Recommendations to grid-tie a small off grid system (with backup)

[josepr]

Greetings,

I have an off-grid system that was installed during an emergency. It’s a small system 24v (375ah) 2,480w PV and a 3kw inverter. I would like to grid connect the PV array (net metering). Currently considering a small string inverter 3kw (can I install a 5kw and use 1 or 2 string and expand later?) or micro inverters. I also would like to have battery backup (but not 100% sure).

If needed (because of hurricanes) the system will be off grid for months. Because of that I think an AC couple system is not ideal in these situations. Also, what I don’t like about AC couple is that (even with frequency shifting) an AC or DC dump load is highly recommended. That’s the reason why I partially in favor of a DC couple system.

I understand that for DC couple I need to buy a Hybrid Inverter. I would like to buy a dependable inverter (48v) and batteries. Most pro recommends AGMs or Lithium. What inverter and battery would you recommend (and why)? I you are an owner of a hybrid system, are you happy with it? Do you think it was worth the additional cost? Are they quiet (and do not have fans like most string and micro inverters)? Thank you very much.

PD: I have several, maybe unfounded, concerns. Some off-grid systems, mainly wind turbine, use dump loads (relays, not PWM regulated). I have heard that it is not recommended because you are basically micro cycling your batteries. My concern is that hybrid inverters use the excess voltage (production) to sell. Does that expose the batteries to micro cycles or to under/over charge situations (or voltages over absorption – AGM)? How the system behaves with lithium batteries? The charge controller needs to maintain the production (float voltage) for the inverter to sell. But as I understand, you are not supposed to float lithium. I know that lithium is compatible (according to the manufacturers). But it is recommended?

[Mike90250]

Stick with flooded lead acid. Li batteries are still not ready for prime time.

Dump loads are only for wind or water turbines. Solar MPPT controllers do not use a dump load, they simply harvest only what they need. The inverter and controller need to be the same brand to simplify data communication and status control. (and of course rated for hybrid sell back use)

[josepr]

Thank you, Mike. I have a lot of questions.

The first one is kinda personal, about Inverters. I know your inverter is a hybrid. Do you recommend it? If it suddenly fails due to natural causes (lightning strike, etc.). Would you buy it again? If not, what would you buy?

The second is about batteries. But I’m not sure if I should ask here so I’ll create a new thread it in the Batteries forum. Thank you.

[Mike90250]

I would highly recommend / endorse the Xantrex/Schneider XW series inverters, Mine ran a contractors 3hp floor sander for 3 days, before I figured out why the batteries were getting lower and lower during the day.. Contractor had to rent 2 different trailer generators to take the load off my system (when they were building the house).
In summer, it runs a 1,000 watt 240V pump for 4-5 hours a day for irrigation . Operation has been flawless . Generator support has been flawless. Firmware updates executed flawlessly,
It's been running 24/7 since mid 2010, with only planned outages, no surprise downtime. Eventually, it will cease operation and I will replace it with the latest XW model, likely the Pro series.

Batteries, plan on murdering your first set due to "unplanned events" Nearly always, during system shakedown, something happens and you find you under sized something to save money, and so you get to spend money twice. Unless you are a experienced power system engineer, it's going to happen. Feeling good does not make physics work, only natural laws.

[Ampster]

Stick with flooded lead acid. Li batteries are still not ready for prime time.....

I guess it depends on how you define prime time. Lithium batteries are the battery of choice of every EV manufacturer that I know of. They are also used in all the large grid support battery systems. California's self Generation Incentive Program does not list any Lead Acid battery systems as eligible for that incentive.

However Lead Acid batteries continue to be the battery of choice in the small niche of off grid users. They are a good starting point since Lithium prices are dropping and starting to be packaged into easy to use pack . After you go through your first set of lead Acid batteries there will be more affordable choices for this niche.

[SunEagle]

I guess it depends on how you define prime time. Lithium batteries are the battery of choice of every EV manufacturer that I know of. They are also used in all the large grid support battery systems. California's self Generation Incentive Program does not list any Lead Acid battery systems as eligible for that incentive.

However Lead Acid batteries continue to be the battery of choice in the small niche of off grid users. They are a good starting point since Lithium prices are dropping and starting to be packaged into easy to use pack . After you go through your first set of lead Acid batteries there will be more affordable choices for this niche.

My guess is that Li chemistry batteries will be the best choice for home owners once they come down in price to make them both affordable and economically sensible.

As long as the price of any battery ends up costing more than what you can purchase a kWh from your POCO (at the Tier level you do not want to use) then I would say don't go for it.

By the way IMO the cost of the POCO's Li systems are probably being transferred to their customers so they don't care how expensive it is.

The same goes for EV's. Except for a handful of states an EV is still not the desired transportation device so they can't be used as a home energy storage system for the majority of the US

[Ampster]

.....
By the way IMO the cost of the POCO's Li systems are probably being transferred to their customers so they don't care how expensive it is.

Except in places like California where the generation of electricity is deregulated. There are contracts in the marketplace that are competitive with natural gas generation. I recently read that the city of Los Angeles recently contracted for electricity from a solar farm in Nevada that included battery storage. I don't know the details but presumably the battery storage allowed that facility to provide power beyond the normal daylight hours of solar generation.

[Mike90250]

When joe DIY homeowner wants to replace his Flooded batteries or trying to set up a new DIY system, beginning with Li will lead to (shall we say) Unhappiness. He has no idea how to read the specs for the charge controller, or the battery or the LVD for the inverter. (what the heck is a LVD anyway ?) While the Li batteries claim to be drop in replacement, I really doubt it because Mr DIY is going for the cheap one, w/out the BMS. I don't need no BMS. What's that smell ? Why is the closet I put the batteries in smoking OMG !!! In my opinion Josepr is not ready to install expensive Li batteries , so I recommend against it. In a year when he knows his loads and how his solar handles recharge, AND the battery tech has improved with more integrated electronics and more charge controller have Li settings, it might be feasible. just because you know the dangers and workarounds, does not mean Mr DIY is going to get it right on the first time, Monthly, we still have neophytes post here with destroyed LA battery banks. LI just has no forgiveness.

[SunEagle]

Except in places like California where the generation of electricity is deregulated. There are contracts in the marketplace that are competitive with natural gas generation. I recently read that the city of Los Angeles recently contracted for electricity from a solar farm in Nevada that included battery storage. I don't know the details but presumably the battery storage allowed that facility to provide power beyond the normal daylight hours of solar generation.

And that may be one way to keep the price down. But again my question is why are the rates so high in CA? And are they going down with the addition of batteries?

[Mike90250]

Rates are high in Kalifornia because baboons are running the state. they have really no idea what they are doing, but another Bond Measure on the ballot and more gas taxes will fix that.

The PUC (pub utility commission) is all appointed political and industry cronies. They can't keep the power lines maintained, so they are now turning the power off in high winds so they won't start another huge forest fire. And we can tax the power people use. And now city's are able ban new installs of natural gas appliances. Electric ranges here we come.

[Ampster]

And that may be one way to keep the price down. But again my question is why are the rates so high in CA? And are they going down with the addition of batteries?

This is getting off topic but I will give you my opinion for the benefit of those lurkers that would like one opinion on the subject.
First off, about 20 years ago we had a flawed deregulation effort that was gamed by Enron. Even though Enron failed the liabilities remained and to save the Investor Owned Utilities, the State of California floated bonds to pay off those liabilities and that debt service appears on the distribution and transmission part of our bills.. In no particular order we also have the costs of decommissioning some nuclear, expensive costs of early renewable energy contracts to meet the States RPS standards and a dysfunctional Public Utilities Commission. I am sure there are other factors that have also contributed.
I agree with Mike's assessment and I am not optimistic about the future either. My philosophy is to overbuild my solar and self consume as much of that as possible. I am not a big fan of natural gas either and have lived in an all electric home in the past. My other long term goal is to get rid of natural gas and replace my FAU with a heat pump and my stove with an induction cook top. I have already replaced my gas water heater with a heat pump. I can't live off the grid like Mike but I can sure try to be as independent of of my electric and natural gas supplier as economically feasible.
Fortunately high gasoline taxes don't affect me because I drive EVs but my registration did go up in lieu of a gas tax. The more I drive the less that costs me per mile.

[Ampster]

When joe DIY homeowner wants to replace his Flooded batteries or trying to set up a new DIY system, beginning with Li will lead to (shall we say) Unhappiness. ..........

I agree with you, that if josephr thinks he is not ready for the complexities of Lithium chemistry then he should start with Lead Acid. There are risks with any battery chemistry. I think the important thing for any DIYer is to understand the risks and pick a chemistry that works for them.

If he or any lurkers are curious about the benefits if Lithium chemistry for storage there are some good stickies. Unfortunately those authors don't post enough for there to be a meaningful dialogue. There is a DIY electric car forum and an electric bike forum where there is a large body of knowledge. Both those niches have abandoned Lead Acid for Lithium chemistry.

[SunEagle]

I agree with you, that if josephr thinks he is not ready for the complexities of Lithium chemistry then he should start with Lead Acid. There are risks with any battery chemistry. I think the important thing for any DIYer is to understand the risks and pick a chemistry that works for them.

If he or any lurkers are curious about the benefits if Lithium chemistry for storage there are some good stickies. Unfortunately those authors don't post enough for there to be a meaningful dialogue. There is a DIY electric car forum and an electric bike forum where there is a large body of knowledge. Both those niches have abandoned Lead Acid for Lithium chemistry.

+1. I agree with both you and Mike. I just hope batteries get less costly and easier to use by joe puplic.

[Ampster]

+1. I agree with both you and Mike. I just hope batteries get less costly and easier to use by joe puplic.

Yes, cost is my biggest issue with the likes if SimploPhi and others like them. With their built in BMS they can be paralleled or put in series in almost any configuration. At $1000 per kWhr they are expensive.

[josepr]

Thank you guys for your responses. I have read multiple articles, trade journals and forums post (thank you Sunking). I currently have an off grid system (for backup). Considering upgrading to hybrid or AC coupled for net metering to put the solar panels to a good use. I know that, because of the batteries, I will not recoup the costs (even with net metering). But I hate generators (because of several reasons ,mostly because of the sound) {but understand they are necessary}. I'm waiting (maybe in vain) for new technologies (alternatives), like propane (not hydrogen) fuel cells.

The most important fact about my questions is that the batteries will be used on a backup system (will be 24/7 floating, unless a disaster strikes). That's the reason I limited my choices to AGM and lithium. For these applications, if the batteries are flooded, the best lead acid 'chemistry' is lead-calcium (telecom batteries). Unfortunately, those batteries, at least where I live, are very expensive. The other lead acid 'chemistry' is lead-antimony (deep cycle golf cart, L16, forklift, etc.). Those are readily available. But perform poorly in float applications (die early, would be best to cycle them to 10-15% daily {not an option}). That's the reason why some pro installers recommend AGMs. Not only because of the maintenance. But because they are usually lead-calcium (like telecom they like to float).

These are my doubts/questions right now. If you recommend a regular deep cycle flooded battery (lead-antimony GC, L16), what maintenance steps should I take to extend their life (prevent stratification, recover capacity lost, not because of sulphation, but because it has been floating for too long). If I should cycle them periodically, how often?

I included lithium because of the recent marketing surge. But honestly, I don't think is the right battery for the application. For off grid, maybe, honestly I don’t know, time will tell. I do not remember the web site, but a couple of months ago I read a master’s or PhD thesis about the loss of life and capacity of lithium batteries in float applications (the batteries lost more than 20% of its capacity in 5 years). Also, several, if not most, lithium battery 'manufacturer' recommends not to float charge them. If that is true, why do people use lithium in residential backup applications? I could understand the needs of an industrial or an energy company. They usually need a lot of energy in a moment notice until the generators are online. They take significatively longer (in case of utilities hours) because they must sync with current production. But that is not the case of a residential application. Time will tell, but in my opinion a lead-calcium lead-acid battery (flooded or sealed) will outperform them (in capacity if flooded/sealed and in calendar life if flooded). For regular off-grid use they are an alternative. Unfortunately, most charge controller are not 100% lithium compatible. Most charge controllers will get damaged if you connect the PV before the batteries. Precisely that is what happens if you discharge them or overcharge them. The BMS will turn them off and bye bye CC.

Please note I'm not an expert. I'm just a guy that is interested in the topic. I've only read a little about the matter. And I would like to know your opinions and suggestions. Specially the opinions of pros and users (with experience) that work every day with batteries.

Also, as a note, current I'm considering a third option. That is preserving the off-grid inverter and install a string inverter and replace the charge controller with a 600v model with a manual 600VDC transfer switch.