AN OFF-GRID SYSTEM

diallodjeri · May 2020

Hello folks,
I hope you guys can help me rectify anything that is not going to work as intended in this diagram. Here's the set up I want to adopt with my off-grid system:
Astronergy panels (260W, 8A, and 37V) in series of 6 => 260w*6 => 1560W, 8A and 222V
4 parallel of those 6 panels put in series 1560W*4 = 6240W, 220V and 32A
The battery bank voltage is going to be 48V. To get the right size of a charge controller, I do this math: (6240*0.7) / 48 = 91, So my charge controller should have an output amperage of at least 100A. This is my basic understanding of how solar system work.
Please tell me if something is wrong with this picture.
Thank you.

BB. · May 2020

For me, I like to start with loads, then define the battery bank, then the solar array to support your needs...

But in this case, we are starting with the solar array... Use that to size the battery bank. 5% to 13% rate of charge suggested. 5% can work for emergency/weekend usage during sunny weather. 10%+ for full time off grid suggested. The math (48 volt battery bank):

24 panels * 260 Watts each = 6,240 Watt array
6,240 Watt array * 0.77 panel+controller deratings * 1/58 volts charging * 1/0.05 rate of charge = 1,657 AH @ 48 volt battery bank maximum
6,240 Watt array * 0.77 panel+controller deratings * 1/58 volts charging * 1/0.10 rate of charge = 828 AH @ 48 volts nominal (full time off grid)
6,240 Watt array * 0.77 panel+controller deratings * 1/58 volts charging * 1/0.13 rate of charge =637 AH @ 48 volt suggested minimum battery bank capacity

Sizing the MPPT solar charge controller, the minimum "cost effective" Amp rating for MPPT charge controller:

6,240 Watt array * 0.77 panel+controller derating * 1/58 volts charging = 83 Amps minimum rated MPPT controller

This is the typical maximum charging current you may see (cool clear spring/fall weather, near noon, a few times a year)... The MPPT charge controller does manage the output current "safely and reliably", but you can go larger if you wish (future panels, derate current to keep MPPT controller a bit cooler for longer life, etc.). If you have well below freezing weather and clear winter days--You could harvest 10-20% more energy and upsizing the controller could make sense, but usually don't worry about that.

If you want to use a 100 Amp charge controller (or parallel 2x 60 Amp controllers, etc.), the size of your system certainly makes sense.

For the AC inverter, typically, somewhere between 500 to 1,000 Watts per 100 AH battery bank (48 volt battery bank). For example (based on "nominal battery bank sizing"):

828 AH (48 volts) * 500 Watt * 1/100 AH (at 48v) = 4,140 Watt AC inverter nominal
828 AH (48 volts) * 1,000 Watt * 1/100 AH (at 48v) = 8,280 Watt AC inverter max suggested

Not sure where you are located, but if you assume that 3 hours per day is "break even" hours of sun per day (like a February), the harvest would be:

6,240 Watt array * 0.52 off grid AC system eff * 3 hours of sun per day = 9,734 WH per day

Your thoughts, location, needs? How far from array to charge controller+Battery bank? Not a small system....

-Bill

diallodjeri · May 2020

I have a question about some of the numbers you used as far as what they stand for. For example 1/58 stand for what in your calculation?

I need to provide more information about my system. The system is not going to be installed here in the States, but in the part of Africa where the temperature never gets below 64F, so there is no concern there other than it can get above 100F in the summertime.
The system in a sense is an off-grid because we don't have such an option of a grid-connected system, but I still can use the grid whenever I need to. I just need to unplug from solar power and plugin to the grid manually whenever necessary. In that part of the world it is almost useless to be connected to the grid because you only have power 10 to 16 hrs a day during summer

After giving a second look to my design, I made some changes to it, and hopefully my maths got closer to reality now.
Astronergy panels (260W, 8A, and 30V) in series of 4 => 260w*4 => 1040W, 8A and 120V
6 parallel of those 4 panels put in series: 1040W*6 = 6240W, 120V and 32A
The battery bank voltage is going to be 48V. To get the right size of a charge controller, I do this math: (6240*0.7) / 48 = 91*1.25 = 113.75 So my charge controller should have an output amperage of at least 120A. The battery size I'm planning on getting is 16KW Lion @ 48V

The loads that are going to be on the system are couple AC, one freezer, one refrigerator, a water pump with a total consumption of 7kW/h all of them being used simultaneously which is never going to be the case. Some of them are going to be used during the day while others only at night.

I just want to make sure I do understand the technical details of the system I'm planning to build.

Thank you!

BB. · May 2020

The "1/58" is the charging voltage for the battery bank (48 volts in this case, or ~14.5 volts equivalent for a 12 volt bank).

We have some rules of thumbs here we use to get a quick back of the envelope system design (battery bank AH/Voltage, Array Size, Inverter Size, loads supported by season, etc.).

Also, we use various numbers and "fudge factors" to quickly/simply model the solar power system. When you get into the details, it does get more complex, and hardware choices affect the system performance...

For example, I use 0.77 (or 0.75 for a couple other folks here) as the "nominal best case" solar harvest you can get from Solar panels and a solar charge controller... And under specific assumptions (properly designed system, moderate to warm temperatures, how deeply you cycle the battery bank, the type of batteries, etc.), for example MPPT and PWM solar charge controllers have similar harvest numbers with the typical assumptions--Even though they are completely different in how they function. But--For our first pass estimates, the numbers work pretty well.

Once you have a paper system design, then you can start picking hardware, making sure it plays together, your actual system requirements (size of array/battery bank/length of wiring runs/options and features/etc. And do several detailed hardware designs and costings--To see what works best for you).

The details, for example, an MPPT solar charge controller... These are basically computer controlled digital switching power supplies (buck mode type--They can only take higher voltage/low current and down converter to lower voltage/higher current for charging the battery bank).

For lead acid batteries, their operating voltage varies from 10.5 to 16.0 volts (for Flooded cell lead acid deep cycle batteries, from "dead" to Equalizing/sub freezing operation). For an MPPT charge controller, they output different amount of current based on the battery bus voltage (loads, battery type, state of charge, charging/discharging/etc.).

For FLA, a 6,240 Watt array in warm to hot climates (Vmp for solar panels falls as cell temperature rises--Hot sun, lower Vmp/Voc panel voltages... You can lose as much as 81% effective Vmp vs Vmp-standard test conditions. So, roughly, the math looks like this:

6,240 Watt array * 0.77 hot/dusty panel+controller derating = 4,805 Watt output in "real life", cool/clear day near noon, pointing at sun
Power = Voltage * Current
Current = Power / Voltage
Battery Bus current = Available array Wattage / Battery bus voltage
Pbat = Pw/ Vbatt = 6,240 Watt array (freezing condtions) / 42 volts (near dead battery) = 149 Amps
Pbat = Pw / Vbat = 4,805 Watt derated array / 42 volts Battery near dead voltage = 114 Amps (more or less) best case charging current battery bus current (battery charging + DC loads)
Pbat = 4,805 Watt array / 54.4 Volts battery float voltage and DC loads = 88 Amps
Pbat = 4,805 Watt array / 58.0 volts "average charging voltage for 80-90% state of charge" = 83 Amps
Pbat = 4,805 Watt array / 59.2 volts "battery transition from Bulk to Absorb charging" = 81 Amps

As you can see, the maximum MPPT charge controller output depends on the temperature of the array, orientation, time of day, battery state of charge, how much time your system spends at XX voltage, etc.

For a FLA battery bank, most of the time, the battery bank will run between ~48 volts and ~59 volts. And most of the time it will be warm to hot weather... So the actual best case (a few hours a year around solar noon) harvest is probably in the 88 to 81 Amp range. I call this a "cost effective" design. Could you justify a 114 or 149 amp charge controller--Certainly worst case, you could--But the chances you would see those conditions should never happen (warm climate in Africa, Nobody should take their battery bank "dead"--Can dramatically shorten battery cycle/aging life.

And, MPPT charge controllers (at least the "good/well designed ones) can safely and reliably "limit or clip" their output current. If you have an 80 Amp rated MPPT controller, it will safely clip its output current to 80 Amps (or less, as controller temperatures rise for self protection). And those clipping events will be relatively rare, in the above example (perfect sun/weather, discharged battery bank and/or heavy DC battery bus loads, etc.).

Since you are choosing a Li Ion battery bank--You need to look at its specific operational voltage rage (say from 20% to 90% state of charge)--And pick the hardware that best meets those needs (bus voltage, array size, etc.). I would still use the 77% (or 75%) derating for the array and the charge controller (~81% derate for arrae 500-y, ~5% for charge controller)--Simply because they are "close enough" for government work.... They do not "over estimate output" (some vendors will tell you that MPPT controllers can harvest ~10-20% more energy vs PWM--But they forget to tell you that is in sub freezing weather, low battery bus voltage). And expecting any closer than +/0 10% accuracy in harvest predictions--Just worth the extra math.

Defining your loads--Job #1. Refrigerators and freezers--Probably 1-2 kWH per day (depending on size, specific efficiency, how hot the room is--Hot rooms, higher fridge energy usage). Water pumping--There are "solar friendly" water pumps (smaller pumps that have almost no surge current) and solar "unfriendly" pumps (simple 1-3 kWatt induction motor well pumps that take 2-3x more starting surge current). And Air Conditioning, that can be done nicely with inverter based Mini-Split A/C units (low/no surge, can run on "low power" vs "standard/high power" cycling on/off).

7 kWH per day in summer--That sounds low, especially if you will be using a significant amount of A/C, larger commercial Fridge/Freezer, etc.... Without the A/C, and a reasonable water pump, 7 kWH per day is probably quite reasonable.

But I do not live there, and I have no experience... So, what I think is right for you is probably just a wild guess. Energy usage is a highly personal set of choices.

Is there anyway you can run and use an energy meter to check the usage before you buy/build the solar power system? Anyone near you with a similar (and successful) system nearby you can get more numbers from?

-Bill

diallodjeri · May 2020

Once again thanks for your wonderful input.
Here I tried to sum up my daily consumption from my array. The ACs are mini-split with an inverter.
AC#1: 220V, 11.8A => #hrs of use 8 a day => 220*11.8 = 2596W

AC#2 :220V, 5.9A => #hrs of use 8 at night => 220*5.9 = 1298W

Refrigerator: 110V, 5A => #hrs of use 8 a day => 110V*5 = 550W

Freezer 115V, 2A =>#hrs of use 6 => 115V*2 = 230W

4 fans with total consumption of 250W => #hrs of use 6 a day

I have don't know yet the consumption of the water pump but, I can safely assume it's going to around 1-2KWh and it will run for maybe 6 hrs. These are the heavy equipment I will have on the system and some of them are being used just during a day.

I can adjust my direct consumption from my array during a day to anywhere between 5 and 7KWh. My goal is to harvest at least 5 to 7KWh from my array.
I'm also planning to use only Victron products because I do have a local Victron dealer.

Raj174 · May 2020

Take care using 4 panels in series because the VOC of the 4 series string will add up to almost 150V and some charge controllers will not work over 145V and most fail over 150V. Add up the exact VOC of your panels and look at the specs on the charge controller you will be using for maximum VOC. You really don't want to be even close to max VOC. This would not be an issue with 3 panel strings.

Dave Angelini · May 2020

Your daytime usage numbers sound really low. With some of the most efficient mini-splits (in many homes) I see numbers for an offgrid home in the 20 to 30 KWH during summer sunlight hours in hot places. Even more depending on the differential you program.

BB. · May 2020

diallodjeri said:

Once again thanks for your wonderful input.
Here I tried to sum up my daily consumption from my array. The ACs are mini-split with an inverter.
AC#1: 220V, 11.8A => #hrs of use 8 at night => 220*11.8 = 2596W

AC#2 :220V, 5.9A => #hrs of use 8 a day => 220*5.9 = 1298W

Refrigerator: 110V, 5A => #hrs of use 8 a day => 110V*5 = 550W

Freezer 115V, 2A =>#hrs of use 6 => 115V*2 = 230W

4 fans with total consumption of 250W => #hrs of use 6 a day

I have don't know yet the consumption of the water pump but, I can safely assume it's going to around 1-2KWh and it will run for maybe 6 hrs. These are the heavy equipment I will have on the system and some of them are being used just during a day.

I can adjust my direct consumption from my array during a day to anywhere between 5 and 7KWh. My goal is to harvest at least 5 to 7KWh from my array.
I'm also planning to use only Victron products because I do have a local Victron dealer.

I think you have a confusion between rate of energy usage (Watts and kWatts) and amount of energy usage (Watt*Hours and kWatt*Hours).

The Rate is like km per hour... Watts is a rate (Joules per Second, but we use Hours instead for "reasonable" size numbers).

Watt*Hours is an amount... Like you have driven at 40 km/hr for 5 hours (rate * time) = 200 km total driven.

Or like using fuel... liter per hour for genset usage (size of genset, fuel line, etc.) and Liters total used in a day (2 l/hr * 24 hours = 48 liters of fuel consumed).

220 VAC * 11.8 Amps = 2,596 Watts (rate)
2,596 Watts * 8 hours per day = 20,768 Watt*Hours per day = 20.8 kWH per day

So, just to run that one A/C system, yes, a 3,000-4,000 Watt (3-4 kW) AC inverter would probably work (inverter based A/C usually have very little surge current).

However, you have two battery questions to answer... One is based on how much current/energy the battery can supply on average, and how much energy the battery needs to store for X days of usage (bad weather, etc.).

So, just for a 4,000 Watt 220 VAC inverter running on a 48 volt Flooded cell battery bank--To supply 4 kW running, would suggest a minimum size battery bank of 400 AH @ 48 volts (running A/C + some other small stuff around the place).

And sizing based on the total energy per day (amont) needed... Nominally, suggest 2 days of storage and 50% planned discharge under normal use for an FLA battery bank. Just to run the A/C (assuming worst case rbattery capacity, running A/C at night, charging during the day):

20,768 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge = 2,036 AH @ 48 volt FLA battery bank "optimum"

Then there is sizing the solar array for just the A/C unit support. One is based on battery capacity and 5-13% rate of charge (will use 10% rate of charge for FLA battery full time off grid):

2,036 AH * 58 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge =15,336 Watt array nominal (for battery bank charging spec)

And sizing based on hours of sun per day... Lets assume you are in a part of Africa with a very good amount of summer sun (don't need A/C in winter?). Guess 6 hours per day. Just to run the 1x A/C system:

20,768 WH per day * 1/0.52 off grid AC system eff * 1/6.0 hours of sun per day summer = 6,656 Watt array minimum summer "break even"

So, I would be suggesting around a 6,656 Watt to 15,336 Watt array based on using standard Lead Acid batteries... Other assumptions and changes in usage can reduce some of the battery/array needs... But this is what a full stand a lone, charge during the day, use A/C at night would look like for just one of your systems.

And notice the 0.77 panel+controller "losses", and the overall 0.52 end to end deratings (from solar panel "marketing" numbers through all of the heat/controller/battery bank/inverter losses)... Yes, you only get about 1/2 of the solar panel "marketing" numbers at your 220 VAC loads when all is said and done. Different battery chemistries, running A/C during sunny days only, vs over night, etc. changes the numbers some--But still need to do the load calculations correctly first--Before heading down the next paths...

More or less, your above numbers sort of look like all your loads run 1 hour per day (7 kWatts * 1 hour per day = 7 kWH per day).... When you run your loads 8 hours per day, then their energy needs (size of array, size of battery bank) go up by 8x too. (sort of like a 10 liter tank on your genset to run loads for 1 hour, vs an 80 liter tank to run your loads 8 hours per day between refills).

Does the above make sense?

-Bill

diallodjeri · May 2020

Dave Angelini said:

Your daytime usage numbers sound really low. With some of the most efficient mini-splits (in many homes) I see numbers for an offgrid home in the 20 to 30 KWH during summer sunlight hours in hot places. Even more depending on the differential you program.

The number you see for the ACs are hourly usage and they need to be multiplied by 8 which is 2596*8 = 20,768W during a day and 1298*8 = 10,384W at night.

diallodjeri · May 2020

BB. said:

diallodjeri said:

Once again thanks for your wonderful input.
Here I tried to sum up my daily consumption from my array. The ACs are mini-split with an inverter.
AC#1: 220V, 11.8A => #hrs of use 8 at night => 220*11.8 = 2596W

AC#2 :220V, 5.9A => #hrs of use 8 a day => 220*5.9 = 1298W

Refrigerator: 110V, 5A => #hrs of use 8 a day => 110V*5 = 550W

Freezer 115V, 2A =>#hrs of use 6 => 115V*2 = 230W

4 fans with total consumption of 250W => #hrs of use 6 a day

I have don't know yet the consumption of the water pump but, I can safely assume it's going to around 1-2KWh and it will run for maybe 6 hrs. These are the heavy equipment I will have on the system and some of them are being used just during a day.

I can adjust my direct consumption from my array during a day to anywhere between 5 and 7KWh. My goal is to harvest at least 5 to 7KWh from my array.
I'm also planning to use only Victron products because I do have a local Victron dealer.

I think you have a confusion between rate of energy usage (Watts and kWatts) and amount of energy usage (Watt*Hours and kWatt*Hours).

The Rate is like km per hour... Watts is a rate (Joules per Second, but we use Hours instead for "reasonable" size numbers).

Watt*Hours is an amount... Like you have driven at 40 km/hr for 5 hours (rate * time) = 200 km total driven.

Or like using fuel... liter per hour for genset usage (size of genset, fuel line, etc.) and Liters total used in a day (2 l/hr * 24 hours = 48 liters of fuel consumed).
220 VAC * 11.8 Amps = 2,596 Watts (rate)
2,596 Watts * 8 hours per day = 20,768 Watt*Hours per day = 20.8 kWH per day
So, just to run that one A/C system, yes, a 3,000-4,000 Watt (3-4 kW) AC inverter would probably work (inverter based A/C usually have very little surge current).

However, you have two battery questions to answer... One is based on how much current/energy the battery can supply on average, and how much energy the battery needs to store for X days of usage (bad weather, etc.).

So, just for a 4,000 Watt 220 VAC inverter running on a 48 volt Flooded cell battery bank--To supply 4 kW running, would suggest a minimum size battery bank of 400 AH @ 48 volts (running A/C + some other small stuff around the place).

And sizing based on the total energy per day (amont) needed... Nominally, suggest 2 days of storage and 50% planned discharge under normal use for an FLA battery bank. Just to run the A/C (assuming worst case rbattery capacity, running A/C at night, charging during the day):
20,768 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge = 2,036 AH @ 48 volt FLA battery bank "optimum"
Then there is sizing the solar array for just the A/C unit support. One is based on battery capacity and 5-13% rate of charge (will use 10% rate of charge for FLA battery full time off grid):
2,036 AH * 58 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge =15,336 Watt array nominal (for battery bank charging spec)
And sizing based on hours of sun per day... Lets assume you are in a part of Africa with a very good amount of summer sun (don't need A/C in winter?). Guess 6 hours per day. Just to run the 1x A/C system:
20,768 WH per day * 1/0.52 off grid AC system eff * 1/6.0 hours of sun per day summer = 6,656 Watt array minimum summer "break even"
So, I would be suggesting around a 6,656 Watt to 15,336 Watt array based on using standard Lead Acid batteries... Other assumptions and changes in usage can reduce some of the battery/array needs... But this is what a full stand a lone, charge during the day, use A/C at night would look like for just one of your systems.

And notice the 0.77 panel+controller "losses", and the overall 0.52 end to end deratings (from solar panel "marketing" numbers through all of the heat/controller/battery bank/inverter losses)... Yes, you only get about 1/2 of the solar panel "marketing" numbers at your 220 VAC loads when all is said and done. Different battery chemistries, running A/C during sunny days only, vs over night, etc. changes the numbers some--But still need to do the load calculations correctly first--Before heading down the next paths...

More or less, your above numbers sort of look like all your loads run 1 hour per day (7 kWatts * 1 hour per day = 7 kWH per day).... When you run your loads 8 hours per day, then their energy needs (size of array, size of battery bank) go up by 8x too. (sort of like a 10 liter tank on your genset to run loads for 1 hour, vs an 80 liter tank to run your loads 8 hours per day between refills).

Does the above make sense?

-Bill

The number above for ACs are meant to be an hourly usage, therefore they need to be multiplied by 8 in my case.
2596*8 = 20,768W during a day and 1298*8 = 10,384W at night. The big AC will not run on battery, but the small one will at night.
There's another thing I need to mention is the number of day of back up power can be zero because we rarely have a day with no sunshine and even we do, I can manage that.

The battery I'm thinking of getting has these specification:

Voltage: 48V

Capacity: 16kWh

Configuration: 7S8P (G1)

BMS Integrated

Maximum Current Discharge: 480A

Maximum Current Charge: 320A

Fuse Protection: 8/100A

Capacity: 16kWh

Configuration: 7S8P (G1)

BMS Integrated

Maximum Current Discharge: 480A

Maximum Current Charge: 320A

Fuse Protection: 8/100A

Dave Angelini · May 2020

Makes a bit more sense now thanks! It was not clear, to me...However if you have anything in Africa like most of us do in hot places, your 8 hours of daylight cooling runtime, is low. When we get hot it often is closer to 12 or 13 hours. Alot depends on the building size, efficiency, and the number of the people. The quality of the Split plays a large part. For offgrid you want the best model. There are often 4 price ranges on the same size split from the same company.

With a big enough arrays, and some virtual tracking to get long hours of sun power, you can stay off the battery longer. Probably over 8kw of solar to start and more later to fine tune. You really do not have numbers until the house is running. The math will help but offgrid is often different because it is so personal and situational. Bill is amazing at trying to eek out good numbers but an envelope is what it is.

The battery has support where you are located? I try to use 2 or more banks to get some sort of failure protection if it is critical. To me cooling is in that area but many can get by. It should at least be thought out on how you will get by. Especially if you are remote like most of us offgrid are. Victron integrates well with it?

Since you have the grid 1/2 the day or more, you might use another strategy if the grid is daily dependable. You can charge with grid and reduce the solar. Victron has self consumption modes or zero sell modes that are there. Check them out.

diallodjeri · May 2020

You're right about my day time cooling runtime is a bit short, but I don't have much of choice otherwise the temperature routinely goes well above 100F during a day and only goes down to 90F to 80F at night.
Unfortunately, the battery won't have any local support where I live. The mini-split I will be using is SHARP destined to the Middle East market.
In your opinion, it is better to get two 8KW battery or one 16KW battery or it doesn't really matter?

Dave Angelini · May 2020

LFP batts, I assume have a BMS, electronics in a battery are a fairly new way to lose power offgrid. So, yes I always divide a LFP battery for a spare, just in case. With your utility power being intermittent you do have other options that a Offgrid home does not. It is a personal decision that you have to make. What would you do? Be hot until the power comes back, run a genset, put a lock on your refrigerator?

We have 100F+ and wildfire smoke 2 to 4 weeks a year. It is a no brainer for me to have triple back-up to anything important. Just like a boat on the ocean. Many of my offgrid clients feel that way. A few are content to just light a candle and worry about it later. People are all different.

Good Luck to you!

diallodjeri · May 2020

Dave Angelini said:

LFP batts, I assume have a BMS, electronics in a battery are a fairly new way to lose power offgrid. So, yes I always divide a LFP battery for a spare, just in case. With your utility power being intermittent you do have other options that a Offgrid home does not. It is a personal decision that you have to make. What would you do? Be hot until the power comes back, run a genset, put a lock on your refrigerator?

We have 100F+ and wildfire smoke 2 to 4 weeks a year. It is a no brainer for me to have triple back-up to anything important. Just like a boat on the ocean. Many of my offgrid clients feel that way. A few are content to just light a candle and worry about it later. People are all different.

Good Luck to you!

Thank you!!

AN OFF-GRID SYSTEM

Comments

Categories