Backup Power with Grid-Tied Solar
We built a all electric grid tied solar home on semi forested fire interface land in western Canada. With last years heat dome and subsequent catastrophic fire season only to be followed by an atmospheric river event that destroyed key infrastructure, I am actively looking at back up power options to keep my well pump going for fire protection and other critical seasonal loads. I have a 240v gas generator and have it interconnected to energize key circuits (ie well) but have been looking at inverters/LiFePo battery storage options to trick my grid tied solar into providing power when the grid goes down (and disconnect from utility power). Any suggestions out there on what options I should consider?
Thank you.
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The SolArk 12K and soon to be released 15K are a good option if you're looking to DIY the install. You can pair the SolArk with any 48 Volt battery chemistry. I'm not sure that it would be an easy task to 'trick' your AC coupled grid-tie system into providing power during an outage. Does your system utilize microinverters?
I love the solark. The 15k has a 200A grid feed (that is an auto transfer switch if the grid goes down). If you have DC coupled solar, just connect that to the MPPTs build in. Then you connect your 240v generator to the ‘generator’ input and the solark will fire up the generator if your batts get low… if instead, you have micro inverters, you can connect those to the ‘generator’ input and the solark will manage those to charge your batteries. Its a pretty flexible and powerful setup.
There's a variety of solutions for this, so there's not a clear "right answer", but it can definitely be done. There are a couple key things that need to be addressed:
-From a code compliance and safety standpoint, you need a way to isolate your house from the rest of the grid, interlocked with your inverter so that it won't attempt islanded operation unless it's definitely disconnected from the grid -- I believe they call this a Microgrid Interconnect Device (MID). You already have a generator, so I assume you're familiar with transfer switches -- this is basically the same deal, except that (unlike a generator) your solar inverter will always be connected, so it needs to know the state of the transfer switch before enabling off-grid mode.
-With existing grid-tie inverters, they generally assume that the grid is a limitless energy sink, and just push out all the power they can, in sync with the grid. This means that you need an inverter that can (a) provide synthetic inertia / "grid forming", and (b) absorb up to the maximum output of the grid-tie inverters (or a way to shut them down). This is all "just" a controls problem though.
At one end of the spectrum, there are commercial systems that are becoming more available that do all this -- Enphase just launched their MID and IQ8 series inverters, which work with their AC-coupled batteries and can also interconnect with a generator -- the MID monitors all the power flows and controls everything. Pika Energy (now Generac I think) has a DC coupled system that IMO is technically superior, but would be a ground-up redo. There are likely others, but I haven't looking into it for a couple years. At the other end of the spectrum, you can piece together your own system -- there are inverters like the SMA Sunny Island that can perform some combination of {transfer switch, inverter, battery charger, generator interconnect, etc.}, combined with various other off-the-shelf parts.
I suppose I haven't answered your question, other than to say it's definitely possible -- it's just not straightforward to say which solution is best for you. That depends on what equipment you already have, your comfort level with setting up and monitoring this stuff, and local code / utility requirements.
Enphase’ s new system finally allows their batteries to be charged from generators. But right now only permanently installed backup generators are allowed. Most portable generators produce power that has too much distortion for electronics to tolerate. Portable inverter generators are an exception to this - possibly they will be allowed to charge Enphase batteries at some point. Tesla hasn’t shown any interest in charging the Powerwall battery from anything but their own solar panels. You can’t even buy a Powerwall by itself anymore.
Most solar manufacturers offer a setup that will work. I would check your specific one first, you are searching for AC coupling.
You need a couple of key items to make this work.
The setup needs a transfer switch to disconnect the backed up load from the grid, this is sometimes built into the inverter itself (ie Schneider XW, Outback Radian) or an external one (Enphase IQ). Since off grid battery power is expensive, only backup what you must.
The inverter also needs to do frequency shifting while off grid to control battery charging. When off grid and AC coupled, the PV inverters can only dump power into loads or the battery (through the inverter/charger), so the inverter/charger needs a way to control this to avoid overcharging the battery.
You also need PV inverters that can respond to this frequency shift to curtail the PV. Some units just go off grid instead of gradual power taper which you don't want.
The battery charger/inverter system also needs to be bigger than your AC PV, so if you have a very big array it might make sense to only connect up part of your PV array to the backed up panel instead of getting a very big inverter/charger.
Setting this up sounds simple, but all the ones I've done have involved a lot of RTFM, lot of tweaking settings and a lot of swearing. I don't know if your average PV installer would be up for this.
Today storing electricity is expensive the capacity of the storage devices is small in terms of kilowatt hours and the devices tend to have relatively short life expectancy.
If the main concern is having water when the power happens to be down the smart move maybe to build an underground cistern in ground higher than the home and keep it pumped full when you do have power. Then you can use gravity when the power is down. Seem likely to cost less last longer and be more reliable.
Walta
True, but nowhere near as cool as playing with some solar battery backup.
I must point you in the direction of this BS* + Beer Show episode: Solar Power Storage and Backup. I’m certain it will yield ideas for your situation.
The main thing to remember is that there are only a few circuits that cannot be backed up with solar and batteries in summer. A range/oven and heat pumps draw too much energy to be economically backed up. One has to accept that. In winter it would probably be impossible to back them up with solar and batteries all the time with all the money in the world. In summer everything else is probably doable, even electric hot water heaters, and of course that water pump.
The other thing to consider is whether you really want or need to automate changing over to backup power. If the transfer switches are located outside the conditioned space, it will discourage you from actively managing the backup system. It's best to have the transfer switches located where you can conveniently select what circuits your backup power is being applied to. Then you won't feel the need to automate the switching. In addition, as Akos hinted at, it is really fun to manually use solar backup power and apply it selectively every day, even when you have grid power.
Using my own backup power as an example, I've got a transfer switch unit located in my kitchen that has 4 single phase transfer switches that run, and are connected to, 4 separate circuits. One circuit goes to ventilation including ceiling fans, ERV and all lighting. Another goes to the refrigerator. A third one goes to the microwave. And the last one goes to one outlet circuit in the kitchen. This is for making toast, coffee etc. I have most of my basic needs covered in an emergency. Hot water is the only one I don't have but that is just because I didn't plan well enough and not because it can't be done. I'd need to upsize the generation and storage for that.
The system is wired so just one of my 14 400-watt solar panels on the roof is wired through a charge controller to the battery. The 4 transfer switches are in turn selectively fed from either the battery and then the 120v inverter or from the grid. That one solar panel is not grid connected but the other 13 are. Those 4 backed up circuits are fed either "from" the grid or from the battery. They never feed the grid. If I had it to do over again, I would double the battery capacity and move 1 more panel off the grid so that I could use it for water heating.
The best reason for switching backup circuits manually, (besides being way more fun) is that you can get maximum use of your solar generation and battery even when you are connected to the grid. Conversely if you transferred power automatically only when the grid went down you would be wasting the power of your backup PV panels (and battery) that are dedicated to backup power. It would only be used when the grid went down.
Thank you all for your insights. I am running a 7kw array with APSystems microinverters, net metered and whatever I do going forward needs to be approved. We are within 10% of being net-zero. From what I observed last year, if we experience a catastrophic event then so be it we move on but the majority of communities impacted here were without utilities for weeks either due to fires or flood damage waiting for systems to be restored (reminiscent of the Texas experience). Other critical electrical loads are freezer/refrigerator and a septic sewage pump. Forget about running our GSHP, water heater, stove etc. Our house is high performance with wood heat back up, well is a soft start VFD unit so many or the key pieces are in place for resiliency. What I am leaning to is leaving the current system unmolested and add another solar array the use an all in one grid tied inverter and battery for when the power is out. In winter when panel production is low then run the generator to charge the batteries. Configuration, sizing and cost will necessitate using pro's, just want the expertise from this group in my decision making process. Thank you again and look forward to any further comments.
A few years ago, after a nearly weeklong outage, we looked into battery backup.. It was too expensive. But another big negative was that the batteries couldn't be charged with a generator. This was true of a Tesla Power wall 2 as well as another brand. So make sure you can charge the battery with a generator.
We ended up getting a 7000 watt Honda gasoline generator that can be plugged into a receptacle that is wired to the electrical panel. If power goes out, we need to shut off the main breaker, which moves a mechanical plate that then allows a 30 amp breaker to be turned on and be energized by the generator.
The generator runs our minisplits, induction cooktop, well pump and the usual other stuff. It'll run our 4500 watt water heater if we don't have any other big power users on.
I would definitely suggest looking into the SolArk 12K. It is capable of AC coupling up to 9.6kW of microinverters.
If you rewire your PV to go dc-coupled, you could use a solark to be grid connected, with auto cutover to batts if grid goes down and have the solark fire up the generator only when batts run low…. The solark is designed specifically to handle all this!
You generally don't want dc-coupled PV for a mostly net metered setup. DC coupled setups have an MPPT to the DC bus than an inverter to go from DC to AC for the grid. Because of this extra power conversion, you have more or less an extra 5% conversion loss.
DC coupled works better for off grid or zero export setup, even there, a mix of ac and dc coupled is your most efficient (and lowest cost) option.
I took a quick look through the Solark and it is pretty much an inverter charger with built in MPPTs. Since I haven't worked with it, I can't comment on how easy or hard it is to set up, but definitely not simple. Like any of these types of setups, it is not a box with two wires that you connect to your breaker panel.
I also couldn't find if the SolArk inverter supports frequency shift. Without this you can't AC couple PV during backup. The idea to connect the AC PV to the generator input won't fly as there is no grid there for the PV inverters to sync to thus they won't produce any power. Even if the generator is on, almost no off the shelf inverter will sync to it as the frequency and voltage is not stable enough.
I was recommending dc-coupling for two reasons:
1. On the Sol-Ark 12k or 15k, if you have ac-coupled panels, those need to be connected via the "GEN" input so it can control the ac-coupled panels in a grid-down situation (and yes, it does this via frequency shift).
2. If you dc-couple instead, you free up the "GEN" input for the generator input... and the Sol-Ark can do a remote call to start the generator, then can even "limit the load on gen" based on settings so you don't overload even a smaller generator.
So my recommendation for DC Coupling was less to do about the absolute efficiency of solar-to-AC and more about allowing the overall system to work with batteries + generator + solar, all together via the Sol-Ark hybrid inverter.
Hopefully this helps to understand my thought process, just looking at it as an overall system based on the original request.
Depends on what you want or need to run. If you need to get your system approved, do that first, then think about adding on a small off grid set up. I agree with the others to keep it separate and manually operated. If something automatic fails there could be some damage done. Adding a small sub-panel for those circuits you would like to operate can be considered, this way a flip of a couple switches isolates it from the main panel and operates what you need and ensures that there will be no "feedback" when power is restored causing potential problems
Once you figure out what you want to run, how long you think you will run it and how much power you need, then you can decide on what type of batteries and inverter you may need. You can dedicate a few outlets for lights/lamps around the house or other necessities such as pumps or your boiler for heat depending upon their loads. Some electronics such as newer tv's and such may require a true sine wave inverter.
As far as batteries, you can go all high tech or just use some deep cycle batteries such as marine type or even fork lift types(a bit more expensive), especially if they are only to be used sparingly. Use several batteries, in series/ parallel, depending on your inverter input, 12/24/48V. Keeping it 24V or less allows you to buy off the shelf components, breakers etc. A proper charge controller for the batteries can be used with either your PV, a couple dedicated PV panels, or regular house power to keep them charged.
Once set up, you can use it year round to keep the system operational, cycling the batteries, and save a few bucks.
>" Keeping it 24V or less allows you to buy off the shelf components, breakers etc."
48v is pretty standard too. You get higher efficiency with higher voltages, so you generally want to use the highest voltage you can in this case. There are lots of switches and brakers available that are rated to 60v DC, and that's because the telecom industry has used a 48v nominal DC plant voltage for over a century. The "48v" plant runs float voltage in normal operation, which is around 54.4v or so and varies a bit depending on the battery chemistry in use (which is usually some flavor of lead acid batteries, just very, very large ones). Airpax is a common manufacturer of these DC rated breakers, and they are readily available in ratings up to 100A. For higher amperages, you're better off using DC rated fuses.
A good middle range battery is the "front access terminal" type batteries made by companies like C and D. I've used those many times commercially. It's probably going to be easiest to setup the solar system to be able to be switched over to "backup power mode" instead of trying to do it all with net metering. If you do that, do be sure that the batteries are kept at float voltage so that they don't degrade too much. If you leave the batteries idle too long, they will slowly deteriorate.
Bill