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Why 100 watts are not equal to 100 watts?

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  • 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, [B]my 120W panel can't power a 60W laptop[/B]. 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 "[I]100W panel can't power 90W electronics, not even 80, it doesn't work that way[/I]". 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.

  • #2
    Well, to try and keep this simple, there are a handful of factors you need to overcome:

    1) 120W panel. If this small "hobby" panel is similarly spec'ed to large residential and commercial PV panels, you'll rarely see 120W from this panel as it is only spec'ed to produced this amount under lab based STC conditions. Real world, you only get 70-80% of this under peak sun assuming your panel is oriented at 180 degrees and the appropriate tilt for your latitude. Even then you'll only get this for a couple hours at most on the sunniest of days.

    2) Charge rates. Not my area of expertise, but depending on how big your battery is (capacity, voltage, type, etc.) you must charge the battery with a certain fairly narrow range of voltages and currents, And the output of a single panel (or even a small handful of panels) may not be able to meet these requirements. JPM or others can give more details here. Long story short, if you're hoping a small, cheap 100W panel will charge a car battery (or deep cycle marine) battery from which you are running a small 300W-1000W inverter. No, this will not work. These panels will not be able to keep the battery charged. Additionally, batteries (depending on their chemistry) go through different charging stages and "willingness to charge" preferences depending on their state of charge (SOC). Long story short, is that you need to oversize the PV array to overcome this -- trickle charge won't cut it.

    3) End to End efficiency. I have no doubt at least 50% of your PV power will be lost / wasted. So your 120W STC panel is only ~96W at NOCT. And this is only true for 1-2 hours per day. In the morning and afternoon, it is less. Let's say your DC to DC coupling efficiency is 90% (for your MPPT) and your DC to AC inverter efficiency is 80%. So now, best case, you're down to 69W. BUT, that's assuming you're not trying to charge the battery. Pushing electrons up hill into a battery is very inefficient process. Much power is wasted when the battery SOC exceeds 90%. (that's why batteries and battery chargers get warm while they charge).

    Don't be confused. There are lots of vendors who want to sell you stuff. Goal Zero and the like. No small portable solar array will be able to charge much of anything. For comparison, my 330W full size panels, each only produce >150W for maybe 4 to 6 hours per day. On a typical sunny day (June 3rd for example, in NJ, 175 degrees, 20 degrees) one of my 37 330W panels produced at most 2.06kWh across the entire day -- from 6am to 7pm. That's 158W on average across the 13 hours.

    The first question to ask is:
    1) How many watts does my laptop use when it's running? 60W is a lot. My Lenovo uses ~24W max. How many hours per day do I need to use it?
    2) If the laptop battery is fully depleted, how many Wh/kWh does it take to fully charge it? If the laptop is off? If the laptop is running simultaneously?
    3) How many days do I need to be disconnected from the grid?
    4) Given the use pattern and consumption required by 1,2,3 how big would the battery bank need to be to meet these requirements? For one day? For three days?
    5) How many panels are required to charge the battery bank identified in #4?

    Given all of that. For short term (<1 week) use. It's likely cheaper and easier to just buy more batteries for your laptop. A small generator, or a large Lithium Ion battery pack from Anker.

    The real question is, what are you hoping to accomplish? Do you really need a laptop? iPads are MUCH MORE energy efficient, for example.
    Last edited by JSchnee21; 06-04-2019, 04:18 PM.


    • #3
      Thanks JSchnee21 for your time and detailed answer. I understand what you say here, so me saying loosing almost 50% of energy was too much... was actually an underestimation, as you describe a very detailed scenario on why this is actually the case, thanks for that. I knew for a fact energy is lost, wasn't informed in detail how this much.

      I have several pieces and parts for experimenting, the most detailed scenario (tested) that I can describe here is one small 12V 7ah battery + UPS inverter keeping one 24" TV on for 1:35, that's one hour and 35 minutes, but that's just one case scenario that I tried to get some measurable results. The panel and charge controller do their job charging the battery while the inverter is OFF, no load. So far so good, the charger does the job avoiding over charging and discharge. After a week of testing the system doesn't last 1 full hour now, so that's a separate problem, probably battery quality or the charger controller quality, separate issue, just wanted to write some details about an specific scenario that I measured.

      About your questions, sometimes I do some demanding work on my laptop (I have a few). For other purposes I use low energy devices like my iPad. I'm very aware of power loss and energy wasting on such devices. My laptop (largest one) takes 30 minutes for a full charge (grid) on standby. Consumes more energy than my 24" testing TV, how much? not sure, charger is rated 19V - 3.5A, I'm not sure if that translates into 19x3.5=66.5W steady, constantly.

      In terms on how much energy I need for how many days, it's not that much. I'm currently working on replacing the lights at home for DC LEDS, instead of inverter I'm using a boost converter, quite efficient (more efficient than using an inverter). I made some tests and I get what I need with this system. Why? during the day I get enough sunlight to illuminate the house and get enough power on the solar panel for specific lights+work. At night the use of electricity in terms of lighting matches what the system produces, no problem. I'm not moving the panel to a better place, I got direct sunlight but not as many hours as I'm going to get on the new place.

      This one panel was enough to charge the battery, as said, small, testing battery.

      As for the iPad, yes, I'm doing most of some work there, not even using the laptop, way better, efficient, smart use. I know I can get a better energy output with more batteries, will do after more tests to have an idea of how much that benefit would be.

      Thanks, I have a cleared idea now.


      • #4
        FYI, just as a sanity check, using grid power, when available, will always be cheaper than a comparable solar / battery solution. So while having a hobby is fun, like boats, they are holes in the water where you throw money (-: if you're lucky, they float.

        Well designed grid-tied PV systems -- WITHOUT STORAGE -- still take 6-10 years to break even in most cases. Given "normal" grid power costs ($0.15-0.25 / kWh) I don't think an off grid, battery based solution would ever break even. Folks only use off grid solutions when the grid is either not available or too costly (>$50K) to connect to.

        A 12V 7ah battery (I have several myself for my FIOS ONT, portable PA system, motorcycle, etc.) is a very small battery. Most lead acid batteries are not designed for repeat discharge/charge cycles. Any lead acid battery, when used in this manner, should not exceed a 50% depth of discharge or you will kill the battery very fast.

        High quality batteries from Trojan and the like, can last for many years, hundreds, even thousands of cycles when properly maintained, charged, and not over discharged. But these are usually used in multi battery arrays, generally in series (24V to 48V), and are typically 2 or 6 volts per cell. Such systems are usually designed for a 30 to 40 % depth of discharge under normal conditions, and these batteries are specially designed for this purpose.

        www dot trojanbattery dot com

        Lithium Ion batteries support much broader depths of discharge profiles. But they are more expensive, require more sophisticated charging solutions, and, historically, haven't been as scale-able of a solution. The also generally support fewer discharge/charge cycles than properly maintained FLA batteries. But, most folks aren't diligent enough to properly maintain their FLA's so that may be a moot point.
        Last edited by JSchnee21; 06-04-2019, 04:51 PM.


        • #5
          A couple of things:
          1.) A 100 W panel will almost never produce 100 W output for 2 big reasons:
          a.) Even under full sun on a clear sunny and cool day, it will almost never "see" a 1,000 W/m^2 irradiance input that's used for its rated claim of 100 watts.
          b.) It will ususually have a cell operating temp. > its STC design point and testing temp. of 25 C., sometimes considerqably so. Output will drop above a cell temp. of 25 C. by ~ 0.5 % per deg. C. above 25 C.
          2.) Other devices such as inverters are not 100 % efficient and will reduce the input to any powered device.

          If the direct normal global irradiance with a 0 degree angle of incidence with the sun (that is, pointed directly at the sun) is, say, 1,000 W/m^2, and the angle of incidence between the panel's surface and the beam radiation is not pointed directly at the sun, but is instead at, say 45 degrees, the irradiance on the panel will probably be slightly greater than 710 or so W/m^2, or ~ 300 W/m^2 less than the panel saw when designed, rated and tested The incident global irradiance is an approx. func. of the cos. of the angle of angle of incidence. All else being equal, for that 45 deg. incidence angle, that will reduce the panel's output to ~ 70 W +/- some. Then, if the panel's operating temp. is, say, 50 C. (a not uncommon operating temp. or maybe 25-35 C. above the amb. air temp.) the efficiency and output of the panel will drop by ~ 10 -12 % or so, putting that 70 W output down to maybe something like 70*0.88 = 62 W or so, maybe a bit less. A small inverter may operate at maybe 90 - 95 % eff., say 0.95. That'll lower output to 62 * .95 ~ = 58 W. Shade on the panel will also reduce the output, as will dirt on the panel, perhaps substantially more than you might think, and most likely more than the % of the panel surface that's shaded or than the panel may appear dirty to the human eye.

          Bottom line: Don't believe everything you read or are told and don't assume thing are as they may appear.


          • #6
            Consider this for example:


            It's rated at 434Wh
            . While I love Anker products dearly, they (and everyone else) "inflate" their numbers. This is the capacity of the batteries inside. It is not usable capacity. Usable capacity is only 60-70% of this when the efficiency of the Anker inverter and your device's battery charger (and battery charge curve profiles) are taken into consideration.

            So for example, 434Wh * 0.7 = ~300Wh. My laptop has a 24+72 Wh battery. So this $500 device could charge it from flat about 3 times. Maybe 4 times if I'm not using my laptop at the same time, and I'm careful to only charge it to ~90-95% SOC.

            Similarly, if you buy a 10000 mAh battery from Anker or others you won't be able to charge your iPad twice. If your iPad has a 5000mAh battery, you might get 1.5 full charges from a 10000 mAh battery. (once again, you need to reduce the vendor's capacity rating by ~40%).


            • #7
              [B]JSchnee21: [/B]Yes, grid power is cheap. In my case it all started with some emergency automatic-night lights at my old house, here at this one the house is well built, very efficient, lights are rarely turned on. Decided to install a small solar system (very small) and it's good, long story. But got fed of charging cellphones, sound system, tablets, Ipad, etc. Then I decided to replicate the system but instead of lights decided to use this second one only for charging those small devices, and photography light systems with 18650, small in this case means slow, but I'm in no hurry.

              I got the 120W at half the price, cheap, and decided to give it a try, besides I also designed a hydro pneumatic water system that uses 12V. I use no grid electricity on that one. Anyway my intention was (is) to get the most out of this panel. That's the long story of the situation.

              That Anker product looks awesome, I don't live in the US so that means = expensive, and I'm not a fan of buying such heavy expensive stuff online to get it delivered here. Yes I understand the example. I'm really tempted to build a 18650 based battery system with BMS but perhaps in the future, I understand they are tricky but in my experience they have proved very useful and tolerating multiple charges and discharges. At present time most my work happens on a laptop but more and more on the iPad so that means less energy.

              [B]J.P.M. [/B]Thanks I'm quite aware of what you describe, but the details on the difference were still floating in my mind because after all, that difference is not small at all (supposed input vs output).

              I'm also clear on not believing what I read, that's why I keep reading more and more specially trying to find practical examples, unless I can replicate the scenarios and measure them myself. Other than that, solar panels are not cheap in my country, cheap enough to buy and experiment with them unless it's about very, very small solar panels. I've seen some videos claiming impossible stuff based on solar panels, some examples are actually based on the charge of the batteries and not the solar panels.

              My case, aficionado-style is more motivated towards using zero energy on my work, at least computer work.

              Thanks for the information.


              • #8
                A "100W" panel rating is based on the panel operating at its max power voltage (Vmp, probably about 18V) and max power current (Imp, probably about 5.5A). Power = volts x amps, in that case 100 watts. I'm guessing you are using a PWM (cheap) charge controller, which effectively connects the panel to the battery in an on/off fashion. When connected, the panel is brought down to the battery voltage, which will be around 12-13V. That times the charge current is only about 65 watts. So even in ideal conditions, your panel is only 65-70W with a PWM controller. Add drops due to temperature, angle, dust, whatever, you might only be producing 50W from the "100W" panel, and that only during the best hour or two during mid-day.


                • #9
                  When a PV panel is aimed at the sun properly, for Max Power output, in 20 minutes, you will need to re-aim it, or suffer a loss of power as the sun moves across the sky and off axis.

                  I generally tell folks in RV's or Caravans, that their panels, flat on roof, are NEVER aimed right and they can expect only 50% of nameplate power, less if it's a hot day. Same with Boaters, with masts, stays, antennas, rolling and pitching in the water - never aimed right.
                  Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                  || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                  || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A



                  • #10
                    Originally posted by hroldan View Post
                    The solar panel is located at full sun power,
                    besides all the other comments about aiming etc. This concept of "full sun power" is bogus. There is not set measurement and the lumens do vary from day to day (as well as hourly).
                    It most definitely varies by location as well. Weather has a huge part in this variance with particulates and humidity in the air that the light has to travel through

                    Then you have the issue of temperature. All PV modules have a temperature coefficient that makes them less efficient as they get hotter. As they sit in the sun, they tend to get hotter.

                    All of this is why we have a standard set for measuring PV output: STC (Standard Test Conditions).

                    There are simple rules of thumb to predict actual output in locations as well as full models to get very accurate estimates.
                    OutBack FP1 w/ CS6P-250P


                    • #11
                      Thanks for all the information. I'm quite aware of the differences, but I wasn't informed (or clear) on how big those can be. I understand lab conditions but this is different, it explains why lots of people have issues setting up or planning their solar systems, because the sum of the energy consumption compared to the supposed output of the a given system will not go along. Differences are understandable, but the reality, the real life differences are too big to underestimate.

                      Thanks again. I'm clear on this now.


                      • #12
                        If you found a DC cable for that laptop you can bypass the inverter.
                        And not be argumentative but I have gotten 1800 watts out of 200 watts of panels ,One Day by moving the panels around manually with my ground-mounted sled to power a swimming pool pump.
                        Now 400 watts , seasonally adjusted ,currently nearly flat . To gather as much sun all day without moving them.
                        Last edited by Brian53713; 06-17-2019, 04:21 PM.


                        • #13

                          A DC cable for a laptop is not that simple, none of mine have been 12 volt, some 16.8, 19, 20 volts, I built two converters for my Lenovo idea pad 10-15 volts in to 20 volts, 4 amps out boost converter and 24-32 volts in to 20 volts 4 amps out buck converter to charge on the go or from my solar system, none of the computer stores that I deal with has seen such a thing. There may be some but they sure are hiding out there, I had to build my own


                          • #14
                            STC ratings are nothing more than a level playing field standard that all panels are rated by. In the real world you rarely see panels putting out their rated output. BUT... in high altitude cold and clear conditions you can see the panels output exceed the STC rating. Heat is the biggest factor in reducing panels output. provided they are oriented correctly, or better yet on trackers.
                            2.2kw Suntech mono, Classic 200, NEW Trace SW4024


                            • #15
                              When I was looking, Dell appeared to have options. These days major brand on the internet should have options. And the talented resources here should have tips.
                              Now I use tablets, laptop only when necessary.