Introduction and questions about my off grid setup

caronte01

First of all, a quick introduction. I´m Santiago Molina, live in La Calera and Bogotá Colombia, with my wife 1 child and 2 dogs.I came here and finally registered after multiple google searches regarding solar setups directed me to this forum. So, thank you for this great resource. 

We own a small house in a very rural site. There´s no electric grid nearby (about 1km away, there´s electricity, but dealing with them is quite a mission, so we´ve discarded connecting to the grid for now). We got this house about four years ago. Previous owners used it as a weekend retreat, and ran electricity out of a 500 watt inverter and a 60ah agm battery that they charged at home during weekdays. Very minimum. We did the same for 2 years, until said battery got killed by leaving the inverter on a couple of weeks.

We decided after some research to test the waters of solar by building a small solar setup. It consisted of 400 watt of solar, 2 100ah 12v agm batteries (in parallel), a renogy mppt 40 amp cc, and a hand me down, 12v/1500w inverter. It worked fine for our basic needs (laptop, cell phones, and occasional kitchen appliances. No fridge. It even ran power tools for short periods of time during the day, no problem.

We plan to move here full time, and based on our power consumption at our apartment (60 kw / month), upgraded our system a bit. Went up to 10 * 100 watt solar panels (2 strings of 5), added 2 12v-100ah batteries (the previous ones had about 7-9 months of use) and switched our trusty renogy cc+no brand inverter, with an integrated cc-inverter, at 48v, 50 amp, 2000 watt inverter 

I´m seeing longer charge times for these batteries than before. Previously, our controller would show float at around 10-11 am, from batteries that went down to 12.6 early morning.  With this setup, I´ve seen 49 v in the morning, and batteries going up to 53v is normal, only getting 55v every other day. I will try and disconnect one of the strings and see if there´s something weird there. I´m seeing around 150-350 watts going in on cloudy days.

I set up my charge profile based on the one from the renogy rover. (charge 56.8, constant volt charge time 2h, float 55.2)
I do believe the inverter part of this cc is less efficient than my previous one. I can make it turn on automatically when loads exceed 60 watt, but normally, our house idle load is way below this figure, so it´s more of an inconvenience right now. I may consider adding a secondary small inverter, just for the lights+electronics, so the big one only turns on when "big" loads are needed (refrigerator, tv, etc)

I´m concerned about my batteries, first of all, and also, not sure if this system can handle the load of a full time house. Our biggest load is the refrigerator, at around 30kw/month, measured with a kill-a-watt. There are no heating or cooling loads, minimum tv use.

I´ve found a lot of skepticism from many who have no first hand experience in the topic. So in a way, I´ve been pioneering in my local community, based on a lot of internet, youtube, etc. I would love for you, based on actual real world experience, to give me advice on what I could change, fix, do over, etc.

mcgivor

Welcome to the forum 

The transition to full time off grid will expose the weaknesses of a part time system in a very short period of time, the importance of accurate load calculations is paramount. As you already know the refrigerator is a significant load, the 30Kwh monthly figure is consistent whth most reasonably efficient units in itself, however the inverters self consumption and efficacy must be considered, which may, in worst case scenarios be equal to, or greater than than, the refrigerator itself.

Adding new batteries to an existing used bank is not something that's recommend especially if the older ones were deeply cycled as their charging needs will differ often leading to the new ones being dragged down by the older, this may be what is causing the problem you are referring to.

As it stands the 4 × 12V 100Ah bank should allow a maximum of 2.4Kwj of usable capacity based on a maximum 50% depth of discharge if in good condition, this is, in my opinion, not sufficient to support the loads mentioned and the array is undersized to support the larger battery capacity required.

Don't be discouraged by skepticism, it is possible to build a reliable off grid system if everything is designed correctly, albeit not cheap. Collect and post all details and specs of the equipment on hand for more accurate feedback as it's important to understand what the hidden losses are.

caronte01

Hi mcgivor, thanks for the reply.

Agreed on part to full time system. This is kind of a special circumstance, related to covid 19. In Colombia, quarantine measures are quite strict, and are forcing people to stay at all times inside their homes, with only specific exceptions allowed to go out. No kids outside, no elder. This will go on, for now, until the end of this month (for now). Since we live in a relatively small apartment in Bogotá (around 700 sq feet), we chose to come here to the country, since even though the house is smaller (500 sq feet), there's ample space outside to move around, with one kid and two dogs. We are planning to move to the country eventually, but this situation has forced us to come here sooner. We don't have a refrigerator or washing machine right now.

Ths is what we plan on using at La Calera. This is the minimum, and it's close to what the apartment electricity bill tells us. I use power tools, but I am ok with using them off the gas generator, since I work outside most of the time. The figure you got is ideal, how much should one consider over sizing for average? There's no snow, just sunny and rainy seasons. Our panels are about 15 deg, facing south. According to PVWatts, It's the best orientation from january till march, then, the sun moves north. Maybe I could over panel a bit, to try and compensate this situation.


The original batteries were purchased in may-june last year, and have been used sparingly,  during weekends for the last year. I understand, and have read that adding old to new is not ideal, but, I could not justify disposing of the old ones or buying more batteries. The ones I got are the same model. I did a quick voltage check at the end of today, to see how unbalanced they were. There was a .15 volt difference between old an new. Old ones at 13.05, and new ones at 12.9. They were all connected in series, for about two weeks. How serious could this be? For now, I connected the old system to the new batteries (parallel 12v) to try and get a full charge, before reconnecting them together again. I checked my cabling, and 1 of the leads is maybe 50% longer than the others. I'll try and correct this. I have no way of disconnecting and charging batteries separately, without getting the house offline. Nor do I have a separate charger. I'll try and get them as close voltage wise, before re connecting them in series, tomorrow, or the day after, depending on weather. Getting hardware right now is close to impossible, given the situation.

I did not take into account inverter inefficiencies... Reading the manual, it says it is around 85% I've opted for turning it off after 9-10 pm, and back on in the morning, for now. It's an off brand inverter, unfortunately no info on the internet, but it's what I have for now, besides the original system. (renogy rover 40 amp + no name inverter) I can scan the spec sheet if needed.

One option I can think of, to apply, using existing hardware, is to create a secondary system. The main 48 v can be used for "heavy" loads, like the refrigerator, and the washing machine, with the existing 1000w power array, (and turn on the inverter auto on feature, so it is only used when needed), and create a secondary 24v system, with my trusty rover, for the rest of the house. That system could be around 600 watt/ 150ah at 24v.  I would need a new inverter, but can probably get away with a simpler 1500 watt tsw inverter. Some extra cabling needed.

caronte01

I just try and edit, and accidentally lost my reply. I'll try and remember.... 

Thanks for your reply. Regarding transition, yes certainly hard. We were kind of forced to come here and stay full time, with this whole covid19 situation. Our apartment is around 700 sq feet, and quarantine measures in Colombia are quite strict, forcing most people to stay inside their homes 24/7, with few exceptions. No kids or elders outside We're 2 adults, 1 child and 2 dogs. This house in the country, while smaller than our apartment (500 sq feet), has ample space outside for playing, walking, etc. We do plan to move here eventually, this year, but this situation has forced us to do it sooner, and in a way, we're grateful. Right now, we don't have refrigerator or washing machine, and rely on the generosity of some neighbors for these chores, but of course, we do plan to have these.

I do understand, from what I have read online that new and old is not ideal, but I could not justify the extra expense of all new batteries. I did check the voltage of new and old batteries, and they are off by around .15v. New ones at 12.9, old ones at 3.05. This measurement was done at the end of a sunny/cloudy day. One of the cables is a bit longer than the others, but I can correct this with what I have available right now. (It's almost impossible to get supplies other than food and medicine right now) What I did was connect the old 12v system to half my panels, and parallel the two new batteries and I will try and get a full charge for a couple of days before series connecting them again to the new system. I'll try and match voltage the best I can.

Our panels are tilted 15 deg facing south. According to Pvwatts, this is ideal for December-May, and for the rest of the year, the sun moves to the north. I could over panel a bit to try and compensate. This roof is the one with better access.

As far as loads is concerned, agreed it's a bit low. What would you consider over sizing for average weather. There's no snow here, just rainy and sunny seasons. We're about 9000 feet above sea level, so solar radiation is good, if not harsh.


One option I've been contemplating is creating a secondary system, using my rover controller, and around 600 watt of new panels, and 150-200 ah of batteries at 24v, with a new regular tsw inverter of 1000 to 1500 watt.

My refrigerator and washing machine would run of the 48 v system, and I can turn on the auto off feature, so it's idle draw is smaller (it will only turn on with loads of 60w or more, so it's  useless for anything but "heavy"loads). It's a no name inverter charger, that, according to it's manual is around 85% efficient, so no good. I can scan or photograph it if needed. It would need new batteries for the new system, and some extra panels and cabling, but we do intend to move here full time.

706jim

I know what it's like trying to monitor electrical loads. Some like the fridge are pretty easy to calculate. Others like pump loads are intermittent and who sits there with a watch to compile the data.

That said looking at your loads I think you need a 4kw array and batteries to match to keep things going comfortably.

Dave Angelini

More solar is always a good thing if it is done correctly. I spent quite a bit of time in Cartagena Santiago. We enjoyed the the 3 months there in the harbor on our sailboat. Did not like the chocosano winds...You know about those?

My advice to you if you do end up doing this long term is to invest in electronics from a name brand like Outback power systems or Schneider electric solar. These systems have entry level costing and do one thing that can really help you in monitoring your system.

The ability to see the system over 24 hours or weeks and months will be much easier for you to be successful. An example below shows a day with clouds and the system really did not complete charge that day. The next screen shows a good day and an EQ charge done later in the day. You could go back and look at other days, see the refrigeration loads at night, and see the output current of the array/battery during the day

These systems are networked and designed to work on a smart phone or monitored anywhere in the world for no cost to you. This is not needed now but it can be a good way to encourage young people to "see" the solar power system. There are many more screens and displays like temperature and state of charge can be added.

That is a nice altitude to live at in the tropics!  Enjoy and stay cool!


Bad day above with clouds. See the refrigerator cycling at night? The battery never gets a steady charge which it needs.

Good day below with an EQ charge around 4pm.

caronte01

hi Dave and Jim. Thanks for the replies.

An update on our current situation. We've been at our house for about 6 weeks, without issues. I reverted back to the previous solar system, at 12v, with the renogy cc 600w of solar, and 4 batt in parallel. Not ideal, but since getting anything now, other than groceries is impossible, it will have to do. I'll check individual voltages next week, but from what I understand, parallel will keep batteries balanced. I wired the batteries using the scheme found elsewhere in this forum, that supposedly evens out the load. But with our modest needs, we've had no issues. April and May are the worse months solar wise in our location and panel orientation.

The first upgrades after everything settles will be a 24v inverter and better cabling (I'll probably get a crimper to make my batt cables, current ones are soldered #4awg). This way I'll run the current system with 900 watt of solar (3s3p). I agree with getting better name brand equipment. The inverter charger was a mistake, but I'll try and give it some better use in an aux system, it's not very efficient, plus it has very loud fans.

 I will create an aux system for my fridge and washing machine, dimensioned accordingly. I will measure power consumption of my washing machine once I can go back to the city. It is a very efficient front loader, direct drive Samsung unit, and I do plan on running it during the day only, with some kind of timed switch.  This will probably be a 3600 wh battery with 400 watt solar.

Reading elsewhere, our power consumption is about 1\10th of the average American home. our climate is very even through the year, no heating or cooling loads, plus no microwave or TV, so that may explain. Our power bill back at the apartment shows an average of 63kw\month.

caronte01

One question about mppt vs pwm. Is it really that big of a gain? With current prices here, the difference between both at 30amp will get me about 400 extra watts of solar. I'm using 20v 100watt panels, so I have flexibility wiring wise to create more parallel stings 40 volt. This, to charge a 24 volt battery bank.

706jim

You ask a good question. I've run a Trace C40 since 1996 and it has been faultless the entire time. I'm replacing it with an Outback FM80 to improve charging efficiency. Generally, "up to" 30% improvement in charging efficiency is claimed but it will be a couple of months before I can confirm this number. And a good MPPT controller is not particularly cheap.... 

mike95490

MPPT allows you to run panels in series and higher voltages, for more efficient power transmission from the solar array to the charge controller.  MPPT does a very clever watts in = watts out DC-DC conversion so this maximizes your solar.

PWM works by Amps in = Amps out:
22v  4a panel  is PWM'd to 14v 4a  and you loose 32 watts from your 88w panel  (8v * 4a = 32w)

So for little systems PWM is OK, but beyond 500w of PV, MPPT makes more and more sense.

The gain is calculated from the mismatch of your panels Vmp to your batterys nominal charging voltage
Volt Difference x Amps = watts lost

BB.

Rules of thumbs and short cuts/engineering assumptions are great for "finding the target". However, that is not the same as hitting the bullseye.

I highly suggest that you do two designs. One with "12 volt panels (say 140 Watt Vmp~18 volt panels and PWM controller(s)), and do a second design with the "cheapest" (good quality, watch shipping costs, typically 250-300+ Watt Vmp~30-36 volts), and see what works out best for you...

Roughly, 140 Watt "12 volt class" panels cost around $1 to $2 per Watt... A ~300 Watt panel "GT class Vmp", typically is around $0.50 to $1.00 per Watt...

And say you want a 24 volt battery bank (typical mid-sized ~3.3kWH per day) system... Say you get a 90 Amp Midnite classic controller--Around $661 list, and 94 Amp output at 24 volts:

• 94 Amps * 29.0 volts charging * 1/0.77 panel+controller derating = 3,540 Watt array (Pmp marketing number)
• $661 controller / 3,540 Watt max array = $0.19 per Watt for MPPT controller
• $182 Morningstar PWM controller / (30 amps * 29 volts charging) = $0.29 per Watt (Pmp marketing number)

For larger system Large Format/High Vmp/High Wattage panels tend to be much less expensive. And Large MPPT controllers tend to be less expensive ($/Watt) for larger systems... The (generic list price) cost for a MPPT 3,540 Watt @ 24 volt system is around (.19+0.75 per Watt= ) $0.94 per Watt.

A PWM system works out to something like ($0.29+$1.50= ) $1.79 per Watt for controller+12 volt panels.

As you can see, there are large swings in prices for the "same thing" 2:1 for solar panels as an example, and USA vs Import MPPT and PWM controllers have wide price ranges too (and range in quality).

You really need to do a  whole paper design cycle for different designs to see the plus and minus of the different designs (besides pricing, there are wiring, size, limits to distance from array to the battery bank, features that "you need/want", power capabilities, etc.).

The above math--Is simple to model different back of the envelope costs, but are just that... A model. And the quote "all models are wrong, and some models are useful".

-Bill

mountainman

Fwiw for 2 years I used 4 6 volt GC batteries 400ah @12 volts.
With 2 arrays 380 watts on mppt 
And 4 100 watt panels on pwm.
I wasn't impressed with mppt.
 for a small system 800 watts and under.
And  if the panels are Less than 40-50 feet from your charge controller.
Spend your money on more  panels And use pwm.
For 24 volts a 30 amp pwm will take 800 watts.
So it's best to use 2 of the cheaper per watt  72 cell 350-400 watt panels in parelell. With 2 panels no combiner box or fusing needed between panels and controller.
In my experience comparing my 2  side by side systems.  spend your money on panels
 That being said. On a 30 amp pwm@24 volts with 8 100 watt panels.
Run them in pairs of 2 with 4 strings (2s4p.)
And fuse or use breakers  between the strings.