Help with off grid pv system

Help!  What are we doing wrong?  Long story short... we bought acreage with nothing on it with the intention of building a house and living off grid...eventually... but circumstances forced us to move to the acreage waaaay before we were ready, and we needed to put things into place quickly and with minimal funds.  So here's the problem: we put together a pv system just to get us started with a little bit of electricity, but with plans to expand.  We don't need a tonne of power, but it seems like we should have a little more power available than we do.  Here's what we have:


 4 x 250 w solar panels wired as 2 separate series, each pair connected to its own 60A mppt charge controller.  Then we have 4 x 6v 230ah deep cycle agm batteries connected as a 2x 2 series/parallel to create a 12v system with 460ah, and finally we have a 2000w modified sine wave inverter.


We live in a place with very hot sunny summers, so there's no shortage of sun right now...direct sun is on the panels from 8am-5pm  We are trying to run our starlink satellite (100w), and a small chest freezer (150w), plus occasionally charging a laptop, and 3 phones. We only charge the phones and laptop during the day when the sun is shining, and we can only run the freezer until the sun is off the panels bc otherwise the batteries are depleted and the inverter gives us an alarm at 9.8 v by late evening.  Even with the freezer being shut off (and the sattelite receiver being the only thing plugged in)  usually we have to shut the inverter off by midnight so we have a little power left first thing in the morning before the sun gets going again.  This isn't optimum, obviously, but it's all we could swing in the time we had.  Suggestions?  Ideas?  Obviously we have done something wrong... but what? 


TIA!

Without more information about the precise setup it's hard to say whether you're doing something wrong, but my guess would be it's probably mostly just a mismatch between your expectations and the capacity of the system you've got.

460 (ah) * 0.50 (usable capacity) * 12 (volts) = 2760 watt hours

90 (watt starlink usage) / 0.8 (inverter efficiency guess) = 112.5 watt usage

2760 (watt hours) / 112.5 (watts Starlink usage) = ~24.5 hours

That's assuming the batteries are new, getting fully charged every day, and you want to discharge them to 50%. In reality you probably don't want to discharge your batteries that far and there probably will be days where they get less than fully charged.

Once you add in the freezer - if it really uses 150 watts on average - it is easy to explain why you'd be running out of power...

150 (watt freezer usage) / 0.8 (inverter efficiency guess) = 187.5 watt usage

2760 (watt hours) / [112.5 (watts Starlink usage) + 187.5 (watt freezer usage)] = ~9.2 hours

Voltage isn't a good predictor of lead acid battery capacity so you can't expect a low voltage cut-out to reliably halt usage at a given percentage of charge. Also, these are best case scenarios. Any wear on the batteries or imperfect care (watering, sub-optimal charge profile, etc) will make things dramatically worse.

Starlink power consumption: https://www.jeffgeerling.com/blog/2021/using-shelly-plug-monitor-starlinks-power-consumption

Best lead acid battery reference (IMHO): https://www.sevarg.net/2018/04/08/off-grid-rv-lead-acid-maintenance-charging-failure-modes/

Em's numbers check out to me (although unless it really hot, the freezer will likely not run constantly). Still, you have too much load for your battery bank. And you should really consider adding an amp meter or battery monitoring system to let you see how much power you are drawing. You can kinda guesstimate based on voltage alone, but its better to have a shunt and a meter, preferably one that tallies all the current in and out.

With a meter, you might also be surprised to find how much power your inverter is using just staying on "standby." This is why an inverter's efficiency gets worse the farther you get from peak load - it takes a set amount of power just to keep the thing on. A 2k inverter that only powers a few hundred watts of stuff is not very efficient. It makes sense from a "someday we will want it" standpoint, but right now I bet it is consuming 20% of your battery power. Some inverters have a "sleep mode" where they reduce their power draw when nothing is running, but if the starlink is on 24/7 then it wont do you any good.

I am actually curious about that starlink dish, as it is something I would like to get someday. Does it take it a long time to make a connection when it is powered up, or does turning it on and off when you want to surf the web just feel too much like the days of dial-up? I kinda miss that weird sound it made if you picked up the phone while it was dialing Reeeeeeeeeeeeaaarrrrssssssschhhhhhhhhhhriiiiiiiishhhhhhhhhhhhhhh. Or was that the sound of a fax coming in?

Hi Em, thanks so much for replying!  Well, i have to admit that in all the reading i did in order to figure out how to put this system together, 50% available consumption wasn't something I came upon.  I mean, i knew you don't drain your batteries down to 0... but i didnt realize that if your battery says it has 230 ah, then you are really only going to get 115ah out of it.  So, ok that explains a few things.  Thank you!

Hi Carl, thanks for replying!  I didn't know that the inverter would be so inefficient with our current usage.  We only bought 4 batteries because that's all we could swing up front.  But we are planing to buy 4-6 more when we are able, as well ad a small wind turbine, and then we would like to be able to run a few more things. So our inverter will hopefully be put to better use then.

As for the starlink... we love it!  Our acreage is in a dead zone for cell phones, and there is no option to tie into any sort of grid here... so sattelite was our only option.  We were lucky that starlink is available in our area!  So far we have had no issues with it.  It works beautifully.  Yes it takes a couple of minutes to get going when you turn it on, but it's not prohibitive. We will turn it off to conserve our batteries, and then turn it on for 2 minutes to check for messages etc, and then off it goes again.  It's definitely faster than dial-up, lol.

Look into Edison batteries when you get ready to upgrade, they are pricey but you can rebuild them and they don't mind getting abused, unlike lead acid.

Tom

Yeah, that sounds like a plan; although I suggest you dont wait too long to add more batteries. As they age, they will lose capacity, and that will drag down how much power the new ones will be able to hold. I have heard with lead-acids that you should not exceed a year in age difference. Also, read up a bit on how to properly wire a series/parallel arrangement: you need to balance the strings so that you are not working the batteries on the ends harder than the ones in the middle (due to slight differences in Since you have two charge controllers, another possibility would be to add a second system. Get more batteries and maybe a small inverter, and run it separately. When off-grid, redundancy is nice! Lead acid batteries will not last for very long, figure 3 or 4 years before they will need to be replaced. When they do decided to crap out on you, it is nice to be able to plug the freezer in to a backup system so you are not scrambling to go get replacements before your food melts.

As for Nickle-Iron batteries, they do have some upsides, but for the price, you might as well get lithium batteries. Lithium Iron Phosphate cells are what I am running at the moment, and they really are the future of off-grid storage, I think. We will see if they can actually deliver the 10,000 cycles that they promise, but the technology gets better all the time (unlike the chemistries from the 1850's, which are about as good as they are ever going to get). I think you are wise to start with lead, though, as it will give you some good experience with batteries. They are pretty tolerant of abuse, and it is nice that you can source a replacement bank from just about anywhere on the same day you need them.

Another route that is going to become a lot more common in the future is to use EV batteries. You can get a massive amount of storage for the money, but it would be a very serious project for a beginner to undertake. I think eventually I am going to try it with a pack from a salvaged Leaf, but its pretty far down my list of projects.

Jennifer Lapinsky wrote:Help!  What are we doing wrong?  Long story short... we bought acreage with nothing on it with the intention of building a house and living off grid...eventually... but circumstances forced us to move to the acreage waaaay before we were ready, and we needed to put things into place quickly and with minimal funds.  So here's the problem: we put together a pv system just to get us started with a little bit of electricity, but with plans to expand.  We don't need a tonne of power, but it seems like we should have a little more power available than we do.  Here's what we have:


 4 x 250 w solar panels wired as 2 separate series, each pair connected to its own 60A mppt charge controller.  Then we have 4 x 6v 230ah deep cycle agm batteries connected as a 2x 2 series/parallel to create a 12v system with 460ah, and finally we have a 2000w modified sine wave inverter.


We live in a place with very hot sunny summers, so there's no shortage of sun right now...direct sun is on the panels from 8am-5pm  We are trying to run our starlink satellite (100w), and a small chest freezer (150w), plus occasionally charging a laptop, and 3 phones. We only charge the phones and laptop during the day when the sun is shining, and we can only run the freezer until the sun is off the panels bc otherwise the batteries are depleted and the inverter gives us an alarm at 9.8 v by late evening.  Even with the freezer being shut off (and the sattelite receiver being the only thing plugged in)  usually we have to shut the inverter off by midnight so we have a little power left first thing in the morning before the sun gets going again.  This isn't optimum, obviously, but it's all we could swing in the time we had.  Suggestions?  Ideas?  Obviously we have done something wrong... but what? 


TIA!

Hi Jennifer, what charge controller are you currently using on the system? Apart from some of the great suggestions listed above I would try to match your satellite consumption to the daytime hours. You do have a lot of solar capacity but with the smaller battery bank it probably cannot take full advantage of it due to the rate that lead acid can absorb. Load shifting to high sun hours can give you a greater percentage of used solar capacity...
Cheers, David

Hi Jennifer.  There are a lot of good helpful posts here.  I live in Ohio and sometimes we don't get sun for days on end.  I designed this little system to run a dc 35w fridge and a few fans and lights.  My goal was to run it for 20 years.  The lowest the batteries have ever got down to is 12.4v.  It has 140 watts solar and a 800 amp hour battery system.  I used a 30 amp hour mppt controller.  I set up the panels in a moon pattern to track the sun better throughout the day so it almost always has enough solar to run the fridge through the heat of the day.

I have one question on the 2 controllers that you are using.  Do they communicate with each other?  I have found running 2 controllers to the same set of batteries only one will charge unless they are designed to work together.

I would add a propane generator, and, as others have suggested, switch the batteries to LiFePO4.

A propane generator, assuming you can have propane delivered in your area, would smooth out the issues of usage vs refill by solar panels. Your battery bank is already a buffer, but it isn't getting refilled by solar when needed by your usage patterns. Switch on the gennie for an hour, refill the battery bank, and switch off the gennie. You'll find the right patterns over time, for the way you use electricity at your location. Also, when the gennie is running, you might be able to do other things as well, as the electricity is flowing.

We switched out our giant FLA battery bank (4 heavy & massive FLA batteries) for two smaller & lightweight LiFePO4 batteries, and now we are able to access more Ah's out of the system. No 50% or more penalty, plus, no more maintenance (of FLA batteries).

At our off-grid location, we started with propane (very clean fuel system) and a gennie (for construction), added an inverter and battery bank (for a power buffer and full-time power), and finally added solar panels (to cut back on propane). We've never run out of "power", and we operate more like a utility company, with one system providing power if another can't.

The choice of the four 250W panels is a good one.  Staying with 12V however is not.  You are really hobbling yourself trying to stick with 12V.  I would also not recommend keeping the modified wave inverter.  They are hard on anything running an electric motor, of which your freezer is one.  I'm assuming you'll also need power for power tools like drills and circular saws, that are also motor driven.

I would recommend switching immediately to a 24V Sine-Wave inverter.  There are many choices once you get into 24V, including models that produce split-phase 120/240VAC.  That's what I have for my off-grid workshop.

here are some choices to look at.
https://ressupply.com/inverters/samlex-pst-1500-24-pure-sine-wave-inverter
https://ressupply.com/inverters/schneider-electric-conext-sw4024-120240-invertercharger
https://ressupply.com/inverters/magnum-energy-ms4024pae-sinewave-parallel-inverter

I have the second, Conext, unit.