Well, since I was mentioned by name...
Relevant background reading here (my blog, trying to offer a good amount of required background for discussions like this):
https://syonyk.blogspot.com/2018/05/why-typical-home-solar-setup-does-not-work-off-grid.htmlAnd, also related,
https://syonyk.blogspot.com/2018/05/so-you-wanna-go-off-grid.htmlThe main point you have to understand from the first link is how solar panels behave - constant current, up to a point defined by the normal illumination, then the voltage collapses. Without storage, you can either run them far below maximum power, or you don't have a stable system - and very few devices like their power supply collapsing under them without warning, then oscillating back up to full voltage.
What I'd really like is a small system that can automatically make good use of solar power, without tying it to a grid. This could be used for renters, for example, as the system could be disassembled and moved.
Such things don't really exist, because they're staggeringly expensive for the small amounts of useful energy they produce.
Even full off grid power systems, like I have, aren't cost effective compared to grid power. If I get lifetime per-kWh costs on my system down to $0.25/kWh, I'm doing quite well - and grid power out here is $0.10/kWh. I think I'm sub-$1/kWh now, based on power delivered, but that's after 4 years of operation. They're not cost effective for a variety of reasons, though panel costs have come down a good bit in the last few years. It still doesn't really change the economics of the system that much, because panels are an increasingly small fraction of the cost of a standalone system.
This led me to considering a supplemental HVAC system. A few solar panels (~1kW) should be sufficient to power a small DC window unit or split heat pump.
"Should be" is not a particularly strong engineering statement. Panel output is rated at STC conditions - which are a set of conditions that are very useful for testing and grading cells in a manufacturing facility, and have roughly no chance of being met in most actual conditions. So that "1kW" of nameplate will produce less in any realistic condition, and far, far less in winter conditions.
I have 2280W of panels hung on my main office array. I rarely see anything past about 1600W input, though this is partly because they're not aimed ideally at the moment. In the winter, on a bad day, I'll see less than 75W - peak.
Depending on the unit, you also have to handle the startup surge. My window unit (which is a non-inverter type) pulls about 700W running, but pulls close to 2kW to start. If you can't meet that startup surge, you can't run the unit. My small backup generator would be able to run the unit, but it can't start the unit, so I can't run the unit on generator. Not a big deal, but it's something to consider. I don't actually know the momentary peak draw, but it's significant.
From my research, the two largest problems with this idea are automation and cost. First, automation: I don't know at what point programmability enters such a system. Can a charge controller be programmed to start a heat pump once the output from the solar panels reaches a certain minimum, and turn off once the output falls below a certain minimum? Are there heat pumps that can change heating/cooking capacity based on received power? Is there another component that can be added to the system to accomplish this goal?
Unless you're loading the panels up, there's no way to tell what the current available power is. The way you do this is to load them up, monitor the voltage, and see when it starts to drop - a MPPT sweep is a common term for this scan. But you can't just look at the voltage and guess. You might be able to put a normal light sensor near the panels, model them, track panel temperature, and guestimate that way, but that's a pretty custom setup - there's nothing off the shelf I'm aware of that will do that.
Then, to make use of it efficiently, you'd need to be able to vary the compressor power consumption, rather rapidly, or you'll collapse the voltage if a plane flies over and shades a panel. It's not impossible, but it's also more of a "custom engineering" project than an off the shelf unit.
Second, there is the issue of cost. One off the shelf package that I've seen that would accomplish what I'm thinking of can be seen at https://www.hotspotenergy.com/DC-air-conditioner/DC-AC-Complete-Systems.php for about $4k all-in. At ~$4.50 per watt, this is expensive, but half the cost is a highly efficient DC heat pump.
A significant chunk of the cost is battery, and those batteries, used heavily, are unlikely to last more than 3-4 years.
I'm not going to rip their numbers apart, but I'll simply observe that I think they're a load of trash, and I wouldn't believe them for a second. They're the sort of numbers put together by people who don't actually have any hands on experience with year round off grid operation, do a tiny bit of research, and then throw together numbers that look decent. They don't reflect any sane reality.
Amount of energy saved in cooling months:
5 months * 30 days/month * 4.5 hours/day * 500W * (20 / 12) efficiency gain = 562 kW*hr of electricity saved by using solar
Amount of energy saved in heating months:
5 months * 30 days/month * 4.5 hours/day * 500W = 338 kW*hr of electricity saved by using solar
Total = 900 kW*hr per year
You have clear skies, all winter long, and are going to be aiming your panels to track the sun every day?
I guarantee your panels won't produce nearly the power you think they will in actual conditions. Minor cloud cover impacts energy produced quite a bit (which is why I'm pretty badly overpaneled - to handle cloudy days, hazy days, etc), and winter conditions vary, but on a bad winter day, you'll produce absolutely nothing. My office system can produce (really, consume - I'm consumption limited most of the time) 10kWh a day in halfway decent conditions, but on a bad winter day, I'll be lucky to get 0.3kWh out of the sky. It's not a big deal as I've got the backup generator, but if you assume full rated panel production all year round, you're going to be very, very disappointed. Cut your production numbers in half, and you
might get close to annual production, though PVWatts would give you a more accurate estimate given your site.
Is this why it was so difficult to find DC powered heat pumps?
They're a weird, niche little product that doesn't have a good reason for existing, so... yeah, they're expensive. I expect a lot of the market is actually to the RV folks, because they seem to largely not care about cost.
Inverters got good about a decade ago. It's now far cheaper and easier to have a large inverter and use AC loads. As soon as anything requires the inverter, you're paying the idle penalty (somewhere in the range of 30W on my 2kW continuous/6kW peak unit), so may as well use the cheap stuff designed for grid power, and save money on wiring. DC systems just don't make a lot of sense for any realistic use case.
If such a system were actually implemented in an owner-occupied home, the excess power from the panels could be diverted to hot water heating to make this a far more economical system. As is, if the system lasted 20 years, with one change of batteries, I calculate the total cost to be about $0.27 per kW*hr – economical in high power locations, but two to three times the price in low-price electricity locations.
Yeah, that sounds sane. Plus or minus some, but you're certainly in the right ballpark.
...as well as suggestions to improve the economics of the system.
It's not possible with current technology. You're missing a bit, but the stuff you're missing would just drive the system costs up. The analysis you've done is mostly correct - it simply doesn't make any financial sense to do it. Which, I'd add, is why what you're looking for doesn't really seem to exist.
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4) and also don't want to be official and get a interconnection agreement with the utility. So taking all that in I suggest look at grid-tie inverters with Zero-export, or also called grid-limiter. it will act like a grid tie inverter but will not create a out-flowing current as seen by your meter, or that's the idea. it has a current clamp that goes in the breaker panel and will provide as much power as it can up to the point that it exceeds the usage at a given moment, I've not used one so I cannot say how well that works. they make a allegedly 1000w that has plenty of YouTube videos.
In most areas, you can't legally do that. If you've got a power source on the home wiring, even if it's "zero export," you still have to meet NEC regulations, and almost certainly still have to have an agreement with the power company - because the system
could backfeed the grid. Even if it's not designed to, if the sensors malfunction, it could backfeed, so they get to know about it.
YouTube videos on solar are collectively godawful advice, mostly put together by morons, and frequently involve non-UL listed equipment that cannot be interconnected to any sane grid legally.
The nicest thing I'll say is that 90% of what YouTubers say is fine, 10% is incredibly hazardous advice, and it takes an awful lot of experience to tell the difference. Some of the "slick" well produced videos have some of the worst advice. But anything found on YouTube can safely be assumed to be somewhere between wrong and actively hazardous advice.
Also, most landlords don't allow renters to make modifications to the electrical system or structure anyway.
...like the 900 cycles or so of lead, which would kill your payback.
Haven't been near a modern solar use lead acid datasheet in a while, have you? Any decent pack will last far, far longer.
There's also the seasonal aspect - in the summer, with properly sized panels, you basically don't use the battery bank most of the day. My office can be into float by 10AM during the summer, and I actually don't even run the absorb cycle every day in the summer to be easier on the batteries. In the winter, you need the batteries more, so having the extra capacity is nice (though lead does have somewhat less capacity in the cold - on the flip side, you don't have to keep them warm like you do with lithium).
But I don't really want to head off into the weeds of batteries. You're not wrong, but you make the usual set of claims that turn out to not matter as much as people like to make them out matter in off grid systems. None of it changes the economics of the system much either way, and I'd rather not have a conversation with insurance adjusters if a home built lithium pack burns a house down. There's a reason my office is separated from the house, and the fact that I do lithium work in there is a major factor.
You're right about automation being a DIY project for this sort of thing, though.
It looks like a AC unit plus inverter is more cost-effective. A 12,000 BTU window heat pump can be found for ~$750, and I found an Energizer 2000W power inverter for $200. The AC heat pump turns out to be just as efficient as the DC unit. That saves $1k on the system compared to what I wrote above.
Correct. And you still need batteries either way. And more panels.
But I still doubt you'll make the math work to save you money on this project.