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I've got a Goal0 Nomad 7m solar panel. It produces ~7 amps @ <22V DC on a regular day.

The MacBook Air has a 45W charger. The markings on the charger say it runs at 14.5V DC @ 3.1 amps. Unless I'm wrong (and I may be, I'm only a beginning EE student ;) those numbers are smaller than the ones on the output of the solar panel. To confirm it another way, P=IV, (P is power in Watts), so theoretically P=7*22=154 watts. 154>45.

The Nomad 7m has a 12v cigarette-lighter style outlet that will fit the MB Airline Adapter. The problem is that it puts out 12v (or so it says, but I'm hoping it's actually around 14) and the Airline adapter needs 14-15 to work right.

What do you guys think? Is this possible? Any recommendations for alternative options (I could always use a 3rd party vehicle adapter if need be). If it puts out this much juice it should also be able to charge a regular MacBook, I think.

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There is no way in hell that solar panel is good for 150 watts. If you look at the specs, it's rated as 7 watts. Where are you getting the "~7 amps @ <22V DC on a regular day." –  Fake Name Feb 3 '11 at 8:54
    
yeah I guess you're right ;) They're must be a typo on the product because while their website says 7 Watts, the back of my product has "~7 Amps" written on it... I probably should have noticed this when I first read 7 amps. That's enough electricity to send someone's heart into fibrillation. –  SeniorShizzle Feb 3 '11 at 17:06
    
And I still respect my Solar Panel... –  SeniorShizzle Feb 3 '11 at 17:16
    
sorry about that accidental close; I misclicked when testing something –  balpha Feb 3 '11 at 17:49
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The Nomad 7 is not designed to charge a laptop. It is for smaller handheld devices. Cellphone, GPS, AA/AAA batteries. Contact us and we can help you get the perfect fit. goalzero.com and click on live chat. –  user3302 Feb 4 '11 at 1:48
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3 Answers 3

up vote 5 down vote accepted

If you look at the page for the Goal0 Nomad 7M (click "Tech Specs") It's rated for 7 Watts.

I don't where you got the "~7 amps @ <22V DC on a regular day."

So the sort answer is - That panel won't even keep the MacBook Air running at idle, let alone charge the battery.

Your post got me curious, so I broke out a Kill-A-Watt power meter.
11" MacBook Air Power Draw, battery already fully charged, measured at the wall:

  • Idle, screen backlight completely off: ~4.7W
  • Idle, Minimum Screen Brightness: ~4.9W
  • Idle, Half Screen Brightness: ~5.4W
  • Idle, Maximum Screen Brightness: ~8.0W

For the active tests, I have the screen set to half-brightness, where I usually use it:

  • Moving the mouse in circles with the trackpad: ~6.5W
  • Scrubbing the Dock (with magnification): ~11.2W
  • HD Video Decoding, GPU accelerated: ~15.6W
  • SD Video Decoding, GPU Accelerated: ~11.2W
  • Flash SD Decoding (not sure if it's accelerated): ~12.7W

Ok, these figures are VERY approximate (the Kill-A-Watt is not a high precision instrument), but they do put things in the ballpark.


The theoretical approach -

Insolation is the term for the amount of energy that falls on a set area per set time.
Basically, we can calculate the amount of solar energy which falls on a specific area.
The Photovoltaic array wikipedia page gives us a ballpark figure of ~1Kw/M².

Then, you have to take the efficiency of the Photovoltaic Panel into account.

Wikipedia gives us a best-case mono crystalline panel efficiency of ~25%, shich means that only 25% of the light which falls on the panel is converted to electrical energy, while the rest is dissipated as heat, reflected away, etc...

Therefore, a 1m² panel will, at the equator and with a mechanism that points it directly at the sun, manage ~250 W.

From this, we can tell that a 154W panel would have to be ~0.616 m² (154/250 = 0.616), far larger than what you have.

Examining the 7W panel you have - How big does a panel have to be to produce 7W?

7/250 = 0.028 m², or 280 cm² or about 14*20 cm, which seems pretty accurate, judging from the pictures on the product page.


Anyways, I went of on a bit of a tangent here, because you mentioned you're an EE, which is one of my main interests.

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Thanks for the reply. I think calling myself an Electrical Engineer would be a bit of a stretch (as you can probably tell by now ;). On the other hand, Goal0 makes a 13.5W pannel (and many larger than that). I imagine you could use this to charge a more substantial battery (like a Hypermac or a Sherpa), which you could in turn use to charge the macbook. Some day I'm going to figure out a way to charge my Macbook Air dammit! –  SeniorShizzle Feb 3 '11 at 17:13
    
Excellent answer, +1 –  conorgriffin Feb 3 '11 at 18:32
    
Probably your best bet is to buy another power adapter, cut the cable, and connect directly to the MBA's power connections. Using the airline adapter means you're going: PV Panel -> DC-DC converter to 12v -> DC-DC Converter to 14.5v MBA PS Voltage -> DC-DC converter in MBA to charge batteries. You want to go PV Panel -> DC-DC Converter to 14.5v MBA PS Voltage -> MBA. –  Fake Name Feb 4 '11 at 0:19
    
Basically, inductor based DC-DC converters (everything > ~3W) have efficiency of ~75-90%, so 2 DC-DC converters is 81-56%. Basically, you want to minimize the number of conversions. –  Fake Name Feb 4 '11 at 0:21
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I'm the EE for Goal Zero. The Nomad-7 is 7 watts, as already noted. The printed W must have been confused as an A. All Nomad panels have 12V output ports, which are actually ~15V outputs, which is a good voltage for charging batteries. I generally don't recommend charging sensitive electronic devices directly from solar panels, because of the potentially inconsistent nature of solar weather. Instead, use the solar panel to charge a separate battery, which doesn't mind inconsistency, and then use the battery to charge your sensitive electronic device.

I have a Macbook Air and the airplane adapter. I have attached this cable to an adjustable power supply, and tested voltages from 13 to 16.5V. None of these, when driven through the airplane adapter, will alow the Macbook Air to charge -- only run. There are some electronics in this cable which signal to the laptop not to take a charge, and I have not yet figured out how to reproduce these. I'm waiting on another magsafe that I can cut apart and experiment with.

I have run my Macbook Air directly from our 12V batteries and from our larger panels, through the airline adapter.

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So the "12V" output is really just the unregulated panel output? –  Fake Name Feb 10 '11 at 11:13
    
Good question. On the Goal Zero Nomad-7, the 12V output is a boost regulated output, since the native panel voltage is 6.5V, optimized for USB and charging AA batteries. The Nomad-13 and Nomad-27 panels are native 15V, and so the 12V output is the unregulated solar panel output on those. –  user3334 Feb 18 '11 at 16:47
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Thanks for helping GZ, it's nice when a company cares that much (or at least its employees :). To clarify, you say that with the Airline Adapter I CAN power the MacBook Air, at least partially right? I don't really care about fully charging it as long as I can power it during like ~3 sunlight hours. You said you could do this with your larger panels, so could I accomplish this with the 7m or only the 13.5+? (product selling motivation aside ;) Also, would it charge if I used a 3rd party car adapter? Just curious, if it's really 6.5v why does it say <22v? Is that after it's transformed or what? –  SeniorShizzle Feb 20 '11 at 5:30
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Also, if you have time, this website may help you with your Goal0 endevours, too: pangea.stanford.edu/~schmitt/magsafe –  SeniorShizzle Feb 20 '11 at 5:34
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I would also like to thank GZ for responding.

since the question is hanging a little i would like to share my experiences.

most laptop and phone chargers today appears to test the current available before starting to charge. as the magsafe is rated at 45w it is no far stretch to assume that it will test if it can output 3.1 amps 14.5 to do this with a power conv factor of perhaps 90% (can be as low as 88 and up to 95%) it will need 45/0.9 ~ 50watts. at 15v that would require 3.33A For GZ test, it is fairly common that a variable bench power source 0-24 can only deliver 2A ie it can deliver 50w at peak voltage but only 30W at 15v(have had this problem when trying to power industrial pc at 24)

hence unless you put a "buffer" battery between the panel and the magsafe that can deliver 3.5A temporarily to keep the charger happy. it wont work.

what i would like to know is if the usb port on the nomad 7 is 5.0 or 5.4 volt

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