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It's a complex calculation that cannot be well planned for as weather is unpredictable, length of power outage is also unknown.While your math is correct, I would look at things differently. The whole point is to be able to run the house off of solar. The solar power first goes to satisfy the house's instantaneous needs. Whatever's left goes into the Powerwalls. But the goal isn't to fill the Powerwalls all the way from empty to full, the idea is to fill them up enough to run the house when there's no sun on the panels, which might be less than that.
I'd start with whether the solar system can generate enough energy in a day to run the house. Then see if the Powerwalls can hold enough to run the house overnight. Extra solar or extra batteries help (in different ways) by giving you some reserve for cloudy days, unforeseen load, etc.
Bruce.
I am only looking at two power walls. My usage is around 650-800kwh per month depending on season. 20kwh would be enough to run the home for close to 24 hours during a power failure with no solar generation such as winter if panels are covered in snow.I don't think 7kw will be enough to recharge four powerwalls and provide sufficient power for your house. If you had full sun generating 7kw for 8 hours, you are just filling 56kWh of 4 powerwalls, without leaving any extra for house loads.
I just stumbled upon this thread. I may be too late to be useful, and you may have already figured this out, but here goes:I am only looking at two power walls. My usage is around 650-800kwh per month depending on season. 20kwh would be enough to run the home for close to 24 hours during a power failure with no solar generation such as winter if panels are covered in snow.
If I disconnected non essential loads and just used the power walls to run essentials such as furnace, refrigerator, internet and some lights, I could easily stretch that to a couple days.
Is this a daily cycle or only a couple of times a month? If occasionally, go for it. I am on a similar plan with Liberty Utilities in NH. Instead of buying the batteries and getting a rebate, they lease them to me at a discount. They are not daily cycling; they only discharge a couple of times a month to hit the monthly system peaks to reduce wholesale transmission costs. Their discharge events go down to 20%; the batteries recharge nightly during off peak. I did have a brief outage one night after a discharge, but it was quick (<1hr).Another interesting thing I've learned in my research is that Xcel Energy will provide a rebate that covers about 30% of the cost of power walls but in return Xcel gets the ability to control the PowerWalls ... so that during peak usage they can tell your PowerWalls to start delivering output power to feed the grid to help with load.
They won't allow your PowerWall to go below 40% when this is done but I'm not sure how I feel about it... as peak demand could also coincide with an outage occurring after your batteries have been partially discharged.
Thousands of dollars in rebates though.
Same here, had to do this except I needed 3 UPS, modem(different outlet), router, DVR. Small price for less headaches.I had to put my home router and DVR on a UPS when I got Powerwalls. Before the Powerwalls they would just boot up after a power was restored after an outage. When I got the Powerwalls the brief power interruption during switchover wasn't enough to trigger a reboot but would put them in limbo. The DVR would lockup and the router would stop routing. I had to manually reboot them to get them to start working again. Was frustrating to come back from vacation to find my shows weren't recording.
4 UPS units for us. My systems, wife's systems, AV rack with security recorders and NAS, Comm rack with 32 ports, cable modem, PIs and other SBCs, etc.Same here, had to do this except I needed 3 UPS, modem(different outlet), router, DVR. Small price for less headaches.
Yep, wife's happiness is very important.4 UPS units for us. My systems, wife's systems, AV rack with security recorders and NAS, Comm rack with 32 ports, cable modem, PIs and other SBCs, etc.
The UPSes are all the same, 1500VA/1000W. They react very quickly, and the Powerwalls takeover almost instantly. Usually, the only way we know that we loss power is the click from the UPS or message on the app. As far as the AV rack goes, it is getting more and more sparse as we shutoff dead/unused electronics like Tivo, DVD player, gaming systems, etc. With everything online no need for a lot of that.Yep, wife's happiness is very important.
Are they all the same VA rated? That rack may need a good bit for instant transfer and maybe for a bit longer usage while transferring.
12-24V DC buck/boost systems are pretty common. They are used commonly to run military or large truck electronics on normal 12V vehicles, and vice versa.Folks may want to start looking into portable power stations going forward for many use cases - technically most are EPS, as their switchover time can be greater than 20-50ms so not technically UPS. Won't handle the switchover quickly for a desktop computer, but works great for router/modem, switches, and other things. I ran a UPS for router/modem for years, but tired of them dying every 2-5 years (and of course the replacement batteries are usually as expensive as a new UPS), now have it on a power station.
Mostly being LFP these days, the power station batteries will last way longer than lead-acid UPS technology. Slightly downside that they might have slightly more power consumption from keeping an AC inverter powered (still only like10 watts or so), even in passthrough mode, compared to a UPS - but for my modem/router setup, I avoided that by removing their wall warts (saving even more conversion inefficiencies) and using common adapter cables to power directly from 12V DC. Now in the process of seeing if I can switch over my aquarium pumps too - they have 24V AC adapters. But just purchased a USB-C PD dongle that outputs 18-20V DC from USB-C, will see if the pump can run off it. Hopefully it works, esp as efficient, variable-flow aquarium pump like mine are actually DC-based pumps.
But that's just nitty-gritty - the portable power stations work just fine as EPS powering equipment from the standard AC outlets as well.
Ah, good to know, thanks!12-24V DC buck/boost systems are pretty common. They are used commonly to run military or large truck electronics on normal 12V vehicles, and vice versa.
Prices for the 12V to 24V step up versions start at about $10 and go up.
There are other sources for the UPS batteries, and if you don't sweat the exact battery model, often a few Ah up or down may be much cheaper, but also may, or may not, fit in the box, and might need a few inches of adapter cables.Ah, good to know, thanks!
I did have a chance to try the $7 USB-C dongle though, and interestingly it works! It looks like a short USB-C adapter cable to 5521 plug common on many 12 and 24V AC power supplies, so it plugs right into the controllers for the aquarium pumps. But since USB-C power supplies need some signaling on what voltage and amps to supply, apparently there's a bit of circuitry in the dongle that specifically signals USB-C PD 18-20V (meant really for laptops and some high-end phones).
I used a compact USB-C PD 65W wall wart for testing, and was able to power not just one, but simultaneously two separate pumps together drawing 35-40 watts. I guess their bulky AC adapters were fairly efficient though, using a watt meter only saw 0.5W less consumption from the wall compared to the USB-C wall wart. Of course the long-term plan is to use the portable battery station in DC mode to provide the USB-C PD power/backup.
Speaking of AV backup, I do have one of those big expensive APC AV Power Conditioners / UPS units that protected my home theater equipment. It has also been sitting in a closet for 10 years, after the internal batteries died within 2 years, and just wasn't willing to shell out big $$ for the two APC brand battery replacements every couple of years.
Regarding the UPS batteries, I have Cyberpower UPSes, and as you experienced their replacements are expensive. But I checked on Amazon and found generic batteries for a fraction of the costs. Cyberpower wanted something like $150 for the one on a $250 UPS. I found a 3rd party one for around 1/4 the cost. It has been working for over a year.Ah, good to know, thanks!
I did have a chance to try the $7 USB-C dongle though, and interestingly it works! It looks like a short USB-C adapter cable to 5521 plug common on many 12 and 24V AC power supplies, so it plugs right into the controllers for the aquarium pumps. But since USB-C power supplies need some signaling on what voltage and amps to supply, apparently there's a bit of circuitry in the dongle that specifically signals USB-C PD 18-20V (meant really for laptops and some high-end phones).
I used a compact USB-C PD 65W wall wart for testing, and was able to power not just one, but simultaneously two separate pumps together drawing 35-40 watts. I guess their bulky AC adapters were fairly efficient though, using a watt meter only saw 0.5W less consumption from the wall compared to the USB-C wall wart. Of course the long-term plan is to use the portable battery station in DC mode to provide the USB-C PD power/backup.
Speaking of AV backup, I do have one of those big expensive APC AV Power Conditioners / UPS units that protected my home theater equipment. It has also been sitting in a closet for 10 years, after the internal batteries died within 2 years, and just wasn't willing to shell out big $$ for the two APC brand battery replacements every couple of years.