The holy grail for small PV installations?

First a quick note. Psyonyk I just found your blog right before you shut it down, and I wanted send you praise, its one of the best source of info i’ve found for small PV installations. I love the text + picture format, youtube style videos are obnoxious garbage in comparison.

I’ll try to keep background short, keep this at a forums post size not personal blog length.

I live on a few acres with cheap grid connection. I got FOMO that PV panels are as cheap as they’ll ever be thanks to subsidies, found 6x 305W panels on local classifieds as leftovers from a big project that bought by the pallet. Paid just under $0.50/watt.

live in northern cloudy area with an estimated 15% capacity factor for PV, i’ll knock it down to 10% to include a bit of pessimism. On a yearly average, 1800w of solar panels may generate 180w continuosly. Doesn’t come close to satisfying my current ultra wasteful lifestyle but if “TEOTWAWKI Grid Down Prepper Fantasy Scenario” happens i guess i’d probably appreciate it more than 0 watts.

However, what is running through my mind now is actually non-civilization ending scenario and a goal called “not being stupid with your money”. On their own, if those panels could offset power i would otherwise buy, that generates a value of ~$156 / yr at current rates of $0.10/kwhr. Pays for themselves in ~6 years, or a few more if you want to add in a discount rate, IF i can actually use the energy for something i already would use grid electricity for.

Batteries seem to ruin the economics of it, i’m served by a simple utility that offers no Time Of Use rates or peak shaving incentives, so essentially i’d be cycling batteries and wear them out powering a fraction of the house’s electrical loads, just for my own amusement - while the reliable $.10/kwhr grid is sitting there most of the time.

Now, I think I’ve found the holy grail on this quest of efficiently utilizing PV energy, and its not really talked about much, and like all the highest quality info on the internet its wrapped in a primitive looking website.
I present for your consideration:

This solution offers:

  • A huge capacity battery* I already own and regularly charge. It just happens to be thermal battery rather than electro chemical.

  • Built in over-“charge” circuit (via ECO), and built in automatic load transfer switch to use grid power for demand if needed at night or in low solar times.

  • A circuit that requires an absolute minimum of additional electronics compared to anything I’ve seen.

I’m really blown away, this seems awesome even too good to be true. Is there a catch i’m overlooking?

Beyond "You will never get an inspector to consider this sane…* problems, I don’t see any real dealbreakers with it.

They discuss how to match your panels handwavingly to your resistor, so other than not actually being anywhere near peak power (if you’re within 20% or so, you probably are having a really good day), it should certainly generate heat.

I don’t see a huge issue from a technical sense. Wiring and panel mounts are far from free, though. And depending on your power company, they may come snooping around if they see a lot of solar panels and no net metering agreement. I’m not sure what your local code types would think of the setup or if they’d consider it “isolated” from the power grid, but as long as you’ve got things run to only one element, with no way to feed the grid, I can’t see that they’d have a reason to complain. Make sure your install is tidy and correct.

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Well I hope to keep my real identity anonymous on your forums, and can take advantage of the fact that I live in one of those places where the golden rule is don’t give your neighbors a reason to complain and you probably won’t need to worry about the other rules. Nor am I looking to risk fire or electrocution going to excessively extreme measures to attempt to cheat the system or save a few bucks.

I’m very interested to hear more about what I could expect in terms of efficiency with a fixed resistive load compared to a “theoretically perfect MPPT connected to the grid or other infinite load”?
One of the other smart resources on this subject ive found is this guy “electrodacus”, he’s on some forums under the name dacian as well. If you don’t know of him his project is a fully PV house using DC direct heating (he gets a lot of sun, and did the work on his house to minimize HVAC consumption, makes sense for him).

He has a very iconoclastic opinion on MPPT - basically that its a bandaid over a mismatch between PV array and load, and its always less cost effective than investing the MPPT capital into more panels matched properly to a load.
Further discussed here, sorry its a video i recommend skimming the transcript Is the MPPT in OffGrid Solar obsolete ? - YouTube

So, do you buy it? I think I’ve read on your own blog, that for a given amount of incoming sunlight, a panel acts like a current source within a boundary of operating constraints (V < Voc, I < Isc). If that simplified model is valid, then it seems converting to an equivalent Voltage source, and using a load impedance equal to the output impedance you’d have maximum power. Waterheater guy says to use R_ideal = Voc/Isc from the panel data sheet. Now obviously those values are for the 1000w/m^2 @ 25c or whatever ideal test conditions, the actual short circuit current and open circuit voltage are different in real operating conditions, in practice have you measured how much the ratio of actual Voc & Isc change with varying intensity of light and temperature?
edit: Or put another way, how much of the perceived benefit of MPPT is because its catering to the needs of a battery bank which are very different than a fixed resistor?

Ah, yes, that guy. I’m faintly familiar with his work, certainly an interesting approach. Lots of thermal mass, IIRC?

I can see his argument about an ideal resistance, and Voc/Isc don’t vary wildly enough to make it lose that much power, though I’d have to think pretty hard about Voc/Isc vs Vmp/Imp (max power points).

On the other hand, I’m not sure it matters. For my panels, it’s the difference between 3.7ohm and 4.1ohm, not a huge gap.

I’m sure it works, I just don’t see why one would go through the hassle of PV to heat when one can get solar heat directly, with far lower technology levels - Build It Solar has plenty of writeups on this, including solar hot water. I don’t see the point in converting 20% of the energy hitting a panel into electricity to route it somewhere else and make heat, when you can collect a lot more than 20% of the energy as heat, if your end goal is actually heat. Electricity, IMO, is a far higher value form of energy than low grade heat, and while it will do it, it’s like asking a sports car to tow. Yeah, they can, but it’s not what they’re good at. I mean, if you want to heat a space on PV, look at some of the DC heat pump options. 1kWh electrical will pump 3-4kWh thermal in a typical setup.

I’ve not measured the differences with Voc/Isc in varying conditions. My office bank is 4S on the main array, and I let my charge controller sort it out. On the other hand, my morning panels are 2S, run into a 48V battery bank with a PWM controller, and I’m not convinced there are many gains to be had with MPPT there. It is good enough for my needs.

But on the house system, the MPPT benefits are significant, because I can run 600V to the house (max voltage is upper 500s in the winter, theoretically). I can run cheap 12AWG for my runs and be fine (actually, quite overkill for how they run, but the math works out to show I need 12AWG for my panels).

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Sorry, I think I got my wires crossed anyways and said Voc Isc but meant Vmp and Imp, but like you said, small difference.

I love the build it solar website, big inspirations for me, and have done a few projects from there and have lots more on the (want) to-do list. My end-goal would be to reduce electricity consumption that I could easily power daily life from a small array. But I am very far from that goal, typing to you from a mostly original 70s double wide that’ll probably be buried in snow for the next 4 months with an electric furnace thats fed with 2 double breaker 220V circuits, 40A and 60A.

So the logical practical course of action for me still remains the usual, insulation, upgrade windows, install heat pump, use more biomass heat, hang dry the laundry, learn to enjoy cold showers, use/store thermal energy from the sun directly. Theres a LOT to do before installing enough PV to go “off grid” becomes the lowest hanging fruit, if it ever does.

So I agree that it is profane to use the most refined form of energy (electricity) for the most lowly form of energy demand (heat). But also because I recognize how high value electricity actually is, combined with the possibly FOMO that now is the cheapest panels will ever be, I consider letting PV panels leapfrog many other projects that would have a better ROI otherwise. I’d like to get to where if an extended grid outage happens I’d power on an inverter, and start charge/discharge cycling batteries to keep the fridge running overnight.Electricity is so nice to have its worth having a redundant supply of it.

For now that scenario is quite unlikely, I guess I see solar water heater as by far the cheapest possible way to usefully idle the panels with minimum amount of complexity or investment. Learn how everything works in practice, and keep an eye on local used classifieds to build out the system bit by bit.

For now its baby steps and maybe one day I will work through enough of Build it Solar, achieve zen, un plug the water heater for good, boil water in a kettle on a wood stove, and be more discerning in use of PV output.

EDIT: HMM, well actually you may have changed my mind, I had no idea about DC heat pumps.

Have you considered a modern doublewide on the same spot/foundation? They’re massively better on power. We heat a recent doublewide on a heat pump and our backup coils are only 16kW (the “electric furnace” that it came with is demoted to backup duty with the heat pump install). I’ll suggest that if you’re worried about any sort of environmental or cost impact, focusing on your heating will be radically, radically more useful than a couple solar panels in terms of return. I don’t even run the 10kW backup coil (mostly because heat pump defrost cycles kick them on, and this is silly). In a cold winter here (highs in the mid-teens, lows a bit below 0, all F), we used 2MWh/mo (mostly to heat), and that was before I worked out details of the backup coils.

But fundamentally, there’s not a huge amount you can do to improve a 50 year old, 1970s doublewide’s energy use. The standards changed radically in the 80s, and if you’re starting to consider going down to the studs to improve stuff, just put a new one in instead. Seriously. Taking a grid-dependent house off grid is really, really hard.

You can still do a lot with heat pumps, and IMO your money would be far better spent with an inverter drive grid heat pump or two (mini split units). I would wager you’ll need more than one to heat in the winter, but… seriously. If you’re on coils for heating, that’s the easiest thing to fix.

If you want to build a backup power system based around solar, absolutely, go for it. And the DC water heater system may make some sense, and offset some of your energy use in the spring/summer/fall. Though if you’re going to roof mount them, they still probably need rapid shutdown capability (and NEC 2017 won’t let you just shut down the array, you have to shut down each panel).

But even then, the low hanging fruit is heating the house with those coils. I don’t want to know what your power bill is, but it can’t be any fun to pay.

I’m not entirely sure what your goal here is, but it looks, to me, like you’re falling down a bit of a sunk cost fallacy here. You think panels are as cheap as they’ll ever be, so picked some up - that’s fine, no objections. But you’re now chasing that down weird paths in order to find some way to use them. If there’s no obvious way to use them, put them in storage for a while - they’ll be fine. My panels sat in my shipping container for over a year before I got them up on the house system. Focus on the low hanging fruit if your concern is energy use or emissions or such. Of course, if you’re up in Canada on a pure hydro grid with cheap power, maybe this doesn’t really matter, so… OK, go for fun stuff. I just can’t see the sense in buying a DC heat pump when, for the same money, you could get a couple grid power heat pumps that will have a radically larger impact on your use.

Also, in general, off-grid power is both high cost and high impact compared to grid power. The energy reductions to get there are useful, but I generate far less energy from my office panels than the house panels will, because I’ve nowhere to put it. And the battery lifespan means that my ongoing costs aren’t zero either. I think I’m down to around $0.50/kWh used in my office in 4 years, but grid power is $0.10/kWh out here. I put it in because I’m interested in the stuff, but I don’t pretend it’s particularly clean, and it’s certainly not cheap.


Thank you for such a detailed reply. Your intuition on my state of mind and goals is pretty accurate. I think you’re getting my head straight about this strange solar panel fixation :slight_smile:

New doublewide would be a hard sell, despite the age this one is fairly solid seeming, “engineered foundation” the rails are sitting on blocks and are strapped down with steel straps. When I got it, it had a newer-ish cool membrane roof that sheds snow really well despite being low pitch. Replaced single pane aluminum frame windows with double pane vinyl, and using the old windows to enclose in a “better than nothing” sunroom thing on the south side vaguely inspired by a few of the projects on Build it Solar. Also had some sealing work done on the furnace to duct connection, replacement of a shredded apart by animals crossover duct, and sealing duct-to-register connections. Supposedly thats a big source of losses on older homes, that was just done over the summer so we’ll see if there’s any improvement on heating energy use. Also have a pellet stove, when we’re not being lazy usually we set the electric thermostat to 55, except it comes on a timer at around 6am for a few hours to make getting out of bed not painful, then turn on the pellet stove for the rest of the day. The big project on my to-do list but also procrastinating since it seems like a totally miserable project, is blowing cellulose in the attic. That will probably be as far as it makes sense to go with this house, I realize sadly it’s probably impossible to retrofit it to a totally awesome insulated airtight passivhaus type thing that just needs the heat output of a candle or incandescent lightbulb for warmth.

I did think about adding grid connected heat pump, got an idea of costs looking at various online equipment sellers, thinking ok maybe 3-6k in parts, assume approximately same for labor, but the quote I got was over 20k. Didn’t even really pencil out with very optimistic assumptions about energy savings. Maybe it makes more sense if you do more if it DIY and find a tech to hook up the refrigeration lines under the table? The “DC” heat pumps I saw also had an AC 220 connection to keep them running at night if you want, but also had about a $500 markup over equivalently sized AC only mini splits so I gotta wonder if I wouldn’t be better off with a 220VAC inverter (that also could run a well pump maybe) wired to a small not especially made for solar heat pump.

I’m in central WA, pretty cheap hydro power and probably not quite as cold as you, but possibly more cloudy overcast. Winter electric bills do end up kind of high, maybe 3MWh/$300 in the coldest months, but in all other ways cost of living is pretty ok. I don’t think i’m in quite the ideal location for all of the Build It Solar projects, some of the colder + clearer places can pull off a lot more impressive solar gain based projects imo.

Look at the mini splits - I don’t think it makes sense to retrofit the house, especially if you’re getting “I don’t want to do this work…” quotes. IIRC the heat pump for our house was $4k-$5k, but it was installed while they were putting the house together. There exist some DIY kits with pre-purged lines, but I’ve also heard the claim that the tools aren’t really that bad - a vacuum pump and gauge is far, far cheaper than paying someone to come out.

Define ‘small’.

1.8kw is pretty tiny, tbh. Just run it pure offgrid. Shed/aux whatever power is perfect for that amount of panel.

I mean ‘throw the power into heat’ is just about the least efficient use, but hey, it’s > nothing.

If you go pure offgrid, it’s mostly inspections-free.

1.8kw to start with, maybe one day growing to 5 or 10 kW depending on what deals show up on used and/or on local classifieds. There are a few spots that get relatively good solar exposure for my location, but I just don’t live in a place on the globe where it makes sense to go All In on solar, like in the US southwest.

If I had a shed or office that needed the power and didn’t have it, it would be perfect. but right now everything is only an extension cord away from cheap grid power.

So if I pick something, like, “I wanna run my fridge on solar”, now I have to buy an inverter, buy a battery, and probably buy an automatic transfer switch when the battery dies to go back to grid power…

I guess what it comes down to, is that I agree “fridge, lights, computers” are a more noble and fitting load for high quality electricity, and if the grid weren’t there thats what I’d use solar for. But in a hybrid situation, offsetting energy usage the savings on your electricity bill don’t care what its for. And heating a water heater SEEMED like a very good fit for this use case. But thanks mostly to the discussions on this forum I’m starting to believe i’m falling for the trap of “premature micro-optimizations”

If your goal is to offset energy, energy efficiency optimizations are a far better place to spend money. A hybrid (heat pump based) hot water heater will save you an awful lot on your water heating costs, mini splits vs your furnace for home heating, etc.

But in most areas, piecemealing together solar is more trouble than it’s worth, at least if you touch the grid at all.

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Watch out, though. Some jurisdictions requires that your dwelling be within the definitions of “temporary” and that may include how many days you actually use it, among other issues. I have known people who got into serious legal and governmental hot water over precisely these sorts of requirements - yes, it is in fact effectively, if not literally, illegal to not maintain a utility hookup in some areas – which is in many cases used to intentionally prevent or at least severely restrict off-grid or alternative-powered dwellings.

Almost certainly more “trouble”. One case where it currently doesn’t seem to be too hard to break $-wise even with off-grid is when the following factors align;

  • Personal interest/edification incentives
  • Low target steady-state usage e.g. 10 - 20 kWh/mo
  • High initial grid hookup cost e.g. $5 - $10 K
  • High monthly base charge e.g. $40 - $50 / mo

The amortized hookup cost + base monthly rate would yield an average rate of ~$3.10/kWh over 20 years at 20 kWh/mo. (Assuming a generous $0.07/kWh rate from the power provider.)

At 200 kWh/mo the average becomes much more enticing at ~$0.37/kWh over the same 20 years.

Finally, at 3 MWh/mo monthly base charge and initial hookup cost are mostly negligible over the same 20 years. e.g. ~$0.09/kWh or ~$0.12/kWh if the power provider charged an average of $0.07/kWh or $0.10/kWh respectively over the same timeframe.

At risk of duplicating content more eloquently stated at So, you wanna go off grid… and elsewhere, I’ll say buying into the grid is a bit like paying into a collective insurance pool - you get peace of mind and nice predictable costs. (Assuming the systems involved stay in a relatively steady state.) It works out in one’s favor as long as usage is average or higher. Those with anomalously low usage effectively pay a potentially excessive premium.

Obviously, your milage may vary by jurisdiction.

Speaking of solar setups, most of the parts just came in for my truck-camper one. I won’t write up as nice of a documentary as Syo, but I’ll at least show some pics.

Yeah, even off grid power systems in Idaho require going through the plans review process from hell, such that off-grid installers have started using “portable” power systems that interconnect to the house via a plug. The structure is wired with a big generator plug, which ends NEC, and then the “portable” power source (inverters, batteries, solar charge controllers, etc) isn’t required to go through the nonsense review by someone who just doesn’t like batteries.

“It’s not grid tied” doesn’t clear you from an awful lot of stuff, sadly.

It really depends on the area. At least out here, the base charge is only $5/mo or so. It’s better than going to demand charges for residential, I think…