation so these plant can go in almost anywhere and much closer to end use. There is no line lose in a gas lines. There is a fairly substantial loss on wind farm transmission and on the whole grid.
Given the glut, there is not much change of ng doubling let along tripling. The "experts" predict at least 10 years of low gas prices.
As more ng power plants come on line, (partially to stabilize the grid because of alternate sources) I would expect that North America power prices will be lock step with ng.

So lets try to do the math without saying "if"

I'll agree that the wholesale price of ng is not likely to triple within the lifecycle of a boiler, but the average will be higher than it has been in the past 3-4 years or the gas developers will go bust (some already have.)  If/when carbon emissions get taxed there WILL be a bump in the retail prices of gas, even if the long term picture for gas prices are flat, at something somewhat above the current spot-market price.  Buck-twenty residential retail gas in IL is more than likely to happen within the lifecycle of heating equipment, it has already made that price point both in 2005 & 2008, just as the shale gas glut got going.  The current price simply isn't sufficient to sustain continued shale gas development over the long term and demand WILL catch up, but whether it'll average $1.20/therm or not takes a crystal ball much clearer than mine.  (Full disclosure: I have a close relative in the gas exploration & development biz.)  The rise of the retail price of natural gas isn't an "if", but a "when", the significant question being "how much". Fracking shale isn't cheap- the number of wells required for gas field development is orders of magnitude more than in traditional gas sources.

The pumping & liquids-separation costs and the leakage losses are much higher than you might think- there ARE significant distribution losses and costs to the gas grid, and the losses are a serious issue, since methane (the primary component of pipelined gas) is a powerful greenhouse gas.

The grid losses for distributed power are known, and easily calculated into the lifecycle cost of wind, and Illinois has fairly substantial undeveloped wind resource to tap. In neighboring Iowa wind now accounts for more than 25% (and growing) of the annual kwh delivered on the grid, with increasing amounts being sold into the IL market. This is not pie in the sky, and the unlike gas, the lifecycle cost/Twh is known before they even break ground the construction, since it's all in the upfront cost & financing.  That cost is still falling year on year as the capacity factors improve with incremental improvements in the technology, and it's at near-parity with cc gas now. And it's not a static picture- there are many moving parts. As better/cheaper grid storage is being developed and deployed (recently mandated in CA), the ability of PV and wind to further cut into the capacity factors of gas fired generation is increasing.

But for now PV/wind and gas generation still "play nice" together, as gas is more flexible than sources such at thermal-coal or nukes- even combined-cycle gas output can be ramped up/down in reasonable time scales while maintaining reasonably good efficiency.  The price of wind & PV power will eventually be what determines the capacity factors of those gas plants though, since all projections for the lifecycle cost of new wind & PV in 15 years is below the per terawatt-hour of cc gas at current gas fuel prices.  Utility scale wind at today's prices are below even the FUEL cost of nukes, which makes me less than sanguin about the newer mini-nukes currently under development.  In CA the wholesale cost of utility-scale PV under recent power purchase agreements is about the same as the fuel cost of nukes too.

If you're going to heat the place with gas and are NOT cutting your heat load to under 25KBTH/hr, it's probably more cost effective to go with the Honda-cogenerator based FreeWatt system (either hydronic, or forced air) than with solar thermal, even at buck-twenty gas. (Full disclosure: My biz partner heats his large drafty 1840s antique with a hydronic Freewatt, but his heat load is pretty  gia-normous compared to new construction.)  Finding contractor might be tough, now that ECR International (who bought the original developing company Climate Energy a few years ago) seems to have removed their web pages on the product.   HVAC companies still seem to list them though.

We have had this discussion but wishful thinking seems to prevail with Dana and his close relative. Gathering pipelines represent the limit on natural gas production, rather than prices, for three reasons. Given high sunk costs and low production costs, it makes sense to flow gas to make the monthly nut regardless. And if other producers are working the same field, you have to get yours while the getting is good. Going broke means the next owner has minimal costs. The last gas bubble lasted 20 years. Old timers will tell you high prices are the anomaly rather than low prices.

Posted By toddm on 12 Sep 2013 01:05 PM
We have had this discussion but wishful thinking seems to prevail with Dana and his close relative. Gathering pipelines represent the limit on natural gas production, rather than prices, for three reasons. Given high sunk costs and low production costs, it makes sense to flow gas to make the monthly nut regardless. And if other producers are working the same field, you have to get yours while the getting is good. Going broke means the next owner has minimal costs. The last gas bubble lasted 20 years. Old timers will tell you high prices are the anomaly rather than low prices.

I'm not particularly sanguine about the prospects of natural gas. The high cost of production is just one aspect limiting the profitability, and the cost of expanding the gas-grid infrastructure difficult to finance getting it to market is also substantial.  While the wholesale price may remain relatively flat, not much above current prices, the retail price will continue to have upward pressure due to pipeline constraints.  New cc gas generation proposals haven't made the financial cut in some parts of New England due to the cost of the necessary expansion of grid infrastructure.

Old timers applying that history to future performance would have to be clueless about the actual cost of developing tight gas resources.  The relative in the biz is convinced about the order of magnitude of the supply, but having lived through the volatility of gas pricing since the mid-1970s he's seen the glut bubbles of low pricing followed by a cycles of increasing price multiple times.  His last coal seam gas project in OR went bust- for now, due to the glut. The reserve is proven as well as the quality of the resource, but can't compete against already developed resources that are selling the gas at below the cost of the development, hanging on ultra thin margins on the  liquid fractions. When the price picks up that field would be able to garner financing for development, but for now it's mothballed- the exploration wells are capped, the permits for developing the field are kept current, but nobody is collecting a paycheck.

The investors (including him) are in a holding pattern, and it may last a decade or more.  The demand side pressure is increasing, exploration is flagging. The movie has played multiple times over the past 3-4 decades.  To stay busy he moved onto a project in Zimbabwe about 4 years ago until politics and financing put that project on hold (but he collected a salary, and owns shared in the South African company formed to legally do the project) and has since gone on to an oil project in Alberta (and is currently living in Calgary.) I'm not sure where "wishful thinking" comes in on his part either.  Sometimes he seems drunk on the frackwater Kool-Aid, other times less so, but he has no illusions about the cost of doing business in gas field development, and his take that the current price of gas won't support it, but it won't take a huge price uptick to get there. Maybe that's where he's being wishful, hoping that it stays far enough above $4/MMBTU on a sustained basis to make developing the resource at his OR project worth something? If it had a high liquids fraction like some of the richer shale gas they'd probably develop it now, but most coal seam gas is dry, some of it too contaminated with CO and CO2 to be directly pipelineable, but this project would not need processing, and the pipeline is nearby. It'll be viable someday- can't say just when though.

Going broke does NOT mean the next owner has minimal costs when you're talking shale gas. It still takes X number of boreholes to produce Y tera-BTU of gas, and that number seems to be increasing, not decreasing with time. Buying up operations that go bust may save SOME of the exploration cost, but essentially none of the development & production cost.

The cost of PV & wind and the rates of development have been consistently mis-judged for decades- all projections for wind development by 2020 made in 1990 were exceed by the year 2000, and even some of the wing-nut pie-in-sky types undershot.   Conventional wisdom about clean energy is still way of date.  The levelized cost of PV is below the retail residential rate in MOST of the US, and where third party ownership of rooftop residential PV is allowed, it's a profitable business, a business that has seen and will continue to see exponential growth unless/until blocked by regulators.  If the solar utility can offer substantially lower rates to the homeowner than the local grid utility can in exchange for allowing the installation of PV on the roof, everybody wins, at least in the short term:  Exponential growth is  a funny thing- what seems like a trickle and easy for the grid-operator to manage in the beginning quickly becomes a torrent- this WILL change the net metering deal structures (and probably within the next five years), to give utilities a chance to adapt their business models and survive in the face of an onslaught of cheap distributed generation that they don't own, but it won't easy.  We're already beyond the thinnest edge of the wedge on this and some large utilities are vigorously fighting it, viewing it as an existential threat.  Are they wrong? Stay tuned.

But what cheap renewables means for the price of natural gas is in the intermediate and long term suppression of gas price inflation, as ever larger renewables-fractions cut into the capacity factors of gas-fired generation, freeing up that gas for other markets (including the export market.) But even then it can't be sold below the cost of production forever, the way it has been for the past several years.

About conventional wisdom - in the 90's nobody could foresee that governments would apply so much market altering subsidies to the alternate energy field.

To now use todays figures to look to the future misses this point. Subsidies were placed to move away from foreign oil dependancies. With the glut of oil and gas on the domestic market, this no longer holds so expect huge cutbacks to these subsidies.

Without subsides and without high energy prices, alternates will no longer do as well as they have in the past decade.

Wind gens still need backup plus with the growing uneasiness of the general public to live close to turbines the cost of transmission will become higher.
This means that there will be a decent market for pv panels but maybe not so much for the rest.

if we stop subsidies for alternative energy, we'll need to stop subsidies for fossil fuel industries which not only include the hundreds of millions annually, but the give away prices we're charging the oil, gas, coal and other minerals for taking and profiting from what belongs to all of us, plus the public costs for cleaning up their disasters and the stuff they normally leave behind. I think solar and wind will do exceedingly well with those terms.

All generation needs backup (ever tried to refuel a nuke while it's running? :-) ) That's the standard red-herring on wind, but completely beside the point. The fact that wind isn't a dispatchable resource makes it a management problem, but it's proven to not be as big or expensive a management problem as everyone feared (or some fear-mongers still purport, contrary to the evidence.) And as more grid storage and demand response get implemented, the less that variable output on non-dispatchable resources will matter. But for now, it's demonstrably easy (and economic for the ratepayers) to hit 50% and higher penetrations of renewables with just combined cycle gas and demand-response as the only "backup". Taking the hit in capacity factor increases the cost of the cc gas, but not to the point that it has a negative impact to the rate-payers, since the marginal cost of the capacity factor being eaten up by wind is just the capital cost of the unused gas capacity, a cost already built into the rate structures, with zero fuel cost. I expect cc gas to be around for the forseeable future, but once the uneconomic coal & nukes are all shuttered the grid share& capacity factors of cc-gas will be monotonically decreasing over time.

You don't have to site wind in housing developments to take advantage of mid-western winds, and dual-use of agriculture + power is the most common. It might take a huge infrastructure effort to ship Iowa power to NYC from the midwest, but not to Chicago. West TX wind development is constrained by the isolation of the TX grid from the rest of the US and thin transmission capacity to the larger TX markets, but the economics of hooking a high capacity DC link from TX wind zones to the western grid are looking pretty good. The SD/IA/MN wind growth doesn't need extensive infrastructure improvements to keep feeding more to the major midwestern cities, and citing wind resources near the existing grid resources isn't all that tough- it's how it's done, and will continue to be done for quite awhile. What's new is that companies with large computer-server loads are being re-located into the midwestern wind zone to take advantage of that nearly-free primarily wind-sourced off-peak power, but they also already build in their own power storage backup to avoid the peak rate sting and to guarantee reliability.

The current cost of PV is below residential retail in much of the US even without subsidy. The production tax credit subsidy for wind only changes the slope of the wind-implementation- it's cost-effective already even without it, but keeping the subsidy in place to develop the market and not crush the nascent wind industry in the US is a reasonable policy decision.

The 800lb gorrilla in the room is that 60% of all electricity in the US is just outright wasted, and if the Shaheen Portman bill gets written into law along with the executive branch efficiency push under Moniz, the economics of ALL power generating sources will need a serious re-write for the next couple of decades. New-negawatts are still much cheaper than any generation resource when you have that much fat that can be economically cut, using discount rates in the present-value calculations that long term investors like utilities use. At the residential level homeowner behavior implies very RIDICULOUS discount rates of even 50% or higher, but when third-parties are allowed to invest in site-efficiency (be it at a home or commercial/industrial location) financial reason can start to take place. Demand-growth (or lack thereof) is already a big problem for grid operators, and it's only gets worse (for them) when a large fraction of the growth in generation capacity is from distributed small generators. Large vertically integrated utilities NEED demand growth and the guaranteed profit margin on power generation plant investments, but with a major focus on efficiency and the swarm of small or independent wind generators coming onto their grids, they can't rationalize new generators to their regulators, and it becomes a death spiral from 1000 cuts, unless they can re-make their business models. This business model problem isn't going away any time soon, but in some states there is better regulatory financial alignments between utilities and rate payers. But even there a major policy shift toward the efficiency economy will force adjustments.

The glut of oil and gas can only affect the price so much. The world price of oil is the primary determinant what we pay for oil-based products, not the domestic production. (Texas now outproduces Iran in total barrels, and the US is now 87% self sufficient on oil- did you notice the huge drop in gasoline prices when that happened?) Gas is harder (but not impossible) to transport internationally, and is less subject to the world price, but if the glut continues unabated the only price relief for the producers will com from LNG transport to Europe and Asia where the spot prices are much higher. I don't expect the subsidy for these industries to get cut too drastically, given the economic benefits of how that affects the balance of trade issue.

The history of subsidy to the oil & gas biz that has led to the glut and low prices is really pointless. Even if it were possible to produce gas at $2-4/MMBTU forever, the cost of PV is still going down, along with the cost of grid storage, as well as the cost of wind. If regulations allowed residential scale grid-attached storage, gas peakers could economiically go away in many areas. (Most grid operators aren't set up for islanding local grids during distruptions, and back-feeding the grid in an uncontrolled manner is dangerous.) But that's coming too- a smart-charger on a plug-in electric car that was allowed to compete in a capacity market would be cost competitive at retail using already developed technology. Currently there are pilot test sites in both TX and CA using only Prius-sized batteries (wall-mounted, not in cars). A decade ago when the RMI folks were first running that up the flagpole it seemed like some green-dreamers fantasy, but reality is proving it true. Don't expect the grid mix and control in 2025 to look a whole lot like it does today. It'll be incremental at first, and not in every region, but with utility-regulatory reform it can happen pretty fast- economic impediments are no longer the driver.

But the current arc of PV in the US has little to do with the subsidy history, and a lot to do with the development of third party ownership models, which now dominate the rooftop solar installations.