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Why 100 watts are not equal to 100 watts?

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  • MPPT_Rob
    replied
    I will simplify it for those non-technical people - there are basically 4 components to a Solar Power system:

    1) Solar Panel
    2) Charge Controller
    3) Storage Battery
    4) DC to AC power Inverter

    From my years of experience with Poly type panels they produce about 81-83% of their STC rating in full sun and 77 degrees F air temperature. In the winter on a clear day and very cold the power can approach the STC rating but that is not the norm. Some areas of the world receive more than 1000 W/m^2 sunlight and can be as high as 1300 W/m^2 near the equator or at high altitudes but again that's not the norm.

    Where you can really loose power is in the other 3 components:

    Charge controllers come in 2 types - PWM and MPPT with the later doing DC-DC power conversion and operating at the Maximum Power Point which moves during the day. PWM controllers do not do power conversion so they never operate at the most efficient point and typically loose 30-50% or more of a panels power depending on configuration over what an MPPT controller can extract from the panel. A good controller matters and the ones we make are 98-99% efficient on power transfer.

    The Storage Battery can also loose power if its a Lead Acid type (Deep Cycle, AGM ect.) and I have measured their Energy Return ratio to be at best 70%. This means if you put in 100 amp hours during charge you will only get 70 back on discharge.... Another type of battery I have been using is the Lithium Iron Phosphate for 7+ years now. I also measured this Energy Return Ratio... this battery chemistry is 99.8% at returning what you put in. I metered this carefully several times to make sure I was not getting bad data. After over 7 years our 600 amp hour pack comprised of 24x 100 amp hour lithium Iron Phosphate cells in a 4 series 6 parallel configuration (12 volt setup) was metered at 608 amp hours. brand new it measured 615 amp hours. Can't say enough good things about this battery chemistry. There are many other benefits I will not go into here.

    Finally the DC-AC inverter matters.... cheap ones are maybe 85% efficient and good ones are 90-95%. The Xantrex Pro Watt 2000 I use measured at 95% at 1000 watts output which is half power and the most common average load. At 200 watts is measured 92%. The spec is 90% from the Manufacturer. The no load draw is also real low and measured 0.35 amps on a 12 volt setup (13.4 volts actual due to the Lithium Iron Phosphate Battery)... So about 4.7 watts draw in standby (No Load).

    So in summary using a cheap controller, inverter and Lead based battery you end up with:

    Controller (70%), Battery(70%), Inverter (85%) = 41.65% of the Solar Panels Energy makes it to the AC output !

    Using a good MPPT controller such as ours, Lithium Iron Phosphate Battery, and a good inverter at 92% or better:

    Controller (98.5%) , Battery (99.8%), Inverter (92%) = 90.43% of the Solar Panels Energy makes it to the AC output - or more than 2x the output in Watt-Hours with the same solar panels.


    Cheers,

    Rob
    < moderator deleted signature > Rob, please leave your self promotion at home
    Last edited by Mike90250; 10-08-2019, 02:10 PM.

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  • Supernova532
    replied
    Lucky for me I was not launching the NOAA satellite, just testing my $75 solar panel, typing a message late at night and made a typo. Yes, even engineers make typos. Time to move on. I corrected the typo in the post. Thanks.

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  • Ampster
    replied
    Originally posted by hroldan View Post
    Already checked to unsubscribe from notifications about this thread, but still receiving them... unsubscribed again.
    Could it be a typo?LOL

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  • hroldan
    replied
    Already checked to unsubscribe from notifications about this thread, but still receiving them... unsubscribed again.

    [COLOR=#008000]Mod - I tried, but my level of moderator does not allow me to alter your subscriptions. Sorry, Mike[/COLOR]
    Last edited by Mike90250; 09-08-2019, 02:10 PM.

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  • J.P.M.
    replied
    Originally posted by Supernova532 View Post
    "Originally posted by [B]Supernova532[/B]
    I experienced the same things with my first 100 watt panel having a 18 watt rating
    and 5.6 A maximum current output."

    Yes, typo, not big deal. Iif you read the context of the post anyone would know I was referencing voltage not power.
    Maybe not a big deal to you. Typos or not, others not as astute as you who may take your written stuff as accurate can go astray by your error.

    Typos happen to all of us, me probably more than most, but considerate posters still check a post before pulling the trigger on it and know why.

    I suggest you take the constructive criticism, proof read your stuff more and move forward.

    Leave a comment:


  • Mike90250
    replied
    Originally posted by Supernova532 View Post
    "Originally posted by [B]Supernova532[/B]
    .......Yes, typo, not big deal. Iif you read the context of the post anyone would know I was referencing voltage not power.
    Maybe in horseshoes, typos don't matter. With engineering, terminology has to be precise, if you want precise answers. Several years ago, a joint ESA & NASA Mars lander crashed, because some of the software measured in meters, and some in feet. I'm sure everybody knew what was supposed to happen, but the terminology never made it to the spacecraft properly.

    [I]Hey Bob, I borrowed some of the hold down bolts last night :

    noaa_n_prime_satellite_on_the_floor_700x621.jpg[/I]



    Leave a comment:


  • Supernova532
    replied
    "Originally posted by [B]Supernova532[/B]
    I experienced the same things with my first 100 watt panel having a 18 watt rating
    and 5.6 A maximum current output."

    Yes, typo, not big deal. Iif you read the context of the post anyone would know I was referencing voltage not power.

    Leave a comment:


  • bcroe
    replied
    Originally posted by Supernova532
    I experienced the same things with my first 100 watt panel having a 18 watt rating
    and 5.6 A maximum current output.
    Perhaps you mean 18 VOLT rating? It starts by getting your numbers and your units right, or you are lost.
    Then its about efficiency, energy gets away every step of the way. As someone once said

    You can not win

    You can not break even

    You can not get out of the game.

    Bruce Roe

    Leave a comment:


  • Supernova532
    replied
    Well, I just joined the forum and I have read most of the responses here and I think the moderator, SDOLD summed things up pretty well to answer this post. I experienced the same things with my first 100 watt panel having a 18 volt rating and 5.6 A maximum current output. I typically only got 4-5 amps out of it. Maybe 5 in the cooler weather, but closer to 4 in hot weather. At 4 amps I was doing 48-52 Watts. I recently found using a MPPT controller really improves my efficiency because it converts the unused voltage into usable power with up to 35% Amps. I definitely see a big difference charging at low voltages. So that has helped me. Also, you may check your inverter efficiency. It may be low and costing you extra wasted power. Mine is around 90% efficient, so not too bad. I'd encourage you try the MPPT controller as it will really help your efficiency.
    Last edited by Supernova532; 09-07-2019, 11:57 PM.

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  • hroldan
    replied
    This is tricky, the initial question was clear and while nobody owes nobody anything, it was... a bit of work to see the thread going the wrong direction. The tricky thing is how this can confuse a reader with the same question. I guess at the end the story will repeat itself over and over until the reader buys the stuff whatever ratings are printed, do the math, calculate some safety margins and then see how in real life the numbers just won't match. And it has nothing to do with lab ratings or lab conditions. It's was confusing to me, not it isn't, but it's tricky to explain without realizing it would be easily taken out of context or off topic.

    notifications off. See ya.
    Last edited by hroldan; 07-29-2019, 10:45 PM.

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  • PNjunction
    replied
    Originally posted by hroldan View Post
    The previous situation induces error when people don't know about it and buy solar panels, because those ratings don't work as battery ratings (even if we connect a battery) that's why at the end of the day 100W are not equal to 100W when we consider those ratings.
    That's right - and as Mike has shown, *real world* applications usually de-rate the nameplate ratings for a variety of reasons.

    This is also a common stumbling block for beginners - relying solely on paper / textbook calculations without taking the real-world experience of solar projects into account. There are many such arguments all over the place in other forums aplenty from "armchair engineers" who have no real-world experience, but will defend to the death their paperwork calculations. But yes, sometimes it is so over the top wrong, that this works - it's when it gets in the ballpark that experience comes into play and will save you time and energy.

    Fortunately Mike and many here are not armchair-engineers!

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  • Mike90250
    replied
    Originally posted by hroldan View Post
    ...... that's why at the end of the day 100W are not equal to 100W when we consider those ratings.
    First 100W always is 100W. It's a scientific fact. You can take a 100w panel and repeat the very same test at the same illumination intensity and temperature and get the same 100w

    Now when the salesman opens his mouth, get prepared to be lied to, because it's happening.

    If you have a mis-match between your loading and power your panel can produce, you get a failed system

    PV panels do not behave the same as batteries. Batteries are a voltage source. PV panels are a current source. Two very different things. Unfortunately, you need a electrical engineering background to understand the difference of why they are different..
    reading - https://electronics.stackexchange.co...voltage-source or search " difference between Voltage source and current source"

    In real life, a panel rated at 100w will, in well aimed rooftop conditions, seldom produce more than 80% of nameplate..

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  • hroldan
    replied
    That's useful information.

    New batteries can give X ratings without a load and yes numbers will go down a bit under a load. Older batteries with different internal resistance will give X nice ratings without a load but will go down quite noticeable under a load. It's almost the same with solar panels, they are rated X but it's not a real number. This is way different from what many members discussed here about lab conditions, we are talking about something different. Yes lab conditions are different but that's not the main topic I was referring to.

    The previous situation induces error when people don't know about it and buy solar panels, because those ratings don't work as battery ratings (even if we connect a battery) that's why at the end of the day 100W are not equal to 100W when we consider those ratings.

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  • PNjunction
    replied
    No sweat - we all went through this at one point or another.

    It actually takes TWO volts or more difference to produce current in enough quantity to actually charge. So that simply means to get a battery up to 14.4v and HOLD it there (can change depending upon your battery spec), you need at least 16.4v to get anything really flowing up to that point. In reality, that's a tad low, so most nominal 12v panels have the ability to produce their rated output at 18v.

    Historical note: in the 70's, some panels were purposely set low to say 15 to 16v in an attempt to "self regulate". They failed miserably. Nice idea, but too simple for reality.

    Neeedless to say, batteries don't like to be charged to that high of a voltage - hence the need for the controller.

    Speaking of which, a common mistake you've seen are those who buy a controller, attach it to their panels, and try to make a voltage measurement on the controller output without attaching a battery. Seeing nothing, they proclaim it is bad and go through endless returns.

    Although less of a problem today, some will attach the system backwards and not get any output or poor performance. Attach battery FIRST, and then the panel. Do it backwards, and most controllers will think you have a zero-volt battery and refuse to charge to prevent a safety issue, or go into a failsafe "float only" mode, or just have their controller brains confused. Battery first - then panel.

    I'll say this - to avoid a lot of confusion or speculation, just know (P/I*E). That's it - you'll be 90% ahead of the game. Grab a decent voltmeter, and possibly a small "clamp on" ampmeter that can do DC, and you'll be able to prove to yourself what's going on.

    Leave a comment:


  • hroldan
    replied
    PNJunction, true. The topic is quite complex due to the terms used all around the web.

    [B]Example[/B]: I was looking for a 12V solar panel and a 12V solar charge controller. I was wrong. I know the basics of battery charging and it's clear to me to charge a 12V battery we need at least (minimal) 13V. Cases will vary but in short: whenever we want to charge a battery we need a higher voltage, -always-. Some chargers have boosters or some sort of joule thief that allow using a lower input voltage to charge a larger batter, just like the garden light we have using a tiny 12V battery using a tiny 3V solar panel.

    But let's continue with the example, I knew about those basics, yet in terms of solar products I was looking for a 12v thing to hook up another 12v thing, etc.

    This is where things get interesting, I got my panel rated at some 18V something and 22V something, it was some ultra cheap product on sale, not a fully planed acquisition. So I thought "damn, my 12v solar charger will not work". But later due to research I found most panels for 12v work are rated 18 to 21. [B]Didn't make sense to me[/B]. Then I found the panels have some interesting ratings, like open, closed, etc, meaning they produce whatever voltage without a load, and that's a maximum voltage we can't count on for daily use or battery charge. It's the beginning of the confusion.

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