Help understanding MPPT controler and watt output

ggeisler
ggeisler Registered Users Posts: 4
I am trying to wrap my brain around what my final output would be in the following scenario.

first a little background of what i currently have

I currently have a small system with 3 12v 100 watt panels

this is connected to a 30A PWW charge controller

this is connected to 6 35AH 12V Sealed Batteries connected together in parallel giving me a total of 210 AH at 12V

now i am looking into getting a MPPT charge controller and ultimately increasing the number of PV's.

As I understand it my current system as it stands is pretty much maxed out. If i were to add one more panel (12V 100 Watt) it would effectively put my Amperage to 33.3A Maxing out my charge controller

Now this is one of the reasons I want to go to MPPT. but am a bit confused.

the MPPT controller i have been looking at states it is a 40A 12v a 500W charge current at 12V and 1000W for 24Volt

I am assuming that if i keep everything at 12V i could have a total of 5 panels and the controller would be maxed out. (5*100/12=41.66)

Now if i change the panels to series / parallel to a 24 volt system this would effectively drop my wattage in half.

In order to receive the same amperage on 24 volt i would need 20 panels (20*50/24=41.66)

Now Here is where my confusion is.

As I understand the way MPPT controllers work it will sense my 12V battery system and down convert the voltage to the optimum voltage for my system. taking the 24V to somewhere more acceptable for my bank. By doing this will it increase the wattage?

so if i were to take my above example, would it take the 24V input, drop it down to 12V and double my Watts?

If this were true would my maximum wattage be 2000 Watts? or would the maximum 24V wattage of 1000 limit my setup to 10 Panels?

If limited to 10 panels on a 24V system i would still get double the amount of wattage from this configuration correct?

This is all speculation and I'm sure not very clear so if anyone could help clear this up for me I would greatly appreciate it.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Help understanding MPPT controler and watt output

    Welcome to the forum Ggiesler.
    ggeisler wrote: »
    I currently have a small system with 3 12v 100 watt panels

    "12 volt panels" really have Vmp (voltage maximum power) around 17.5 to 18.6 volts or so to properly charge a 12 volt lead acid battery over most conditions (hot solar panels/cool battery bank).
    this is connected to a 30A PWW charge controller

    this is connected to 6 35AH 12V Sealed Batteries connected together in parallel giving me a total of 210 AH at 12V

    You really do not want to parallel that many lead acid batteries together if you can avoid it. I prefer a single battery (12 volt 210 AH) or a pair of 6 volt 210 AH batteries in series for 12 volts at 210 AH. If you have paralleled batteries, 2-3 strings in parallel is about the typical maximum that works well for most people. If paralleling batteries, review this website.
    now i am looking into getting a MPPT charge controller and ultimately increasing the number of PV's.

    MPPT controllers are more expensive and will "match" the Vmp/Imp of your array to your battery bank's working voltage... Sort of like the DC version o a variable transformer (technically a switching down converter or DC switching power supply inside the MPPT controller).

    While a MPPT controller can increase current in very cold weather with a "12 volt" array (by 10-15% in subzero weather)--Where they really work will is when you run a Vmp-array 2x or more battery voltage. Allows you to use much smaller gauge copper wire (save money) and/or install the solar array much farther away from the charge controller+battery bank.

    MPPT controllers also allow you to use GT solar panels with "non-standard" Vmp ratings (typically around 30-40 volts) and efficient take high voltage/low current of the array to the low voltage/high current needed to charge the battery bank.

    Those >200 Watt GT Panels are usually much cheaper (and larger wattage)--And you can save money overall (cheaper panels+more expensive MPPT controller) vs a "12 volt"+PWM system (12 volt panels are usually more expensive vs GT panels).

    There are some other advantages too--MPPT controllers can have many more optional connections and various software configuration choices.
    As I understand it my current system as it stands is pretty much maxed out. If i were to add one more panel (12V 100 Watt) it would effectively put my Amperage to 33.3A Maxing out my charge controller

    You need to look at the Manual for your controller and the Vmp/Imp of your array. I am not sure I understand the 33.3 amps for your array--But really need the exact ratings/configuration of your array to be sure.
    Now this is one of the reasons I want to go to MPPT. but am a bit confused.

    the MPPT controller i have been looking at states it is a 40A 12v a 500W charge current at 12V and 1000W for 24Volt

    Need to know which controller you are looking at... There are many choices--But usually you get what you pay for... $300 to $600 for a full featured/larger MPPT controller. There are some import "MPPT" controllers that are very cheap and either do not work well are are sometimes really PWM controllers inside.
    I am assuming that if i keep everything at 12V i could have a total of 5 panels and the controller would be maxed out. (5*100/12=41.66)

    The Vmp of your panels is probably around 17.5 volts:

    500 Watts / 17.5 volts Vmp = 28.6 amps

    Just under the maximum rating of your PWM controller (check the manual--Sometimes there are other limits/deratings used).
    Now if i change the panels to series / parallel to a 24 volt system this would effectively drop my wattage in half.

    4x100 watt "12 volt" solar panels in parallel:

    17.5 Vmp * 5.7 amps Vmp (for 100 watt panel)
    4*5.7 amps = 22.8 Amps @ 17.5 volts Vmp

    2 panels in series then two strings in parallel:

    2*17.5 volts = 35 volts Vmp array
    2*5.7 amps = 11.4 amps Imp array

    Note that Power = Voltage * Current. If you have 2x the voltage and 1/2 the current, you still have the same power.
    In order to receive the same amperage on 24 volt i would need 20 panels (20*50/24=41.66)

    Same amperage * 2x 12 volts = 2x the Wattage from the array.
    Now Here is where my confusion is.

    As I understand the way MPPT controllers work it will sense my 12V battery system and down convert the voltage to the optimum voltage for my system. taking the 24V to somewhere more acceptable for my bank. By doing this will it increase the wattage?

    Configuring 4 panels in parallel vs 4 panels in series/parallel (2 series, 2 parallel strings)--Does not really change anything between PWM and MPPT. Still about the same amount of power (unless you are talking about sub freezing weather, where the very cold panels and Vmp can extract 10% more power or so).

    The advantage is you can have an array with 2x the voltage and 1/2 the current and basically use 1/4 the diameter (minus 6 AWG sizes) smaller copper wire from array to charge controller. Depending on size of system and distance covered--This may or not be a "big deal" to you.
    so if i were to take my above example, would it take the 24V input, drop it down to 12V and double my Watts?

    No.

    If you put a "24 volt" array on a PWM controller charging a 12 volt battery bank, the PWM+battery will use only about 1/2 the wattage of the array (because of the way a PWM controller works).
    If this were true would my maximum wattage be 2000 Watts? or would the maximum 24V wattage of 1000 limit my setup to 10 Panels?

    If limited to 10 panels on a 24V system i would still get double the amount of wattage from this configuration correct?

    This is all speculation and I'm sure not very clear so if anyone could help clear this up for me I would greatly appreciate it.

    You have asked a lot of questions, and made some assumptions that are not really correct. It makes a bit difficult to give you "the right answer" because of the differences in the way MPPT vs PWM controllers work.

    If you want more information on MPPT vs PWM controllers--We can do that.

    Or, you can tell use about your needs (what loads you are powering, Watts or Amps peak at what voltages, Watt*Hours or Amp*Hours per day, etc.) and where it will be installed (how many hours of sun per day by season). And we can help you design a system that will meet those needs (on paper). Then, at that point, we can start talking about hardware choices.

    The hardware between a "small" DC only solar power system and a "large" AC+DC off grid power system can be quite different... Sort of like talking about choosing options for a truck--Without knowing if the parts will be for a compact pickup or an 18 wheel semi-tractor+trailers.

    If we size the system first--Then we have a better idea of what components to pick for your installation. And it limits the scope of the discussion to those things of interest for your application--And not the theory of the universe type discussions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Help understanding MPPT controler and watt output
    As I understand it my current system as it stands is pretty much maxed out. If i were to add one more panel (12V 100 Watt) it would effectively put my Amperage to 33.3A Maxing out my charge controller

    Not likely.
    A 100 Watt "12 Volt" panel will put out <6 Amps. Four of them would be 24 Amps max, not 33.3.

    Simple difference:
    For a PWM controller the array has to be at the proper Vmp for the system Voltage. Only the current (Imp) is passed.
    For an MPPT controller the array has to be at or above the Vmp for the system Voltage. Wattage is converted to current @ Voltage.

    Four 100 Watt panels with a Vmp of 17.5 and Imp of 5.7 will put out 22.8 Amps on a PWM controller.
    The same four panels on an MPPT controller will put out (400 * 0.77 efficiency {typical} / 12 Volts minimum) 25.6 Amps.

    A bit about array configuration on MPPT controller which may help: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller
  • ggeisler
    ggeisler Registered Users Posts: 4
    Re: Help understanding MPPT controler and watt output

    Thank you for you prompt and informative answers.

    I believe you have answered my primary question. If I have (for example) 2 X 12V 100 Watt solar panels connected in series it will give me an output of 24V at 100 Watts. Once connected to the MPPT controller. The controller will "transform" this energy to a lower voltage for charging my batteries and increase the wattage back to ~ 200 Watts (depending on voltage).

    I live in a rural community in Alaska where power is expensive ($0.59 Per Kwh) My monthly bill is always more $300 a month and I keep everything turned off. All of my lights have been replaced with LED.

    Because of where i live there is no gas alternative so all appliance like the stove is electric. I know that the stove is my biggest energy sucker so i use it sparingly. I have purchased an 1300 watt induction cook top to speed up my cooking.

    I am not looking to go off the grid as there are some things that i will not be able to operate no mater the size of my setup (Boiler, Electric Clothes Dryer, etc) I just want to reduce my day to day usage. I also know that in the winter the sun only comes up for a few hours a day and the solar panels will not produce enough power to keep things running so I will not be able to rely on solar as my primary power source. The summer however we have extremely long days where I will be able to benefit.

    A friend of mine had a few panels that he bought and never used so i got them for a steal and are the same as these on amazon
    http://www.amazon.com/RENOGY-Monocrystalline-Photovoltaic-Battery-Charging/dp/B009Z6CW7O/ref=sr_1_1?ie=UTF8&qid=1408636043&sr=8-1&keywords=renogy

    He has three of these and this controller
    http://www.amazon.com/RioRand-intelligent-controller-regulator-CM3024Z_30A_12-24-V1/dp/B004YSFG6C/ref=sr_1_10?ie=UTF8&qid=1408636541&sr=8-10&keywords=30a+pwm+solar+charge+controller

    When i purchased this kit i started using it right away with an old car battery. the battery had at least one dead cell and did not really do much good as it did not hold a charge very long. so I went out and purchased 6 of these Wheelchair batteries. 12V 35AH Group U1 Deep Cycle Sealed Battery
    http://www.amazon.com/gp/product/B00A82TUW4/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

    I know that it is better to have fewer batteries but in location the local store's cheapest lead acid battery is around $300 and if ordering online or elsewhere the shipping makes it counter intuitive (everything here needs to be flown in). Purchasing the above batteries gave me a sealed battery at a low cost and free shipping.


    The MPPT controller I am looking at currently is the Renogy® Tracer 4210 40 Amp MPPT Charge Controller 12/24V 100VDC Input
    http://www.amazon.com/gp/product/B00E7NI9PE/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

    What is possible with this controller? would 5 X 100 watt panels @ 12V (500W) be its maximum? and 10 at 24V (1000W)?


    I have not done all of the wattage calculations of my load but currently what i intend to use are the following

    4 X 12V LED light strings ~ 50Watts
    2 X 110V LED Light fixtures 1 the runs at 7 watts and the other at 11
    1 Freezer that runs 50 Watts but peaks to 900 watts on start up.
    1 X 1300 Watt induction cook top for maybe 30 min a day. (I run it on a setting that only uses 600 to 700 Watts)

    The below are the things i would like to connect but have not looked up wattage or power draw to know if my ststem is capable (yet)

    My Laptop
    My TV
    My surround sound (probably not as it is 800 Watts)
    My desktop PC
    Some wireless repeaters, routers, wireless access points.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Help understanding MPPT controler and watt output
    ggeisler wrote: »
    Thank you for you prompt and informative answers.

    I believe you have answered my primary question. If I have (for example) 2 X 12V 100 Watt solar panels connected in series it will give me an output of 24V at 100 Watts. Once connected to the MPPT controller. The controller will "transform" this energy to a lower voltage for charging my batteries and increase the wattage back to ~ 200 Watts (depending on voltage).

    Not quite. The Wattage stays the same. Watts = Volts * Amps. Only the Volt * Amps change. Two 100 Watt "12 Volt" panels in series is still 200 Watts. The change is that instead of it being 17.5 Volts * 11.4 Amps it is 35 Volts * 5.7 Amps.
    I live in a rural community in Alaska where power is expensive ($0.59 Per Kwh) My monthly bill is always more $300 a month and I keep everything turned off. All of my lights have been replaced with LED.

    Because of where i live there is no gas alternative so all appliance like the stove is electric. I know that the stove is my biggest energy sucker so i use it sparingly. I have purchased an 1300 watt induction cook top to speed up my cooking.

    I am not looking to go off the grid as there are some things that i will not be able to operate no mater the size of my setup (Boiler, Electric Clothes Dryer, etc) I just want to reduce my day to day usage. I also know that in the winter the sun only comes up for a few hours a day and the solar panels will not produce enough power to keep things running so I will not be able to rely on solar as my primary power source. The summer however we have extremely long days where I will be able to benefit.

    You should make inquiries as to whether or not grid-tie systems are allowed. Off-grid solar is even more expensive than your utility rate, so there would not actually be any economic benefit. The electric bill would go down, but the cost of producing off-grid over the time of the equipment's life will actually be higher.

    With a grid-tie system the production cost would be competitive to the utility rates. Especially if you can make use of the tax credit incentives that seem to be available in the USA.

    That Tracer controller is of questionable quality. Basically 40 Amps on a 12 Volt system would be around 600 Watts and on a 24 Volt system about 1200. But this is the wrong way to go about sizing an off-grid system: the power comes from the batteries and the batteries are recharged by the solar. If you limit the charging you limit the battery bank size and thus the power available.

    For example: that 600 Watt cooktop, if it draws 600 Watts continuously (which it probably doesn't) would consume 300 Watt hours in 30 minutes. On a 24 Volt system that works out to about 15 Amps hours, meaning a minimum 30 Amp hour battery. For which you'd want 3 Amps charging current and that needs about 100 Watts of panel right there. That is just one item.

    Buy a Kill-A-Watt meter and start measuring everything you plug in under normal usage. It will give you some real consumption numbers to contemplate and will help identify any power hogs.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Help understanding MPPT controler and watt output
    ggeisler wrote: »
    I believe you have answered my primary question. If I have (for example) 2 X 12V 100 Watt solar panels connected in series it will give me an output of 24V at 100 Watts.

    NO! I think Bill tried to explain, but the basic equation is Amps x Volts = Watts

    So if you have 2 - 100watt 6 amp panels, in series they make an array of 24(nominal 35vmp) volts at 6 amps or 200watts, in parallel they make an array of 12volt(nominal 17.5vmp) at 12amps or 200watts.

    How I'll read the rest of your post...lol
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • ggeisler
    ggeisler Registered Users Posts: 4
    Re: Help understanding MPPT controler and watt output

    A light bulb just went off. Thanks for that. It is amperage that divides in two.

    What happens when it is connected to a MPPT controller? Does the amperage go back up when the voltage is decreased?
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Help understanding MPPT controler and watt output

    Might look through the Energy Use and Conservation section of the forum. Those of use who are off grid tend to really scrimp on energy usage, for instance you might hang your clothes even inside, to dry, toss them into the dryer without heat for a few minutes will 'relax them' a bit if you can't get use to the crisper feel. I allow food to cool in the storage container before putting it into the cold fridge. Use you laptop for primary computing, even consider giving up your desktop for a screaming laptop, since a laptop is designed to run off a battery even a screaming laptop will run on 1/3 or likely less the wattage of a desktop, you can use a large energy efficient desk top monitor and come out way ahead.

    My next 'solar' project might be to do an insulated roof over adding R12 to my roof, allowing me to stay cooler in the summer, warmer in the winter for the same energy, though in Alaska, I suspect this is old territory.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Help understanding MPPT controler and watt output
    ggeisler wrote: »
    What happens when it is connected to a MPPT controller? Does the amperage go back up when the voltage is decreased?

    Here is an explanation
    .

    I have and use only MPPT at home now, but personally like PWM for a lot of uses. Most explanations don't include the charging cycle of batteries, since most off grid system try to stay in the top 20% of battery capacity, MPPT charge controllers are less effective than most believe, since above 80-85% your battery start excepting less charge and defeats the advantage.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Help understanding MPPT controler and watt output
    ggeisler wrote: »
    A light bulb just went off. Thanks for that. It is amperage that divides in two.

    What happens when it is connected to a MPPT controller? Does the amperage go back up when the voltage is decreased?

    In a word, yes.
    Watts Out = Watts In minus losses and controller power.

    So you may have 35 Volts @ 5.7 Amps going in but 14.6 Volts @ 13.6 Amps coming out (not exact math).

    The biggest advantage to MPPT is not the 'extra' current (typically 10% more during Bulk) but the greater flexibility in array design.
  • Alaska Man
    Alaska Man Solar Expert Posts: 252 ✭✭
    Re: Help understanding MPPT controler and watt output

    Call GVEA and make sure you qualify for their SNAP progam. (I'm assuming you're in the interior) I'm off grid because of the bull-chips you have to get through to hook up with them. Way more expensive then giving them the finger, getting a generator, batteries and buying a gas range.

    However since you are already hooked up to the grid, I would see where else you could save money. It will be far less expensive then trying what you propose. We hang our clothes out to dry, we use propane to heat food and water, bought a Danby fridge, LED lights, LED T.V., everything is on a power strip and all ghost loads on appliances are cut out when not in use. and most of all, don't use electric to produce heat of any kind.

    A Kill-A-Watt meter will tell you what in your house is driving up the electric bill.

    I was happy to get out from under that GVEA bill. On average, after about $13,000 invested over time for my system, we burn around $26/mnth in genny fuel (This is the wettest summer in history up here) to run the genny on the days it's needed. In the winter at the worst part of the cold and dark we burned about $80/mnth in genny fuel last winter charging batteries. So much better than the typical $300-$500 electric bill of most folks in the interior.

    When I was still at the farm we had electric bills up over $750/mnth. It was there I grew to dislike GVEA with some intesity.

    Good luck and if you want some local help, Eddy at http://www.renewableenergysys.com/ is very helpfull. They are also running a special 6 month same as cash financing deal.

    Tell him Rob from up on the Elliott sent you. ;-)

    And I still need more panels, going solar isn't cheap, but in my opinion it's worth it. I like the peace of mind and smiled a lot last winter when all those storms knocked out the power, but my house still glowed yellow.



    Posted on January 20, 2010 by Laurel Passera
    On January 15 the Alaska RCA issued an order adopting finalized net metering rules. As we mentioned in previous IREC posts, these regulations allow net metering up to 1.5% of each electric utility’s average retail demand (roughly equivalent to 1% of peak demand). Systems must be owned or leased, and operated, by the consumer, and have a cumulative nameplate capacity 25 kW or less. Net metered systems must be used primarily to offset part or all of the customer’s requirements for electricity and be controlled by an inverter or switchgear. No month-to-month rollover is allowed of excess credits, rather month-end excess is credited at the utility’s avoided cost rate on the customer’s next bill. Utilities may not assess additional charges or fees for net metering customers and must pay for the purchase and installation of any additional metering equipment that they deem necessary.

    One additional issue to note is that, according to a footnote in the rules, only 5 of the 21 communities served by Alaska Power Company are subject to the rule. For a list of utilities affected by this rule, please see the attached table:

    Alaska Utilities Required to Allow Net Metering
  • ggeisler
    ggeisler Registered Users Posts: 4
    Re: Help understanding MPPT controler and watt output

    Wow, only $80 fuel to recharge in the winter with your Generator. I live in Bethel AK and fuel here is a little over $7 a gallon. Do you have to have a special 12V generator or do you just use an AC generator and hook it up to a charger?

    Also (since everything has to be flown in) there is no way to get propane in the winter. the barges bring them in in the summer but only those small BBQ grill size tanks and it is expensive.

    until a few months ago our utility company was privately owned and cost more then it does now. Fortunately AVEC (Alaska Village Electric Cooperative) has recently taken over. I hope to see reduced rates soon. Im sure as soon as the river freezes and fuel prices go up, so will the power bill.

    Thanks all for the help. I'm sure I will be asking for advice frequently on this forum.
  • Alaska Man
    Alaska Man Solar Expert Posts: 252 ✭✭
    Re: Help understanding MPPT controler and watt output

    Gas last winter was in the high three dollars a gallon range, I would guess about $3.86 on average? I bought a Portable Yamaha EF3000iS 2800 Watt Inverter Generator. It is super efficient running a rated 19 hours at 1/4 throttle on one tank of gas (3.4 gallons). I only bring my batteries to full charge once a week. Under normal conditions running 2-3 hours in the morning and again at night if needed means I can run the gennerator for three or fours days before I have to fill it up again, often more than that.
    On average I bought (3.5) 5 gallon tanks of gas a month is what I figured, plus about 1-1/4 quarts of oil a month. I change the oil like some people change their fish bait.

    I charge with a 150amp charger built into my Xentrex Freedom series inverter.

    I've not been to Bethel, but I would like to visit. Stay Warm up there.