Why 100 watts are not equal to 100 watts?

[PNjunction]

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.

[hroldan]

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.

[sdold]

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.

[hroldan]

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.

[Brian53713]

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.

[SunEagle]

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?

[Brian53713]

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

[PNPmacnab]

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!

[Brian53713]

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.

[Mike90250]

...
And not be argumentative but I have gotten 1800 watts out of 200 watts of panels ,One Day by moving the panels around manually....

You mean you think you harvested 1,800 watt hours from 200 watts of panels in a day ? That's unbelievable, considering that would take 9 max (solar noon) production sun hours with panels held at 70F. Spill the beans, how did you do that?

[Brian53713]

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.

[littleharbor]

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.

[Tecnodave]

Brian53713,

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

[Brian53713]

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.

[hroldan]

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.