Solar Laptop

When I joined the Peace Corps, I received a letter saying that it was likely that I would not have electricity at my disposal once I got to Kenya.  As it turned out, this is not true, since most (but not all) Deaf Schools here are in fact “on the grid,” I nonetheless prepared for the worst, planning to use my laptop even if there was no power at all.  I’ve only used my solar setup once in practice (although I tested it a lot before I left), but perhaps my experience may be useful for anyone in a similar situation.

The first thing to understand about solar is that it is inefficient and expensive, so if you see a cheap and/or small “solar charger” for your electronics, don’t be fooled… it will either not work or it will be so slow that you will go crazy.  Let’s take a kind of “worst-case” scenario and see what it would take to make it work off of solar power alone: a laptop.

It IS possible to run a laptop off of a single flexible (foldable) solar panel that can fit in a backpack.  It’s just expensive.  Let’s take it step by step.  There are two ways of doing it: without a car battery, and with a car battery.  Read on for both scenarios….

(Testing in America– laptop in the tent, solar panel outside)


First things first, how many watts does your laptop use?  This is easy to calculate.  Look underneath your laptop and you should see something like this: “15V 3A.”  This is telling you how many volts and amps your laptop might demand, at most, meaning that if you are playing some 3D online video game with WiFi running, with the screen at full brightness, all while you’re charging the battery.  If you don’t see the numbers there, check the brick on your laptop’s power cord.  Multiply the volts and the amps, and that gives you the watts.  In the case of my Toshiba laptop that I bought specifically for this purpose (Portege A605-P201), 15 x 3 = 45… so 45 watts.  Let’s look at my Macbook Pro: “16.5V 4.6A,” which ends up being 75.9 watts… a pretty significant difference, as we will soon see.

At the time I left for the Peace Corps (Nov. 2008), the largest, most powerful, and most expensive foldable solar panel you could buy was 62 watts.  This rules out the 76-watt Macbook Pro right away, but the Toshiba still stands a chance, but when you consider that the 62W is under “ideal” sunlight, and that in fact the number will vary throughout the day, the 45W laptop is already cutting it pretty close.

The next thing to understand is that you need to ditch your laptop’s power cord right away.  The plug that goes to your wall will not work efficiently with solar.  This is because solar power is Direct Current (DC), as is your laptop, and for that matter so is your car.  But the electricity in your wall is Alternating Current (AC).  So the reason your power cord has that big brick in it is so that it can can convert from AC to DC.  AC has no place in the solar world, because you’d have to use an inverter to convert from the solar panel (DC) to the power cord (AC), which in turn will use the brick to convert back for the laptop (DC).  Each conversion is lossy… you LOSE power in this example in two steps, and when you have so little wiggle room in the first place, forget it!  You’re definitely better off running your laptop directly off of DC, which is surprisingly easy… just buy a car cigarette lighter adapter, which you can get at just about any electronics shop.  This will convert from 12V (the standard for car batteries and solar panels) to whatever voltage your laptop needs.  You lose very little/no power in this step, since it’s DC-to-DC.

OK, now that we’ve taken AC out of the equation, can we hook the solar panel directly to the laptop?  Oh, if it were only that easy!  The problem with a big panel is that its power output can fluctuate so much.  Not only can it dip too low, it can spike!  A spike in amps is not a problem.  Amps are just how much power you have on tap, waiting for the laptop to demand.  Too few amps and the laptop will think its not plugged in (actually, Windows will say “Plugged In, Not Charging”).  Too many amps and the laptop will just ignore them.  Volts are the real problem here.  Most electronic devices have a voltage threshold.  The car adapter for my laptop is designed for 12V, but even a car battery can fluctuate  a little, so it might handle ~9-15V with no problem.  It depends on the adapter, but my panel can peak at 31 volts, and that’s way out of range!

So we need to get a little box to solve this problem: a voltage limiter.  I bought one that caps the voltage at 14.4, which is typically as high as a car battery will normally get.  I also ended up having the best luck with  APC brand car adapter… the others I tried freaked out due to the fluctuation even with the limiter.  The APC also converts to USB power, which is nice for charging a lot of other things, too.

So, I plug the voltage limiter into the solar panel, then I plug the car adapter into the voltage limiter, and then I plug my laptop into the car adapter.  That’s it… a portable solar laptop setup!

Once this is up and running for a laptop, you can apply the same logic to just about any electronics device.  Count the watts to make sure you have a big enough panel, then buy a car adapter for the device (iPod, camera battery charger, etc)..  I can charge my cell phone and my laptop at the same time using a 3-way cigarette lighter from Radio Shack.

Charging phone and laptop simultaneously in Kenya.   Solar panel is outside the window.  Dishwashing soap and USB 3G modem are unrelated.

It’s also worth noting that although I haven’t tried it, if I bought a car adapter for my Macbook Pro, maybe I could charge it from solar while it’s off.  The wattage rating on the bottom of the laptop is a MAXIMUM, so you never know.  But I’m pretty certain it won’t run off of solar full-time.  Even with my Toshiba, although I can run just fine on solar without depleting battery, if I’m really maxing out the brightness and CPU, it may only be able to maintain the charge… it won’t get the battery back to 100% unless I lower the brightness and/or CPU.  An even safer bet would be a netbook, but I really wanted a DVD burner.  That’s up to you.

In summary: The solar panel connects to the limiter, which connects to a gender changer (custom, parts from Radio Shack), which connects to a plug shape adapter (included in solar panel spare parts kit), which connects to the cigarette lighter adapter. 


Now, the more common way to harness solar power is to use a car battery, sometimes called a “solar battery,” but effectively the same thing.  Don’t try traveling with one, but it’s reasonable to assume that no matter where you go, you can get one, even if it’s just a small one for a motorcycle.  I came prepared for both scenarios, with and without car battery, but since I’ve only needed solar sporadically, I haven’t bought a battery to set up this type of installation here in Kenya.

The idea is basically this: whereas in the previous example, the laptop is effectively fooled into thinking that the solar panel is a car battery, in this case we actually involve a car battery.

It’s easy enough to imagine hooking your laptop to a car battery.  If you already have the cigarette lighter charger for your laptop, you just need a cigarette lighter receptacle that clamps to the car battery’s positive and negative posts.  You can get such a thing at most any car parts shop.  Make sure it has a fuse… once you’re dealing with a car battery, the chance of frying something gets much higher.

The harder part is charging the car battery from the solar panel.  It’s tempting to hook the panel up directly to the battery, but remember the voltage spikes?  Multiply that times battery acid.  Yikes!  What you need in this case is a battery charger.  Think about the laptop.. you aren’t actually hooking your laptop BATTERY up directly to the solar panel… you’re hooking up your laptop itself, which in turn has circuitry inside that manages the charging process.  Batteries themselves are dumb… they don’t know when they’re full or empty, they just sit there.  A laptop, or in this case a dedicated car battery charger, keeps track of such things and makes the best use of the incoming solar power to charge the battery as efficiently as possible, and that it stops charging when the battery is full.  You don’t need a big car battery charger since your flexible solar panel won’t put out much power relative to a big roof-mounted system.  Mine for example can manage up to 9 amps, which is more than enough.  The big ones also have fancy features that might actually make you LOSE power for a small setup.

So, the solar panel connects to the car battery charger, which connects to the car battery, which connects to the cigarette lighter adapter, which connects to the laptop!  Easy as can be, and you can leave the car battery and panel out to charge all day, and hook your laptop up at night, for example, which frees you up a bit more.  The installation itself just becomes a bit more permanent, since car batteries weigh a ton.  The other advantage to this scenario is that it would enable you to use more demanding electronics.  For example, I could let the car battery charge for a few days and then hook up my Macbook Pro!  I’ll just drain the car battery faster.

In summary: the solar panel connects to the battery charger, which connects (with a fuse) to the battery clamps, which connects to the car battery (not pictured).  The car battery connects to another set of clamps, which connect to the cigarette lighter adapter (preferably fused, but not pictured).


None of these options are sold in a tidy little package.  Even after I bought all the right parts, I had to make some custom connectors to get everything to plug in smoothly, and that involved soldering, heat-shrink tubing, and some trips to Radio Shack and Fry’s Electronics.  Give yourself some time and test with cheap electronics before you actually plug in your laptop!  Buy a AA battery charger with a car adapter and test it first.

Also, the connector used most often by all these weird solar parts is pictured below.  They’re called SAE connectors.  I just bought anything I could find that used them (online mostly but I also found a car power kit at Radio Shack that came in really handy), and then I took everything home to figure out how to make the adapters I needed doing the least amount of splices and soldering possible.  I prefer the cables that are color coded (leftmost is ideal, but the middle one with the white stripe is okay too), since especially when you’re dealing with the car battery and the clamps, you have be be careful about the directionality of the wires.



I make no attempt to keep track of changing prices, part numbers, etc.  This is simply what I bought in Nov ‘08, and the prices are ballpark from memory.  Don’t hold me to them, just use them to get a general budget idea.

solar panel: Global Solar 62W 12V, ~$800

universal laptop car charger: APC UPA9, ~$100

extra cables, connectors, etc: Global Solar, ~$20 and Radio Shack, ~$20

voltage limiter: Global Solar 14.5, ~$100

car battery charger: Genasun GV-1, ~$100

5 Responses to “Solar Laptop”

  • Paul,

    Small calculation error. “(Portege A605-P201), 15 x 5 = 45… so 45 watts.”

    And the connector is an SAE Connector.


  • hi paul, the usb 3g modem you got there, is that a safaricom modem? just wonderin since you once hinted that you dislike them due to them being locked. do you now prefer them to using your mobile phone as a modem and since you have said you use wimax at home, is the usb modem for when you are out and about?

    • The USB modem is for travel but also it’s a backup connection if with WiMax is down (which happens mostly if the power is out, and the modem is more power/solar-friendly than the WiMax). I never found a good 3G unlocked modem around here, so I use the Safaricom one most of the time and I just tether my phone if for some reason I want to use Zain or Orange or whoever.

  • nice one many thanks to my fellow technologist

Leave a Reply

You must login to post a comment.