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selection help: small inverter for storage, no solar
Hi all -

I'm a new member here, but I've been following energy storage for years.  I recently decided to try building my own energy storage system (ESS), mostly as a learning exercise.  I'm an electrical engineer, but my experience isn't in the energy / power sector -- yet.

After some time researching, I think I've boiled my problem down.

I'm trying to find a battery charger / inverter with the following constraints:
- 120V AC interface
- single circuit for charge and discharge
- no "protected loads"; energy flows in and out on "grid" side of inverter
- grid-tie / UL1741 behavior: if grid drops, the inverter drops
- don't care about backup during loss of grid
- small-ish size and cost

I think the last one is the problem.  Every time I think I've found an inverter (from Outback or Magnum or MPP Solar), I find that the smallest they go is 2 kW or so, and they cost $1500-$2000.  Or it's a small inverter but is only designed for solar self-consumption, and won't sell back to grid.  I'm looking for something that's say $500 or so, and would be happy to find something used.

I plan to use this for simple price arbitrage, charging during low cost power and discharging during high cost power (I'm on a TOU plan).  I do have a solar system (which will NOT be directly integrated with this ESS) and was planning to have the ESS charge in late morning when I've got free power from the solar system but haven't loaded up the air conditioning yet.  Then in late afternoon I'd have it discharge.  The ESS will not be aware of the actual solar power availability, rather I will use a dumb schedule to charge and discharge.  I doubt that the inverter can be scheduled like this, so I'll probably have to come up with a controller (Arduino+serial or whatever) to command those mode changes, but that automation is farther off in the future, and not what I'm asking about here.  First I'm just trying to find a small-ish inverter to build this around, and would be content with manual commanding at first (e.g. pushing buttons on it twice a day to change modes).

What I want to do here doesn't seem particularly cutting edge (no solar/MPPT needed) so I think I might even be able to use an older inverter model, purchased used, to do this.  However it would need to be modern enough to go offline if the grid dropped (grid-tie behavior) and it would be nice if I could set the high and low battery voltage thresholds so that I could try different batteries.

I was looking at a used Outback FX for this, but at ~$900 used they are still too much for my little exploratory project.  Plus I think I would need their Mate controller to manually change the modes, so more cost. I looked at MPP Solar and Victron lines but they all seemed overkill for one reason or another, but maybe you all can correct me.

The exact battery specs are not relevant right now, I think.  Whatever the inverter needs, 12V or 24V, I'll build a battery pack to meet.  I was planning on starting with a lead-acid battery first, and then move on to an 18650-based pack.  In both cases it would be a very small system, so not a lot of wasted cost when I finish playing with the first battery pack.

And so I turn to you all.  What am I missing?  Can you point me to a small inverter that will do grid-tie like this?
The market for grid tie inverters is probably driven by economics & mass market, so factors the cost of installation (by sparky) can't be too big vs returns, cost for the manufacturer to get grid tie approval, etc, are likely to drive grid ties to the bigger end of the market?
This would seem to make any small grid tie units rare at best?

Would a controlled AC coupled battery by an option? eg a BYD 2.5kWhr unit? (You don't get to have fun building a pre-potted solution though...!)

If you start with LA, they will 100% be wasted fast (only few hundred cycles they'll be gone), you can't DoD them to far, 18650's you can DoD them deeply, expand & change configs better.

At least start at 24V, better match to 18650 voltages in 7s configs.
ChrisC likes this post
Running off solar, DIY & electronics fan :-)

I understand about the lead acid drawbacks. I'd still start with them, just to learn and gain experience on the "old" method of energy storage, before moving on to the shiny new method.

You mention a BYD 2.5 kWh unit. Looking this website:

... I see only solutions for protecting a load. If the grid goes down, it keeps providing power to the load. I don't see a capability of discharging back into the grid, nor a mention of UL1741. Am I looking in the right place?

Do you think an Outback FX2024 would fit this application? Do I need the Mate controller to change modes on it?
Sorry, I don't know options in the USA market. Was just suggesting a nominal battery unit, and as you're thinking, there would be more integration detail.
ChrisC likes this post
Running off solar, DIY & electronics fan :-)
Let me see if some research will tell me whether this will work with the Outback FX2024. I'm picking that inverter because I found two units for sale used, and because I have a preference for using Outback if possible (long story). Of those two units I found for sale, one was built in Q1 2003 and the other in Q2 2004. This post will be me thinking out loud, trying to determine if this model will work, and I would greatly appreciate the review and input of those here with more experience.

First, let me find a manual for this thing. The Outback website doesn't list docs for FX series, although some digging in their manuals uncovered that the GTFX series docs appears to cover the FX series (says so in the introduction, specifically the FX2012 and FX2524):
GTFX / GTVX series manuals:
Installation manual, dated 2008:
Programming manual, dated 2007:

One of these manuals says "the non-Mobile FX is ETL listed to UL1741" which I believe means that if the grid drops off, the inverter will drop off as well, and won't sell power back to grid during that condition. But does it mean that the inverter is capable of selling / exporting power back to the grid, under normal circumstances? Or is only the GTFX ("grid tied FX") capable of doing that?

If I google for "outback fx2024 manual" I get a few PDFs:

One of those was from 2006 and says it was for the "'Export' FX and VFX Inverter/Charger", which implies that earlier FX models could not do selling / exporting of battery power back out to the grid. Further, in the MATE menu section of that manual, it says: "The SELL [menu option in the MATE] is used to set the functions that control the process of selling power back to the utility grid. This menu is only operational for Grid-Tie FX’s and is not operational with “Export” FX’s at this time."

Huh, OK, what's an "Export" FX? A later model of the FX series that offered export? It sounds like a 2003-2004 vintage inverter won't work.

OK, now let me look at the MATE controller. First, here are Outback's MATE docs:

All of the MATE / MATE2 models offer RS-232 serial access except for the MATE 2M. The MATE3 does not. I could (relatively) easily automated any mode changes if there's a serial port, so I think I'll be trying to use a MATE / MATE2.

The GTFX programming manual says that "an OutBack MATE is required to program the FXs beyond their default values". I'll likely not be using the default values (in particular I want take it easy on the AC circuit side and not pull the max current), so I'll probably need the MATE to set that up. The seller of the used inverter that I have my eye on has a MATE, so I could ask him to at least read me the present settings, so I know those until I acquire my own controller. See page 24 of the GTFX programming manual for a list of all of the settings.

MATE docs also say: "The MATE can be used to program the FX and then removed. All of the settings in the FX are stored in non-volatile memory inside of
the FX itself - the battery can be removed from the FX and the settings will not be lost." So in theory I could borrow a MATE from someone for initial use, and if it does what I want, then buy one.

All that said, I feel like the FX 2024 is a dead end because it likely does not sell / export to grid. Do you agree?

And now let me also post this separately since it's a completely different question and I didn't want it to get lost in the post above.

What inverters are you guys using for a small pure battery application, tied to grid (so selling back to grid), without any solar directly integrated into it?

I looked around in this forum and didn't find any that did this. I felt like I kept running into dead ends, per my original post above. What am I missing?
(09-27-2018, 02:50 AM)ChrisC Wrote: What inverters are you guys using for a small pure battery application, tied to grid (so selling back to grid), without any solar directly integrated into it?

I suspect no such device exists, certainly not "small" devices in terms of either price or size.

SMA have their Island products that can be standalone or A.C. coupled to an existing grid, (Victron have similar) and as such can be configured to export (sell) - but that's not a cheap product, and you'd have to have an export purchase agreement in place.

Traditional GTIs don't have battery connectivity as standard, hybrids do, but they'll all come with some form of DC input (PV, wind, hydro etc) - unless they are intended for islanding (creation of own grid), such as the SMA Sunny Island and similarly higher end products.

You might find something cheap and nasty that'll grid feed from a battery (if that's what you are really wanting to do ? )
ChrisC likes this post
Thank you. I'm really scratching my head that this hasn't already been tackled in a community called "DIY Powerwalls". Surely lots of people have A) AC-coupled solar systems and B) no loads on the load side of their ESS, rather feeding power back into the panel. So I'm going to review the whole forum again and see what I'm missing here.
Have a look at the Growatt SP range, they might be open to enable your own battery to be attached (but it'll need to talk modbus)

Just out of interest, where do you live, and have you considered what's required before a supplier would enter into a purchase contract for your export ?
(09-28-2018, 07:53 AM)Sean Wrote: Have a look at the Growatt SP range, they might be open to enable your own battery to be attached (but it'll need to talk modbus)

Thank you! So far, everything I've found from Growatt interfaces to AC at 240 V. I'm looking for 120 V so I can connect it to branch circuit from my panel. Got no room for 240 V and this is a small scale research project for me.

Quote:Just out of interest, where do you live, and have you considered what's required before a supplier would enter into a purchase contract for your export ?

I'm in Georgia. Thanks for the tip! Are those purchase contracts necessary if I'm only discharging into my own loads, not sending power back up through the meter? My house has an always-on / vampire load of about 450 Watts, so I was planning to have the ESS discharge at 400 W to ensure total self-consumption.
You don't need to do anything if you are just aiming to offset your baseload, just be safe.

Just be a little cautious as some meters are able to detect, and flag reverse flow, the implications of that will vary depending on your provider.

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