Sizing a MPPT charge controller

[Living Large]

One source says a 1.25 factor in sizing charge controllers is to account for "peak irradiance conditions", and it is required by the NEC.

How do I properly use this factor, if this is true?

The Xantrex 80 600 (rated for 80A output) says connect to arrays of "up to" 4800W. 4800/48 = 100A, and if I multiply that by (1/1.25) or 0.8, I get 80A. Does this reflect the 1.25 factor mentioned, or is it just a coincidence? When they say "arrays up to 4800W" are they in essence saying the peak power coming in can be 4800W, but the nameplate PV would be lower? In other words, the maximum nameplate PV power would be .8 * 4800, or 3840W?

More to the point and put another way, let's say I plan for a 4400W array. Do I divide by the battery bank nominal and multiply by 1.25 to determine the charge controller output capacity required? This would make sense if the purpose is to allow for peak input power.

Edit I saw another example, using the 1.25 factor on the short circuit current of the solar panel array at the input to the controller, and making sure the result doesn't exceed the input current limit of the charge controller (which is 35A for this unit). This makes the same or more sense as checking the output side. Should the factor also be used against the output current, as described above?

It seems like the controller would limit its output, which brings up a whole different important question about these systems - if the array is loaded to operate at its peak output, what happens to the excess power that can't be delivered to the battery? Is not the battery current or voltage carefully controlled per desired profiles for absorb, float, etc? Can the array be putting out maximum power, and the power to the batteries be far lower if they are in float for example? What happens to the excess power (assuming there is no diversion of excess power, and no load on the inverter)? Would the controller move off the curve to a point where only the power required is being delivered by the array?

Thanks!

ps I posted a question a few days ago about a proposed system. I've discovered I skipped over or misunderstood several design details - this is just one. So I'm going back to learn how to walk, before running. Thanks for your patience.

[Sunking]

1.25 is for sizing of the wire for current.

[Living Large]

1.25 is for sizing of the wire for current.

I am not referring to wire sizing. Here are examples of what I saw:

On the input side
Look midway down the page at "Choosing a Controller" in the first paragraph

http://www.homepower.com/articles/solar-electricity/equipment-products/2012-charge-controller-buyers-guide

The phrase "The electric code recommends leaving a 25% margin, that is, for a 47 amp solar array use a 60 amp control rather than a 50 amp control." is on this page"

http://www.backwoodssolar.com/products/charge-controllers

On the output side
Scroll to bottom of page, click "Example 1" which will open a new window. The calculation divides the PV power by the battery voltage, and multiplies by 1.2 to determine output current capacity of controller

http://www.leonics.com/support/article2_14j/articles2_14j_en.php##6

For my setup, I used the Schneider Conext (string sizing) tool, and it allowed me to size a system that exceeded this 25% margin on the input. I have also seen references that manufacturers may incorporate a safety margin in their ratings, so I am bit unsure what to believe at this point.

[Sunking]

I am not referring to wire sizing. Here are examples of what I saw:

On the input side
Look midway down the page at "Choosing a Controller" in the first paragraph
http://www.homepower.com/articles/so...r-buyers-guide

You are talking about this right?

First, limit your choices to controllers that work with your battery bank voltage, which will usually be 12, 24, or 48 V. Then, calculate the approximate maximum amperage your controller will need to handle. Divide the PV array watts by the system voltage to get amperage, then add a 25% safety margin to account for higher irradiance conditions. For example, a 40 A rated controller could possibly handle 480 W of PV into a 12 V battery bank; 960 W into a 24 V bank; and 1,920 W into a 48 V bank. After factoring in the additional 25%, those maximum ratings become 384 W, 768 W, and 1,536 W, respectively.

That is still current on the output side, not input. Any wire size in low voltage systems are determined by maximum voltage drop, not current. When done that way will always be much larger than any code requirement. There design example is complete Fantasy. Using their example of a 12 volt 40 amp controller OK. They call for a STC panel wattage rating of 480 watt panel rating, then down size to 384 watts to add 25% safety factor? I don't what school the writer went to, but the one I went to says 480 watts plus 25% = 600 watts. That will never happen.

I will not bother with the rest of the article because whoever wrote it is an idiot and knows nothing. Both input and output wiring are sized based on maximum current and voltage drop. If you were to use Code Minimum requirement you would loose a significant amount of power on your wiring. Only way around that is to over size the wiring beyond minimum code requirements. Remember Code is not a Design or How To manual. It is minimum safety requirements, not performance based.

Quit reading Home Power Magazine. It is written by readers, not professionals.

[Living Large]

OK, but what is the answer to the question? I do understand about sizing wires, but I am asking about the MPPT.

Let's assume my array has total power Pw, short circuit current Isc and maximum power point current Impp. My batteries are FLA and 48V and x Ah. What capacities in current should the MPPT have on the input side and output side? Thanks! If I get the answer to this question, I can apply it to any design since that is in flux.

[Sunking]

For the input side you take the IMP current at the Vmp voltage and calculate for no more than 2 to 3% voltage drop over the given 1-way wire distance. Example let say you have a 1000 watt panel with an Vmp = 80 volts and Imp = 12.5 amps over a 1-way wire distance of 50 feet and we want to maintain 2% voltage drop maximum. 2% of 80 volts is 1.6 volts. That means the maximum wire resistance is 1.6 volts / Imp, or 1.6 volts / 12.5 amps = .128 Ohms. Now we have to find a wire resistance is .128 Ohms or less at 100 loop feet. That is going to be #10 AWG wire. If you were to use the NEC method you are referring to would be #14 AWG and you would loose about 5% of the power. So what you do is do it both ways and use the larger of the two answers. Only time NEC method will be larger is if the 1-way distance is very short of say around 10 to 20 feet or less. NEC is minimum requirement, not performance. Sometimes NEc works when distances are short, but not for longer distances.

On the output side assuming the distance is short, just take the maximum output current and use NEC tables for open air 90 degree cable. Example 80 amps would require a #8 AWG. If you use the voltage drop method for a short distance of say 5 feet at 80 amps you get #12 AWG which would be very dangerous.

[Living Large]

For the input side you take the IMP current at the Vmp voltage and calculate for no more than 2 to 3% voltage drop over the given 1-way wire distance. Example let say you have a 1000 watt panel with an Vmp = 80 volts and Imp = 12.5 amps over a 1-way wire distance of 50 feet and we want to maintain 2% voltage drop maximum. 2% of 80 volts is 1.6 volts. That means the maximum wire resistance is 1.6 volts / Imp, or 1.6 volts / 12.5 amps = .128 Ohms. Now we have to find a wire resistance is .128 Ohms or less at 100 loop feet. That is going to be #10 AWG wire. If you were to use the NEC method you are referring to would be #14 AWG and you would loose about 5% of the power. So what you do is do it both ways and use the larger of the two answers. Only time NEC method will be larger is if the 1-way distance is very short of say around 10 to 20 feet or less. NEC is minimum requirement, not performance. Sometimes NEc works when distances are short, but not for longer distances.

On the output side assuming the distance is short, just take the maximum output current and use NEC tables for open air 90 degree cable. Example 80 amps would require a #8 AWG. If you use the voltage drop method for a short distance of say 5 feet at 80 amps you get #12 AWG which would be very dangerous.

This is excellent information to have, but not what I am asking. I'm trying to be straightforward - maybe I am asking an invalid question but I don't believe so. My question is how do I size the charge controller, based on the parameters above?

What capacities in current should the MPPT have on the input side and output side?

Charge controllers, at least the ones I have seen, have a rated input current, and maximum output current. This is what I am looking for. Thanks again.

[Sunking]

This is excellent information to have, but not what I am asking. I'm trying to be straightforward - maybe I am asking an invalid question but I don't believe so. My question is how do I size the charge controller, based on the parameters above?

For MPPT Panel Wattage / Battery Voltage = Output Amps period end of story. Input and output are two different voltages and current. Input will always be higher voltage and lower current then the output. The input current depends on how high you run the voltage. Higher the voltage, the lower the current. The only restriction on a MPPT controller is VOC and Panel Wattage. The Panel Wattage is restricted to the battery voltage and output current. For example a MNS Classic 150 is roughly 150 VOC input, and panel input is:

1250 watts @ 12 volts
2500 watts @ 24 volts
5000 watts @ 48 volts.

Charge controllers, at least the ones I have seen, have a rated input current, and maximum output current. This is what I am looking for. Thanks again.

I think you are confusing PWM with MPPT.

With PWM controllers Input Current = Output Current period end of story with an unhappy ending. With MPPT output current = Panel Wattage / Battery Voltage. Only input limitation on a MPPT is the VOC/VMP max limit. I can have 1000 watts input at 150 volts on the input (6.7 amps) to a MPPT controller, and on the output have 12 volts at 80 amps. Do the same with PWM you have 150 volts @ 6.7 amps on the input (1000 watts) and on the output have 12 volts @ 6.7 amps (81 watts). What is wrong with that picture? 1000 watts in, 80 watts out.

Here is the difference with PWM you have to use expensive battery panels. A battery panel operates at 18 volts for each 12 volt of battery. So a 1000 watt PWM system requires a 55 amp PWM controller because 1000 watts @ 18 volts is 55 amps input. We know Input Amps = Output Amps. So we go in at 18 volts x 55 amps = 1000 watts, and on the output 12 volts x 55 amps = 660 watts, or a 37 % loss of power. At 55 amps input is a huge wire to handle the current and limit voltage loss.

With MPPT we can use inexpensive high voltage grid tied panels. I can use 3 330 watt GTI panel with a vmp of 40 volts each for a total of 120 volts input @ 8.4 amps use small inexpensive wire to the input of the controller. So at the input you have 120 volts x 8.4 amps (990) watts on the input, and output 12 volts x 80 amps = 960 watts with only a 3% loss.

A 1000 watt PWM system at 12 volts will cost you $2200 for panels and controller and get robbed 370 watts. A 1000 watt MPPT system will cost you roughly $1600 in panels and controller and save another $200 to $500 in wire, fuses, and combiners because you do not need anything but the smaller wire. In addition you put 1000 watts in and get 970 watts out.

I can take that same 80 amp MPPT controller and input 5000 watts if I want to use a 48 volt battery. The only difference is at 5000 watts @ 120 volts input the current goes up to 42 amps. Output is still 80 amps. Can't do that with PWM.

Takeaway is MPPT is a Power Converter where Power in = Power Out minus around a 3% conversion loss. A PWM controller is a series voltage regulator or a resistor with high losses depending on the input output voltage differential ratio.

[paulcheung]

Dereck,

I believe the OP is try to ask you about the Charge controller size to match the PV array, He read from somewhere that they believe the charge controller should work at 75% of it's rate power.

To the OP, You should not have to worry about the charge controller over load if you size the PV array on there name plate rate and not purposely over size it because you calculated the lost.

You very rarely has the name plate power coming to your charge controller as the PV array always only have about 80% efficiency if you lucky, I live in a tropical country that I only get the name plate power when the sun coming out a sudden rain in the windows from 11am to 1 pm hours, they called it cloud edge. most of the times I only get 60 to 75% of the rated power. So my charge controller always run on it's 60 to 75% capacity.

The Classic 150 is rated 80 amps but it can go up to 96 amps, and the Outback FM60 and 80 can limit the output current to what ever you wants. just size the MPPT controller to the PV array name plate power, you will run the controller at less than 80% most of the time.

I hope this will help you.

[Living Large]

Dereck,

I believe the OP is try to ask you about the Charge controller size to match the PV array, He read from somewhere that they believe the charge controller should work at 75% of it's rate power.

To the OP, You should not have to worry about the charge controller over load if you size the PV array on there name plate rate and not purposely over size it because you calculated the lost.

You very rarely has the name plate power coming to your charge controller as the PV array always only have about 80% efficiency if you lucky, I live in a tropical country that I only get the name plate power when the sun coming out a sudden rain in the windows from 11am to 1 pm hours, they called it cloud edge. most of the times I only get 60 to 75% of the rated power. So my charge controller always run on it's 60 to 75% capacity.

The Classic 150 is rated 80 amps but it can go up to 96 amps, and the Outback FM60 and 80 can limit the output current to what ever you wants. just size the MPPT controller to the PV array name plate power, you will run the controller at less than 80% most of the time.

I hope this will help you.

Yes - this is exactly what I was looking for. Thank you.

[Wy_White_Wolf]

Not all MPPT controllers are made the same so you have to follow manufacturers instructions on sizing and usage. There is no standard/code answer we can give you.

Basic answer is that your array wattage divided by battery voltage has to be less than controller size but some controllers even allow that to be exceeded under the idea that panels seldom operate ate there rated capcity. The other thing to pay attention to is the string VOC of the array. It plus any highlight voltage increase must remain below input voltage of the controller. Here (Wyoming) code requires us to derate the controller input voltage by 1/3 to compensate for the cold temp increase.

WWW

[Living Large]

Not all MPPT controllers are made the same so you have to follow manufacturers instructions on sizing and usage. There is no standard/code answer we can give you.

Basic answer is that your array wattage divided by battery voltage has to be less than controller size but some controllers even allow that to be exceeded under the idea that panels seldom operate ate there rated capcity. The other thing to pay attention to is the string VOC of the array. It plus any highlight voltage increase must remain below input voltage of the controller. Here (Wyoming) code requires us to derate the controller input voltage by 1/3 to compensate for the cold temp increase.

WWW

I was starting to believe this as well - that there may be no standard and digging into a specific manufacturer's recommendations may be in order. Looking at a few, I didn't see guidance. I suppose one could contact a manufacturer for advice, if they are willing to discuss this with non-professionals. My principal concern was I am near the maximum for some models, and wanted to understand correct sizing as well as anticipate the cost of an added or larger MPPT.

As far as the VOC, Amy here helped me understand some things about it that I did not, but I was aware it goes up at temperature goes down and you plan for the highest voltage. Is your Wyoming code procedure a substitute for using the temperature coefficients to predict the VOC at the lowest expected temperature, or in addition to it? For example, let's assume a one panel tall array, and a panel has a VOC of 39V, and a predicted VOC using coefficienct of 45V at -20 deg C. Does Wyoming say you need to handle a VOC of 1.33(39) or 1.33(45)?

[thastinger]

I've been away for a bit but I assume you have calculated your loads? This seems to be quite a large system you're putting together.
I designed and built my off-grid system with help from members here and some books, NEC etc. It has been operating problem free for over 2 years now and performs better than I expected it to but I entered it with the mindset of every watt I could save is a watt of capacity I didn't have to build/pay for.

To address some of your questions. Yes, if the sun is bright but your battery bank is full, your CC display will read very low watts, only what is required to maintain the batteries. Once the absorb cycle is complete at 56ish V, it will immediately drop to the 52V float voltage. Where does the other 4V go? I believe it is dissipated as heat by the CC. CC have large heat sinks and fans and they do get hot to the touch.

[Living Large]

I've been away for a bit but I assume you have calculated your loads? This seems to be quite a large system you're putting together.
I designed and built my off-grid system with help from members here and some books, NEC etc. It has been operating problem free for over 2 years now and performs better than I expected it to but I entered it with the mindset of every watt I could save is a watt of capacity I didn't have to build/pay for.

To address some of your questions. Yes, if the sun is bright but your battery bank is full, your CC display will read very low watts, only what is required to maintain the batteries. Once the absorb cycle is complete at 56ish V, it will immediately drop to the 52V float voltage. Where does the other 4V go? I believe it is dissipated as heat by the CC. CC have large heat sinks and fans and they do get hot to the touch.

Hi. Yes, the very first thing I did was calculate my need. I did this by inspecting the monthly usage for my current house over a few years, and predicting some lighter loads where appropriate in the proposed house. All the appliances are currently propane, but I added a load for an electric refrigerator. I watch tv a lot, but LED tvs lower my current tube tv draw. My largest current (both meanings) load is my well pump, which it will be at this place as well. I use a microwave for 2 to 8 minutes some days. Propane cooking and heating and hot water. Hoping to have radiant heat, with electric circulators. Wood heat supplement. In summer, possibly a mini-split on the lowest setting - 300W. I looked at this pretty carefully, and added some but not a lot of headroom. My intent is to have a "grid-like" existence similar to what I have now, with the addition of the monitoring, maintenance, backup power X 2, etc. Since I currently have two heat sources, I have that backup mentality to an extent already. Making this shift is a bit intimidating though. People here are helping with a lot of advice and I am learning more what to expect and how these systems work.

I currently am planning only 2 days of capacity. I have limited area for panels. Right now, I am around 6.9kWh per day, 4000-4400W PV, 730Ah @ 48V, with 6000W backup generator.

[Sunking]

Dereck,

I believe the OP is try to ask you about the Charge controller size to match the PV array, He read from somewhere that they believe the charge controller should work at 75% of it's rate power.

OK so what? All you worry about is not to exceed the max 100% limit. Any less than 100% is fine. It is not rocket science. A Midnite Solar classic maximum is:

1250 watts @ 12 volts
2500 watts @ 24 volts
5000 watts @ 48 volts