DIY Remote Solar Power System

[littleharbor]

The property will never have services installed there. So we need this system to work exclusively as stand alone.
I would rather not add 2 more panels.
You're right, the panels I have sighted are 37.9 VoC (151.6V 4s2p) 30.4 Vmp. That doubles the price of the CC to handle two more panels...

Charge rate needs to be 10-13% 46.2 Amps of Battery Bank 420 Ah. At that amperage I would need to use #6 cable instead of #10.
Voltage needs to be 1 1/2 of Battery Bank Voltage @72V, 113.7 VoC, 91.2 Vmp.
This wasn't mentioned before.

My charge rate is too low at 24.7A when it should be 46.2A with 420Ah BB.
How can current be increased other than changing cable size?

I am noticing some discrepancy here. My MPPT draws panel voltage at the optimum 30.4V (91.2V) which makes 113.7 VoC. 1500W / 91.2V = 16.4A. That is the actual current, lower than the stated 24.7A.
Am I interpreting the stats correctly?

Each string will be the added Vmp. of 3 panels @ the Imp (amperage = 8.24) so 91.2 Vmp. @ 16.48 Isc. total = 1502.976watts. Real world numbers will be lower due to insolation, angle of panels and temperature.The MPPT controller does the voltage stepdown while the amperage rises proportionately yielding 26.79 amps @ 56 volts for example. Again, real world numbers will be less. Lets say you start the bulk charging process at 50 volts, you should be getting around 30 amps from that 1500 watt array. You might want to add another 3 panel string if you want to hit a higher charge rate. You will need to add a fused combiner if you opt for 3 strings.

[-TX-]

I am sorry to tell you this, but you are in so deep, there is no way to help you make this right. Nothing you have is workable, safe, or will do what is expected. I do not think anyone here will help you because those of us that know are not going to help anyone build a fire trap or a system they know will fail.

The very first step is you have to determine daily Kwh usage. There is no other way. 99% of people grossly underestimate only to find out shortly they lost all their money and fin dout they need to spend 10 to 20 times more than they lost to make it work and be safe.

Good Luck.

SunKing, I have read you a number of times go off on inverter wattage and would like to know why. Better said, I'll be at 1600w PV, 100 amp CC, 690ah/24v bank, and, wait for it....... a 3000 watt inverter. How have I forsaken my project?

Isn't an inverter's wattage, up to? Why is it bad to use an iinverter with a higher wattage that would handle seldom seen spikes in usage, instead of using one that would blow in such a situation?

Sorry to interrupt, OP. No way to send private message that I've seen.

[Sunking]

SunKing, I have read you a number of times go off on inverter wattage and would like to know why. .

Because this is in your future the instant you have a loose connection with 150 amps of current flowing. There is a damn good reason you cannot buy an appliance larger than 1800 watts designed to run at 120 volts. It would be to darn dangerous and the manufacture would be sued into bankruptcy replacing burned down houses and dead burnt bodies. That is why cooktops, hot water heaters, air conditioning, and oven run on 240 volts.

3000 watts on 24 volt Inverter pulls 150/75 amps through an inverter. 3000 watts pulls 13 amps on 240 VAC. Your argument is a Red Herring. You buy a new Corvette, go to your Insurance agent and tell him you wil never go faster then 50 mph. So therefore he should not charge as much on premiums.

[SunEagle]

SunKing, I have read you a number of times go off on inverter wattage and would like to know why. Better said, I'll be at 1600w PV, 100 amp CC, 690ah/24v bank, and, wait for it....... a 3000 watt inverter. How have I forsaken my project?

Isn't an inverter's wattage, up to? Why is it bad to use an iinverter with a higher wattage that would handle seldom seen spikes in usage, instead of using one that would blow in such a situation?

Sorry to interrupt, OP. No way to send private message that I've seen.

Two reasons to not to go with an inverter that can deliver 3000 watts of power.

The first is you have no sure way of limiting the amount of wattage that inverter will try to power so you have no way to limit the amperage on the wire except by installing fusing on the DC side to a value that limits how many watts you can load it on the AC side. The side affect is usually blown fuses and wire insulation that is being abused.

The second reason to not use a large inverter is because most are less efficient when they are not loaded and the watts lost are usually a % of the total nameplate wattage. So a 3000 watt inverter with a loss equal to 3% of the nameplate would be 90 watts for every hour the inverter is turned on (90wh) of energy drain on your battery without any load actually being powered.

[-TX-]

Well, now I feel bad because I do not need a 3000w inverter. I considered something smaller, even an inverter alone (instead of solar inverter/charger) but the efficiency ratings between 3000 and less were all pretty much the same. I didn't see the downside of going larger.

I answered Sunking on another thread so as to not interrupt OP anymore.

[Hiro]

"quote"
The first item on this list is an air conditioner, right? Do you know what the BTU rating is? Because about the smallest residential window unit available is 5000 BTU and they use 450 to 500 watts while the compressor is running. At 4 hours with a 50% duty cycle that's 1000wh or 1kWh which only adds a little over 500wh to your total, but the problem is the surge amperage. The surge can be 4 to 5 times the running amperage. I don't think a 1000 watt inverter can handle that much surge. But I do know someone who runs a 5000 BTU A/C on an 1800 watt inverter. Food for thought.

I used my split system on a powermeter to get that figure. The Hi (surge) Watt reading however showed 1461W. My aircon used 4.6kWh over 7 days. He hasn't bought the aircon yet and I don't know if he will ever install one? The systems I have looked at were 20k BTU at 1.6kWh which is unfeasible for him.
My guess, if I know him well enough, is he will resort to a fan... I've emphasised and reiterated he NEEDS TO, he MUST KNOW what he will be using there, no ifs. Cos once the system is up, you cant add appliances later on.

Currently I'm working on the system providing 5800Wh/day. From the powermeters I have, its safer to calculated kWh not Watts and do it over 7 days as Sunking recommended.

[Hiro]

There are two very good reasons to use MPPT Charge Controllers.

1. It allows you to use much higher voltage GT Panels. Low voltage battery panels cost some 2 to 5 times more watt than GT panels.

2. 95% efficient vs 66% of a PWM controller.

The system will have an MPPT whichever way i go. No doubt about it.
But what do you call higher voltage? If an MPPT is maxed at 50A 150V, that limits what kind of panels you use in what configuration unless you spend bigger $$$ to buy a bigger CC.

I'm beginning to see a quandary occurring here or a math problem for me
My system is low on amperage at 91.2V 16.48A from panels to MPPT. Charge rate for my 420AH BB requires it to be 46-50A. Voltage needs to 1 1/2 x of 48 @ 72V. If I use this formula 72 volts x 48 Amps = 3456 Watts. This means I have to go a #6 AWG Cable.
That is double the wattage from the PV Array.

Adding another 3P string adds 750W = Total PV Array 2250W. Required Watts is 3456W @ 72V. A 3P string still won't work!
Can the MPPT deliver more power than it gets?

[Mike90250]

..... Because this is in your future the instant you have a loose connection with 150 amps of current flowing. ....

Nice pic Thanks. My friend used to melt terminals off the batteries in his EV going up hills, and would have the shop re-cast the terminal the next day,

[littleharbor]

Reread post #61 again. The amperage going into your controller and amperage coming out are not the same. One more series string will give you under 25 amps into the controller and potentially up to 45 amps out to your batteries.

[Sunking]

Can the MPPT deliver more power than it gets?

No Sir. Let's go to extremes here because you are not getting the Math

MPPT OUTPUT CURRENT = PANEL WATTAGE / BATTERY VOLTAGE

There are MPPT Controller out there that can input 600 volts to charge a 12 volt battery. So lets say we use Thin Film panels rated 100 watts at 100 volts at 1 amp. We wire 5 of them in series and end up with 500 volts @ 1 Amp input to the controller. We buy ourselves a 40 amp MPPT controller rated at 600 volt input. DO THE MATH, what is the current on the output?

500 watts / 12.5 volts = 40 Amps. No magic, just simple 5th grade math. We go in at 500 volts @ 1 amp, and out 12.5 volts and 40 amps

Little more math

Watts = Volts x Amps

500 volts x 1 amp = 500 watts
12.5 volts x 40 amps = 500 watts.

No magic, just simple Ohms Law math.

Now here is some fun.

PWM Output Current = Input Current.

What happens if we used those same panels wired in series. 500 volts @ 1 Amp input you get 12.5 volts @ 1 amp output. Do the math

500 volts x 1 Amp = 500 watts input
12.5 volts x 1 Amp = 12.5 watts output.

Where did all the power disappear to? That is exactly what you get with PWM.

Why do utilities use high voltage. Example a common service is 200 amps @ 240 volts. The utility secondary distribution uses several voltages but a common voltage is 13,200 volts. So they hang a transformer on the pole to step sown the voltage. Transformers like MPPT Controllers voltage and current are inversely proportional. Such a transformer would step down the voltage from 13,200 volts to 240 volts or a ratio of 55 : 1, thus it steps up the current to 1 :55. Now think about that for a minute. Assuming you draw the full power of 200 amps, only requires 3.6 amps from the utility. They do it because low voltage SUCKS. Their lines can only carry a few hundred amps. At low voltage the lines would have to be thousands of times larger. That is why utilities transport power at high voltages up to 1,000,000 volts At 1 amp is 1,000,000 watts. 1 amp at 12 volts is 12 watts.

[Hiro]

No Sir. Let's go to extremes here because you are not getting the Math

MPPT OUTPUT CURRENT = PANEL WATTAGE / BATTERY VOLTAGE

There are MPPT Controller out there that can input 600 volts to charge a 12 volt battery. So lets say we use Thin Film panels rated 100 watts at 100 volts at 1 amp. We wire 5 of them in series and end up with 500 volts @ 1 Amp input to the controller. We buy ourselves a 40 amp MPPT controller rated at 600 volt input. DO THE MATH, what is the current on the output?

500 watts / 12.5 volts = 40 Amps. No magic, just simple 5th grade math. We go in at 500 volts @ 1 amp, and out 12.5 volts and 40 amps

Little more math

Watts = Volts x Amps

500 volts x 1 amp = 500 watts
12.5 volts x 40 amps = 500 watts.

No magic, just simple Ohms Law math.

Now here is some fun.

PWM Output Current = Input Current.

What happens if we used those same panels wired in series. 500 volts @ 1 Amp input you get 12.5 volts @ 1 amp output. Do the math

500 volts x 1 Amp = 500 watts input
12.5 volts x 1 Amp = 12.5 watts output.

Where did all the power disappear to? That is exactly what you get with PWM.

Why do utilities use high voltage. Example a common service is 200 amps @ 240 volts. The utility secondary distribution uses several voltages but a common voltage is 13,200 volts. So they hang a transformer on the pole to step sown the voltage. Transformers like MPPT Controllers voltage and current are inversely proportional. Such a transformer would step down the voltage from 13,200 volts to 240 volts or a ratio of 55 : 1, thus it steps up the current to 1 :55. Now think about that for a minute. Assuming you draw the full power of 200 amps, only requires 3.6 amps from the utility. They do it because low voltage SUCKS. Their lines can only carry a few hundred amps. At low voltage the lines would have to be thousands of times larger. That is why utilities transport power at high voltages up to 1,000,000 volts At 1 amp is 1,000,000 watts. 1 amp at 12 volts is 12 watts.

Maybe I'm not asking the questions the right way???

Using my system figures.
Going into MPPT 16.5 amps @ 91 volts (1500W). I need the MPPT to convert it to 48 amps @ 50 volts (2400W). Can the 60 amp MPPT convert this was my question. From your reply it does.

But it requires and there is no way around having to put 9 24V 250W 30.4 Vmp panels 3s3p 24.7 A Imp @ 91.2 Vmp (2247W) into MPPT and 46.8 amps @ 48 volts (2247W) out.
Another string of 3 means $800 plus framing...

Are those figures sounding right?
Why did we come this far to be told I need 9 panels not 6 for a 48V 420 AH Batt Bank?

[littleharbor]

Reread posting #61 again. If you insist on max amperage you need more panels. Not sure where you were told what you have will give you max, optimum amperage. Just because you have a 50 amp controller doesn't mean it automatically charges at 50 amps. You do show a 50 amp controller in your diagram.

[Hiro]

I have re-read post #61. And it wasn't mentioned what amperage I require, nor was any recommendation made by anyone other than formulas provided to calculate certain specs. So I chose the 50 Amp MPPT just from the amperage in the diagram.
Someone could have said a LONG time ago that charge rate should 1 1 /2 - 2 x the Battery voltage. I wonder what else I might be missing?
I hope someone can pick out the problem in my next diagram sooner than later.

But it does mean I can't use #10 wire from MPPT to Batteries...

Thanks littleharbor for clarifying that important detail.

[littleharbor]

From post #55

For GT panels to work in a off grid scenario you use MPPT controllers and series wire panels to a higher voltage, (1 1/2 to 2X) than your nominal battery voltage. When you are pushing this a long distance to your controller you can use higher voltage, up to your controllers limit, to keep amperage low and minimize voltage drop over long distances.

From post #57


Your proposed battery bank, at 420 Ah would like to see from 5 to 13 % charge rate. 5%, or 21 amps, being only good for intermittent ie; weekend use. Any full time off grid scenario wants to see 10-13% so 42 to 55 amps would work best. Charge voltage needs to be set accordingly for flooded or VRLA, (AGM) batteries. Stay away from Gel batteries. They don't work well in off grid situations.

I think these are what you are asking for. I realize this thread is getting pretty long winded but I cant change the chronological order of the postings.

[Sunking]

Maybe I'm not asking the questions the right way???

No you are just not understanding.

Using my system figures.
Going into MPPT 16.5 amps @ 91 volts (1500W). I need the MPPT to convert it to 48 amps @ 50 volts (2400W). Can the 60 amp MPPT convert this was my question. From your reply it does.

You tell me.

MPPT Output Current = Panel Wattage / Battery Voltage.

2400 watts / 50 volts = ??????

I get 48 amps. So a 60 amp MPPT wil work.

But it requires and there is no way around having to put 9 24V 250W 30.4 Vmp panels 3s3p 24.7 A Imp @ 91.2 Vmp (2247W) into MPPT and 46.8 amps @ 48 volts (2247W) out. Another string of 3 means $800 plus framing...

OK so what is your point. It takes what it takes. The sky is the limit when it comes to off-grid budget. That is the choice you are making.

Why did we come this far to be told I need 9 panels not 6 for a 48V 420 AH Batt Bank?

This is getting old, and I am not going to go back and keep running numbers for you. Everything starts with determining how many watt hours you need in a day. You cannot pass GO until you do that.

So let's say you need 4 Kwh per day. Immediately you know you need a 20 Kwh battery. At 48 volts is 20,000 Kwh / 48 volts = 416 AH. So how did you come up with 48 volts @ 420 AH battery? Only way is if you need 4 Kwh per day. It is what it is.

Next you must have enough panel wattage to generate 6 Kwh per day to replace the 4 Kwh per day you use. To find the Panel Wattage you must know the Sun Hours received in the WORSE CASE MOTNTHS. For you that is the Months of June and July. To come up with 2400 watts can only mean you must have used 2.5 Sun Hours.

Again Panel Wattage = [Daily Kwh x 1.5] / Sun Hours

So [4000 wh x 1.5] / 2.5 Sun Hours = 2400 watts. It is what it is.

But I can tell you made a mistake on your website. It is optimized for Grid Tied systems, or maximum Yearly Average. You cannot do that with off-grid, Well you can, it just cost a lot more money. If you change the Tilt Angle to optimize for Winter months, your Sun Hours will be higher than 2.5 hours. That means less panel wattage and less current out of the MPPT. Example if you change the tilt angle and it gives you 4 Sun Hours worse case, then your panel wattage and MPPT go to.

Panel Wattage = [4 Kwh x 1.5] / 4 h = 1500 watts
MPPT Amps = 1500 watts / 50 volts = 30 amps.

If I were you I would look into optimizing for winter months vs yearly average. Could save you several thousand dollars. But it is not my problem, it is yours to figure out. I bet you can probably get at least 3.5 Sun Hours in winter by changing panel tilt. Its your money, not mine.