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  • hroldan
    started a topic Why 100 watts are not equal to 100 watts?

    Why 100 watts are not equal to 100 watts?

    Hi there, I have my own solar panel setup, in fact 3 of them. Have read lots of information, pages, forums, watched videos, etc. I'm still confused. This is the sort of question you can find lots of EASY answers, but depends on who you ask. The thing is this is easy to replicate, stay with me please.

    As I read I kept finding information that built conclusions over power and efficiency, I couldn't help to find lots of other people also confused about this. Turns out despite how contradicting this sounds to me, my 120W panel can't power a 60W laptop. Let's expand this in several different ways. The solar panel is located at full sun power, no problem, connected to a charge controller, a battery and one inverter. Yes I can power my laptop but not constantly. The energy goes to the battery and then to the inverter, then to the laptop, it's not enough so the battery will drain until discharged. I know energy is wasted in the process: charging is not perfect, the inverter also wastes energy, there is nothing 100% efficient without power loss, I get it, I understand this.

    But in no way I can imagine 60W being lost in that process (60W used and 60W wasted). Yes I have measured everything.


    Ready? ok, there are forums, pages and videos explaining solar panels and how to produce energy, perfect, easy, but very few approach the topic of WHAT CAN YOU POWER WITH IT. Yes you can find information about 100w panel = you can power 90w electronics!!! well that's not true apparently. There are videos from professionals trying to explain this and they are long, boring, complex, and actually not clear. This sort of thing confused me until I found some sources being clear and straight to the point, saying "100W panel can't power 90W electronics, not even 80, it doesn't work that way". Some explain how the panels produce X watts (voltage x amperage) but those measurements are valid only without a load, and the moment you plug something directly you see the drop, meaning a 100W solar panel can't really power something near 100W.

    I keep reading and I still find confusing sources of information, and most of it: a lot of people making questions, no real answers. It's not like a powerbank for your cellphone: 5V 1A, you can power your phone for about X hours, no way, in solar panels it's about "plug and test". The information on how to calculate the size of your system are kinda confusing to me, and most of them rely on big battery banks, I get it, but doesn't solve the relation between solar power output vs consumption, the numbers seen odd to me.


    As much as I dislike the conclusion I keep finding evidence that if I have a 60W laptop, I won't be able to power it with a 100W solar panel, even at full sunlight. I keep finding contradicting information anyway.

    Anyone care to expand in human terms? thanks in advance.

  • 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.

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

    Example: 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. Didn't make sense to me. 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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  • PNjunction
    replied
    This was interesting so I wanted to see where this was going wrong myself.

    The devil is in the details.

    Somebody out there was being bright. Aha! If the "nominal 12v panel" is based around your common 18-22v, then so are most laptop bricks! Should be an even swapout.

    So lets look at *best case* for the panel's ability to push current and play with P/I*E (Watts / amps * voltage).

    100w panel / 18v = 5.5A output best case. Ok looks good.

    So they think "what about the brick?"

    60w laptop / 18v power brick = 3.3amp current draw. Looks like plenty to spare!

    BUT, instead of 18v power-brick output, they need to use the 12v battery itself as the source for doing the power calculation:

    60w laptop / 12v = 5 amps drawn *at the battery*. We just might make it if the battery is fully charged to begin with!

    But wait - there's more: Inverters aren't perfect, and we normally apply a 10% derating to them at the very least. So now, instead of 60 watts load we have

    66w / 12v = 6 amps current draw. Uh oh - like firing up the LEM on Apollo 13, we got problems.

    So yeah, not only is the panel not able to supply the necessary current to drive the inverter and 18v laptop power brick, there is nothing left to charge the battery either. Battery dies a slow death making up for the loss all the time.

    Heh, and this is assuming perfect solar conditions and wiring. Add solar variables, and it just gets worse obviously.

    The failing point is that the bloggers and whoever else don't know the basics of P/I*E, and most importantly, where you take that measurement / calculation from to even start your power budget.

    Pretty simple, but fun to see how quickly it went wrong.

    Last edited by PNjunction; 07-28-2019, 07:18 PM.

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  • hroldan
    replied
    Originally posted by sdold View Post
    Glad it's working for you and thanks for the update, most folks never come back to tell us how it worked out.
    Thanks to everyone taking the time to explain with examples and numbers.

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  • sdold
    replied
    Originally posted by hroldan View Post
    In short you can't buy a 100W solar panel and use a 60W laptop, yes 40W are wasted even if you have full sunlight.
    Glad it's working for you and thanks for the update, most folks never come back to tell us how it worked out.

    Leave a comment:


  • hroldan
    replied
    Well this is how the story ended. Thanks for the input.

    I know ratings differ from real life, same for solar panels and even for lead acid batteries, but what I meant on the thread goes beyond ratings. In short you can't buy a 100W solar panel and use a 60W laptop, yes 40W are wasted even if you have full sunlight. The details are kind of tricky and related to the electronics, energy measurements and not so much to brands and lab conditions for the ratings, but now that I have full understanding I know how difficult it is to explain, that's why the videos I found on the web are long and still confusing people, it is confusing. Let's just say besides the tricky stuff, an inverter will steal just too much from your system. It's better to work with direct DC to DC converters, they are far more efficient.

    Here are the details of my recent setup, https://www.solarpaneltalk.com/forum...-other-details I'm getting exactly what I need (and more) from a cheap setup.

    Thanks for the input.

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  • Brian53713
    replied
    Like I tried to describe previous post, now that I have 400 watts I just can tip that ground on its leg back with some supports under it to go nearly flat and have other adjustment on the garage door track I used to build it raising it up and down seasonally before I tip it back with some treated lumber under the front of it. So no need to move it around anymore. It was Heavy ish, girlfriend could move it , but she's gone .replaced her with the smart camera, smart outlet to turn the pool pump off and on.

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  • SunEagle
    replied
    Originally posted by Brian53713 View Post
    Ground mounted movable heavy contraption shown in my profile picture. Used to have two panels now 4. 2 Brand new 100 W Renogy panels. As an experiment left them facing North East for a.m. moved them numerous times during the day to run my small pool pump .2 DC fans. TV. Victron100/30MPPT.
    Wow. Do you move your array every day to track the sun or just on the days you run your pool pump?

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  • Brian53713
    replied
    Not sure if that last comment was directed at me, but the measurement of the wattage collected was off the Victron 100/30 Bluetooth dongle info storage

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  • PNPmacnab
    replied
    Was that measured or just taking the pumps rating. There are $10 wattmeters that are quite handy for this. Going back to laptops, I'm using mine right now with the wall wart powered directly from almost exactly 60V DC from the array. I can't advise anyone to do that, but higher array voltages are lovely. Go grid tie panels!

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