Solar Ratings Explained

Solar Ratings Explained

By David Rosner

Solar power is quickly emerging from the black smoke of the energy sector. It is becoming imperative that we see this growing technology less as a magic sun scoop and more as a realistic power production tool that meets real world expectations. The current disconnect between what we think solar does and what it actually does needs a little explaining. I’ll try to fill this gap by expanding our knowledge of solar panel ratings and how it relates to the power we need and use.


We at Goal Zero have made using solar power as easy as plugging in a cord. Lights flash, numbers scroll, icons blink, and the power of the sun sits at our fingertips. Unfortunately, our incredibly observant customers quickly realized that not everything adds up. We regularly get asked why “my new 30 Watt panel is producing 23 Watts, is something wrong?” The short answer is NO, but let’s look at why this happens across the solar industry.


Panels of the solar variety are rated according to STC, or Standard Testing Conditions. This is done in a lab, not even out where the sun hides! As solar panels are produced, they are all tested at the exact same conditions so we have comparable numbers to know what to expect.

Power is measured in Watts, and sunlight is measured in how many Watts are hitting the earth within one square meter (yes, we’ve gone metric for solar). STC has been designated at 1000 watts/meter^2, which is some pretty intense sunshine. This is done on a horizontal panel with the “sun” directly overhead for optimal angle of interaction. STC is also designated at 25 C (77 F). The important factor here is that the hotter it gets, the less efficient the solar panel becomes.

Finally, STC is calculated at 1 Atmosphere, or ATM. This is a standard accepted number (precisely equal to 1,013,250 dynes per square centimeter) representing how much all the air above us is pushing down on you. For solar, it is how much all the different air particles are bouncing and blocking the sun’s light from getting to us. It is important because when the sun is directly overhead, it goes through less atmosphere than when it rises and sets on the horizon.


So now we build these factors into a relationship of efficiencies. The sunny summer months are great because the sun is close to directly overhead, like in the lab. However, the increase in heat reduces panel efficiency. In the winter months the panels are cold, so more efficient, but the sun is lower on the horizon so it fights through more atmosphere to get to our panel. On top of this, most panels do not point directly at the sun except for a fraction of the day, reducing efficiency again. This compounds with voltage drop, the lost power as electricity moves through wires.

Essentially there is almost always something(s) that prohibits solar production at lab settings, a factor that pulls down the panel from its rated wattage.


It means we can expect to see between 67% to 75% of a panel’s rating when used. So if a 30 watt panel is in sunshine, you should expect 20-24 watts. This is the most you’ll see most of the time, less as the above factors add up to reduce power production. Even less as clouds and shadows block your panel. Less as dirt builds up on your panel. Each connection usually has a minuscule loss as well. All these individual nuances build into a visibly significant lower number than expected.


For this inconvenience, the entire solar industry would like to apologize for any confusion or perceived under performance. We are trying to adjust expectations through education. Some solar companies are even putting both rated watts and “true” watts on their panels to help solve this confusion. It’s like when auto companies started giving urban vs highway miles per gallon because no one was observing the potential efficiencies while commuting to work in stop and go traffic.

The “Bright” side: The opposite is true too! A solar panel placed at high altitude, in the cold, with the sun at mid-summer angles, while directly pointed at the sun, can produce ABOVE the rated watts for a panel! The other 99% of the time, however, you should expect less.


  1. John C Garcia 2 years ago

    You recommend on linking/chaining a maximum of 4 solar panels, why?

  2. David Alexander 2 years ago

    Thank you for taking the time to explain this in a well written article. Although I and a few others understand that we don’t live in a controlled environment, many people just don’t understand why there products don’t perform as expected, and needed a simple and effective answer.

    Thank you for doing this.

    David Alexander
    Yeti 400
    Boulder 30

  3. Ronald Schubert 2 years ago

    Thank you for the honest and comprehensive write up on how Solar Panel are rated. I do like Goal Zero Products!!

  4. John C. 2 years ago

    Thanks for the clarification. My question then is why a Goal Zero 30 watt panel cost over a $100.00 +. more then a similar panel rated at the same power and same kit set up? That’s what doesn’t make sense, when both panels are certified and tested, and are produced of the same material?

    Sort of like buying a pair of Levi’s, vs. Wrangler jeans. Because they have the Levi brand I’m paying $40.00 more then the Wrangler jeans although same material, threads, just a couple of cosmetic differences like buttons. How can Goal Zero justify the higher price for the same wattage?

  5. Aaron 2 years ago

    Great article. My question is, what kind of device could I plug into my Goal Zero Nomad 20 solar panel to measure its current output at any given moment in time? Right now, all I have to go by is seeing if the little blue LED lights up. Seems like having such a current readout would help me in positioning my panel for the best exposure to the sun, which is constantly moving. Not to mention help me adjust my expectations of how fast my connected batteries might be recharging.

  6. Nee bel 2 years ago

    Real good article and well formulated for normal user well be good if more similar articals for flexi panels can be posted

  7. Fred 2 years ago

    My favorite tidbits for optimizing output:

    There’s an iPhone (and probably android) program called “PV Optimizer” It’ll spit out the angle and direction you should aim your solar panel at for the day or an hour. The neat thing is that the program uses you’re phone’s sensors to calculate the heading (/) and angle of the panel so you can lay your phone on the panel and know when you have it lined up.

    Nite-eze makes these bendable “ties” that work pretty well for making a “frame” for a Nomad 7. I fold up one big orange one and keep it in the pouch.

    If you’re really old school, setting your solar panel at the same angle as your latitude (like 45º for 45 north) you could even use the slope or just the degree marks on a compass to estimate this.

  8. Rrussell Harada. 2 years ago

    I recently bought a Yeti 1250 and 2 Boulder 30 panels from the Pearl Harbor NEX. I asked if they will bring in the Boulder 90 panels. The clerk could not say, but put ny name/number on a “To Call List”. She also said they had a few in , but were sold out. That was approx 7 months ago. Whenever I go to shop there I check if any arrived. No luck. However I see a constant restocking of Boulder 30 panels, and Yeti 400/1250 generators. Any way find out if or when NEX (PH) will get some 90’s in?
    By the way, it’s a great product. I have two Yeti 1250’s.

    • Author
      goalzero 2 years ago

      Unfortunately there isn’t a good way to get that info to you. See if they will special order you one and if they won’t you can always order one through


  9. John Musielewicz 2 years ago

    It’s relatively easy, build a bridge using the lower current cells, build it for the voltage, across the bridge use a few higher current cells. You’ll get 10 times the wattage in the same space. Use heat sinks, you’ll need them. Watts, volts, current is additive and cumulative.

    I expect both copyrights and patents out of my design, the goal is being able to operate portable electric lights and electric hot plate/heater while on expeditions, backpacking and camping. Moose.

  10. John Musielewicz 2 years ago

    Watts are also subtractive so if you reverse connections you produce negative and positive motion.

    In fact solar cells are generally incorrectly connected together, if I was big boss like thieving billionaires I would be recalling and replacing every installation, from tiny to enormous.

    The negative and positive on cells is not + above zero and – at zero like everyone and all installations I have seen work but positive and negative where they swing above and below zero volts but never at zero.

    Daylight can be either nightlight usually a negative voltage and negative current.

    Solar is a floating voltage conversion that produces energy whenever there is light including moonlight and starlight.

    Sorry to stat that this is well known since I read it but there appears to be quite the stupid liars and criminals floating around in technology that misrepresent quite a bit of knowledge to the innocent.

    Not from my relatives, but take your pick else where, and kick out the army, they are full of it.


  11. John Musielewicz 2 years ago

    My last comment is on the polarity statement I made. What I described is the polarity-less nature of photovoltaic cells. They are marked + and – usually but due to the dual nature they are a dual or quad, a multiple installation.

    Also, because of the energy and heat produced they are great sources for small efficient power, especially relieving the land from the crazed civil ‘engineer’ and the insane works they build like the hydro dam.

    Hoover dam, the waste of time and money buy European and Asian nut balls roaming and murdering in america is a good example of destruction of natural resources by anti-intellectual fools.

    Look at the slums of Europe and Asia and you see the hypothesis of Stalin, Lenin and the nuts in charge of the nuthouse when it comes to ‘social engineering’. Europeans and Asians are no engineers.

    And not a ship builder in the mix. Americans build the best racers, the fastest, sleekest, ships on the waters, the detriment to all that challenge our path, and that is well recognized, especially due to the last two centuries of war thrown at our ship yards, yet they still stand, for America will once again own the World Cup and there is nothing the world can throw at us to produce a ‘nay’.

    Fear us, fear the U.S. Of A. For the World Cup is ours!!!!


  12. FF 2 years ago

    Thanks for the explanation. You stated that “the test is done on a horizontal panel with the sun directly overhead for optimal angle of interaction”. However, in all goal zero panel user manuals, it is stated that the optimal efficiency of the panel is reached when solar rays hit the panel at a 45 degree angle. Can you clarify?

    • Roz 2 years ago

      Good question, sorry for the delayed response and the confusion. What we want is the sun’s rays to hit the panel to make a 90 degree angle. If you live at the equator and don’t plan to adjust the angle of your panel throughout the year, point it straight up (horizontal). In the US/Northern Hemisphere, we need to angle our panels toward the Equator. The further you are form the Equator, the more angle you will need to achieve this perpendicular interaction with the sun. So the basic rule of thumb is to angle your panel, and the safest number is about 45 degrees. I believe our manual is stating optimal efficiency is reached when sun rays hit a panel that is tilted to 45 degrees, the wording is a little misleading and a very generic rule. Hope this helps.

  13. James DaVanzo 2 years ago

    Thank you for that precise, easy to understand explanation. In my research, I’ve know the factors that reduce the effectiveness of solar panels. This article puts it all in one place.

  14. Gavin Lang 2 years ago

    Excellent, poignant and clear explanation.

  15. George 2 years ago

    As Schultz says, very interesting. Great article,loved it,thanks

  16. Yoshi Yamamoto 2 years ago

    Have you thought about including 2 ratings for your solar panel? Or just go with he real word rating. It’s always better to underrate than overrate. Surefire is great at this. I often turn on my 400 lumen G2X flashlight and it’s brighter than my friend’s 800 lumen chinese light he got off of ebay. Surefire “lumens” are know in the flashaholic world as THE standard. They do not overrate.

    It still not too late to change the public perception however. Start being transparent and put it on the product label. Remember the computer monitor debacle? 27″ but only 26″ is view-able? So companies are now forced to put view-able rating on the package. Maybe this needs to be the case with Solar and batteries as well? a 20,000 mah battery will not have 20,000 mah of useful energy. perhaps 16,000 of that will be useable if even that much.

    Please consider transparency on your product label. You will only disappoint if you overrate to get sales. Example: Nomad 20….. product label reads RATED 20 WATTS (14 WATTS TYPICAL). Or YETI 400 RATED 396wh capacity (200wh usable).

    I am available for consult if needed. Thanks!

    • Author
      goalzero 2 years ago

      Thanks for the suggestion. We have talked about something like that, but in the end we decided to stick with the industry standard so people don’t get confused. We are going to focus on more solar education pieces like this one to clear up any misunderstandings. Thanks!

      • Yoshi Yamamoto 2 years ago

        Got it, thank you for the reply. Educating the public is great and keep up the good work! I love Goal Zero products.

    • Tom 2 years ago

      Yeti 400 rated 398wh capacity (200wh usable) – regarding battery recommendation you should discharge battery only to 50% – that is right but at discharge current regarding C factor of batteries. If discharge current is less then you have more capacity if discharge current is higher you have less capacity. In emergency situations you can/will discharge batteries more than 50% – you can get specified capacity out of battery.
      Last but not least this numbers are only valid when battery is new and 0 cycles discharge.

  17. Duane Miles 2 years ago

    Thank you for giving some straight talk about solar. So many people out there are so uninformed about solar, the challenges it presents, and how to really use it. They just think all they have to do it connect to it and it is the answer to their energy needs. Well, maybe on the space station. But, here on earth, it’s a whole different ball game. They need to remember that you don’t solar at night. First, you need batteries to store the energy you produce. Then once you know the limitations of solar, you’ll know what you can with it. And, have fun using it. Look at RV’ers who like to dry camp and who have solar on their rigs. Charged batteries, an inverter for AC and they are good to go.

  18. GAIL 2 years ago


  19. D Mack 2 years ago

    Well stated
    And is something that needs to be repeated and promoted
    Put this one in rotation

  20. Garry Bryant 2 years ago

    Thanks for posting this and helping us to understand solar tech.

  21. Sébastien 2 years ago

    Another factor that CAN influence is the latitude you are. At the equator, the sunlight hits a specific spot more directly than it does at the pôles, du to earth inclination.

  22. Tom 2 years ago

    Are STC not specifying 1,5 ATM?

    Great article, thx.

  23. Wade Carlan 2 years ago

    I understand your comments on solar efficiency ( although I don’t know why you don’t sell larger panels to compensate), I have a question on the charge rate when plugged into an outlet. I own an Yeti 1250 with an extra battery and just recently bought a Yeti 400 also with a second battery. For the 1250 I bought 8-30watt panels with two tripods and experienced the solar inefficiency. Therefore for the Yeti 400 I bought a Renogy 150 watt panel (much cheaper) and needed to test. So after three months I pull down the load on both units per your recommendations and recharge them. This is where I noticed something odd. When using Renogy 150 watt panel on the Yetei 1250 50had 40% blinking, the charge was aroung 95 watts. This is to be expected based on you explaination. However when I pluged in only the electrical into the Y1250 the charging rate was around 65 watts for one plug in and approximately double for both electrical cables plugged in. Why does the solar charge more when plugged into only one input source than either electrical input? Similar results when doing the same thing on my Y400. In both cases the solar charged more than the electrical. Why at less than 40% charge on either the 1250 or 400 does the electrical charge at a lower rate the solar. Another question is why does the electrical not charge at full 120 watt input when the batteries are below 40 %. I understand when closer to full charge. What do I have to do to see 120 watt input charge? I know not to drain the batteries to zero.

    • Roz 2 years ago

      The limiting factor is the size the “brick” on the wall charger. When electrons move they create heat. The more electricity, the more heat. If we create too much heat that it is a safety hazard, we must decide if we want to make a bigger wall charger or charge at a fairly slow, yet safe rate. We chose the latter assuming you wouldn’t want to carry a cinder block of a wall charger. For the full 120 watt input, approximately 180 watts of solar would be needed to max the system.

  24. Lost 2 years ago

    Hello and thank you for the explanation.
    So with this estimated average use of 70% efficiency rating does that mean I could safely use the new Nomad 100 watt panel with the Guardian 12V Plus charger that has a 90 watt input limit?
    Seems that with what you’re saying that in most conditions the 100 watt panel would not be sending too much current to the Guardian and therefore it would be safe.
    I have four Nomad 20’s and would like to make things much simpler and get a little more charging capacity.

    • Roz 2 years ago

      You are correct!

  25. James 1 year ago

    Just recently bought the Yeti 1250 generator along with 3x90watts solar panel (because yeti 1250 can only input 240watts max). The solar panels was installed on a rotating pole for max sun light thru out the day. 2X90watts solar panel was connected in series with a Anderson connector to the yeti 1250 and 1x90watts solar panel connected directly to the yeti 1250 input (this connection was recommended from goal zero tech support). I was only getting 150watts input max. changed my connections and connected all 3x90watts solar panels in parallel to the yeti 1250. I was still getting 150watts. shouldn’t I get close to 200watts atleast?? I am not even getting close to 70% of what is suppose to be “estimated”. tech support is now saying my cables might be to long. how is that? I bought everything from goalzero. don’t buy the $500 90watts solar panel. you can get the same solar panel watts at a much cheaper price that will work better. yeti 1250 is decent.

  26. Vaughan Miller 1 year ago

    Mixing deep cycle battery group. will this hurt the batterys I have group 24 (2) group 27 (4) and group 29 (1) total of seven.

  27. Matt Meiresonne 12 months ago

    Hey David,

    Good stuff man. Nominal vs. real is a real pain.

    Do you use MPPT controllers to help solve?

    • Author
      goalzero 11 months ago

      The Lead Acid Yeti’s do come with MPPT. Our newer lithium line have an adaptable port that will allow for an MPPT charge controller to be installed.

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