Inverter, controller and cabling questions

PortuGal · April 2023

New here, please be gentle!

I am endeavouring to set up a small house off-grid in Portugal.

So far, I have 2 of 24V Suntech 250W panels, mounted at 45 degrees and facing due south, connected in parallel. Current at PMax is rated at 8.15A. Voltage at PMax 30.7V. 20A fuse rating.
These are running to a (supposed) MPPT 30A controller which looks like this one https://a.allegroimg.com/original/11da60/52f89bb249b69cd41dee5894bbd1. Judging from the reviews on amazon, these type of controllers are not true MPPT, so I would like advice on changing that out for a better one, without breaking the bank.

Then, I have 8 Trojan 105 6V Batteries wired in series as 12V pairs, and then all wired together in parallel to make 1 12V. These are rated at 225Ah at 20HR, 185Ah at 5 HR. I don't understand how to work out what capacity this battery bank may be able to supply to work out if sufficient for my needs. (Both panels and batteries were gifted when somebody upgraded their system and I have no contact with the person who donated them so can't ask!)

I have no inverter yet. I was recommended to get a Victron Multiplus II 3000. https://www.victronenergy.com/inverters-chargers/multiplus-ii. I would like to be able to run a standard EU-size fridge/freezer, lights, 2 desktop PCs (require Pure Sine) with monitors (home office), laptop, occasional printer use, charge phones etc, with occasional washing machine use (not daily as only 1 person in house). The Victron seems over-spec and I don't know if it is overkill for that battery bank?

However, for the last 2 winters we have had extensive, prolonged periods of rain and cloud (13 weeks this year) and I was recommended to get something I could plug my generator in to give the batteries at least a little charge to keep the fridge running.

There are very short cable runs, with controller and batteries in a small hut and panels outside. The inverter can also be housed in there or in the house, which is approx 30m away from hut. What cabling will be required in either scenario for the inverter from the batteries?

I suspect my questions may trigger further questions and highlight my status as completely uneducated on matters solar!

Thank you in advance.

littleharbor2 · April 2023

First off, your charge controller is NOT an MPPT. It will cost you considerable power. You are at a perfect stage to ask questions. I would say first off you really should get a good quality MPPT charge controller I think the EPsolar/EPever brand of MPPT controllers may be available in your part of the world. They are decent mid range controllers. The max voltage/output current will be determined by your planned system size.
Second you really should go with 24 or 48 volt system, meaning your inverter/charger should be of one of those nominal voltages. 12 volt systems severely limit your large battery charging complexity and efficiency. Being that you have enough batteries already for either 24 or 48 it's your choice. 24 volts works well and gives you some redundancy in the event you have a battery bite the bullet. Your battery bank will need to be located wherever you end up placing the inverter
What is the typical residential voltage in Portugal? . if, in the 230 volt range you shouldn't have any trouble with that distance if running the inverter in the shed. If you have lower voltage you might want to run high voltage DC from the panels to the house. Cable size will depend on the inverter output size or DC voltage from the panels.

PortuGal · April 2023

Thanks for the answers.

OK, it seems I can get EPsolar from Germany so that's easy. https://epsolarpv.de/en/product-category/laderegler/mppt-laderegler/ What am I looking for as my criteria to choose one? This is all new to me.

Do I need to rewire my battery bank as 24V then? The panels are 24V.

Portugal is 230V. I don't actually have a washing machine or fridge yet, so have all the joys to come of looking into which models etc to buy to suit my system.

Also, I am still unsure how to calculate how many hours of power I will be able to get from this battery bank. I've read other threads on here but the maths is losing me at the moment. I'm sure there is a simple formula......

Thanks

littleharbor2 · April 2023

Find out the watts consumed by each item, multiply by the expected time of use per day then divide that total watts by your nominal battery bank voltage. This will give you the daily amp hour needs. You will want at least triple that amphour number in Battery amp hours. Do some reading on battery info. Here is a good place to start. Deep Cycle Battery FAQ | NAZ Solar Electric (solar-electric.com) and here, Battery University Homepage .

If you are planning on keeping your batteries in the shed you can go with one of the 150 max Voc models With your present battery bank you need about 45 amps of charging at 24 volts nominal I don't know if they have a 45 amp model but you should go for a 60 amp version if they have one. At 48 volts you can get away with a 30 amp version. Be sure it will charge a 48 volt battery.

You still need to add to your 2 panels. You need to be able to charge at said 45-60 amps. This will require 6 to 8 panels. If 6, then you should wire them 3s2p. If 8 you can wire 2s4p or 4s2p if you don't get extremely cold in your area. There's a whole big explanation regarding Temprature and PV panels. I really don't have the time to explain that. Again you can learn about this online. Google is your friend.

PortuGal · April 2023

Thank you. I have been using search engines a lot to even get this far with the panels wired up via a controller to the batteries!! I can't afford any more panels at the moment so I will downgrade my requirements to fit what I have for now. The pure sine inverter is the most important and a real MPPT controller, not a pretend one. A light in the house and the PC would do me for now!

BB. · April 2023

I will go through some quick math using our rules of thumbs... 8x * 225 AH @ 6 volt batteries wired as 12 volts. Adding batteries in series changes the pair to 12 volts @ 225 AH... Adding 4 parallel strings of batteries is 4*225 AH = 900 AH @ 12 volt battery bank.

Note, to wire up parallel battery strings, this website does a good job of how an why to wire "correctly" (balance battery charging and discharging):

http://www.smartgauge.co.uk/batt_con.html

Also, Lead Acid batteries need to be kept charged... Do not store (for days/weeks) under ~75% state of charge or they will sulfate and die an early death.

Also, ideally, you want to charge at 5% minimum or 10-13%+ rate of charge for off grid systems.

Assuming that these batteries are still good (if they have not be charged in weeks/months, that may not be a good assumption), then the typical off grid design for full time use would be 2 days of stored energy and 50% maximum planned discharge (for longer life):

12 volts * 900 AH * 1/2 days of storage * 0.50 max discharge = 2,700 WH per day (for 2 days of "no sun" usage)

To calculate the amount of solar panels needed to keep the battery bank "happy" and in good shape... Two calculations. The minimum rate of charge for an off grid system used full time of 10% (suggested). And second based on the amount of energy used per day (your loads).

The minimum suggested solar array:

14.5 volts charging * 900 AH * 1/0.77 panel and controller deratings * 0.10 rate of charge = 1,648 Watt array minimum off grid system array
14.5 volts charging * 900 AH * 1/0.77 panel and controller deratings * 0.05 rate of charge = 824 Watt array minimum weekend/emergency system array

And if you were going to use 2,700 WH per day loads, the math would be:

Lisbon Portugal, 45 degree tilt, facing south (ignore the AC energy numbers for now):
https://pvwatts.nrel.gov/pvwatts.php

Month	Solar Radiation ( kWh / m2 / day )	AC Energy ( kWh )
January	3.42	102
February	4.04	107
March	4.49	128
April	5.18	143
May	5.53	155
June	5.60	150
July	6.16	169
August	6.34	175
September	5.56	148
October	4.71	134
November	3.96	112
December	3.49	104
Annual	4.87	1,627

Pick February 4.04 hours of sun as "break even month" (use genset in winter, or less power, etc.):

2,700 Watts * 1/0.52 off grid system eff * 1/4.04 hours of sun = 1,285 Watt array "February Break Even" array

For full time 2,700 Watts of AC power in February (running refrigerator, lights, etc.)--Would suggest a 2x larger array if you want to use as little fuel as possible (reduce genset runtime) for bad weather, etc.

How much power can 2x 250 Watt panels produce on an average February day:

500 Watt array * 0.52 off grid AC system eff * 4.04 hours of sun Feb average = 1,050 WH per day

Which is not really enough power to keep that large of battery bank charged (5% = 824 Watt array minimum, 1,648 Watt array suggested minimum for full time off grid).

Which gets to loads--Were we should start at the beginning.... Say you want 2x 13 Watt LED lights (used 12 VDC bulbs--Save AC inverter losses).

The PC... That is a big question... If this was a desktop, with networking, etc... That could be a 300 Watt system. A lower power laptop, around 30 Watts... Or a Chromebook or similar can get down to 8 Watts... To see how the math works (guesses, examples):

2 lamps * 13 Watts * 5 hours an evening = 130 Watt*Hours per day
1,050 WH per day from 500 Watt array (February) - 130 WH for lights = 920 WH per day "non-lighting power"
920 WH per day / 300 Watt desktop PC + Networking + monitor + printer = 3 hours per day (Feb "average")
920 WH per day / 30 Watt laptop = 30 hours of runtime per day (yes over 24 hours--But remember bad weather, etc.)
920 WH per day / 8 Watt tablet/low power laptop = 115 Hours of run time per charge.

We always suggest defining your loads first (using as efficient as possible loads), then design the system.

If you are lucky, your 12 FLA (flooded cell lead acid) batteries are still good... But if they are not--Then you really should at least start a new design based on your energy needs first... Then buy the hardware, etc. as needed/can afford.

Unfortunately, solar power is not cheap. And it is very easy to have too small of system and too large of loads which takes the battery bank "dead" in a few days of use, and ruins the battery bank. Really need to start the design from the beginning.

A sine wave inverter is very nice--And for any larger solar power system is the way to go (avoid 12 volt / 24 volt / 48 volt DC power systems). But, again, need to know your loads (Watts * Hours of use per day, etc.) to design a "well balanced" system.

The above is really just an example of how the math works... I made lots of guesses and assumptions here. Also, much of above the above may need more explanation for you to better understand what is happening and how to apply the math.

Please feel free to ask... Just jumped in with both feet here as a starting point.

-Bill

Inverter, controller and cabling questions

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