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Solar Powered Laptop Computer

A while back I got an email from a writer who likes to work most of the day out in a large shed, some distance from his house.  According to the email, the shed has no power and the distance is so great that the guy said that running a long extension cord all the way out there was impractical.  So, the guy wanted to know what it would take to power his laptop computer and a small fluorescent lamp for 8 hours each day using solar power.
Laptop Computer    15w Desk Lamp
The guyís request is not unusual.  This sort of thing comes up all the time.  Letís see what it would take.  This kind of thing might be needed by someone camping or staying in a cottage by a lake where power is not available.
His laptop computer averages about 40 watts.  The fluorescent light is a 15 watt unit.  Both run off 120vac.  He would like to be able to work even if the sky was cloudy. So, some kind of battery will be needed to store the some energy for dark days.
40 watts plus 15 watts is 55 watts.  Letís round it off to 60 watts.  The guy may need some other small electrical loads in his shed, so perhaps we should bump the power up to 75 watts.75 watts times 8 hours is 600 watt-hours.  So, the guy needs 600 watt-hours of energy for his 8 hour work sessions out in his man cave.
Next, the writer will need solar panels large enough to supply power for his electrical loads plus charge his battery for later use.  My philosophy is to size the solar panel so it can generate at least twice the average power needed.  A bit more would not hurt.  The extra power would allow the solar panel to charge a weakened battery at the same time it was powering the normal loads.  This puts the solar power at 150 watts.  A typical solar panel for 12v systems will have the solar cells configured in a 4 x 9 thirty six cell series string.  The open circuit voltage would be about 18v.  However, a single 150 watt panel, which can crank out 10 Amps or more of current, is unusual.  A better solution would be to buy two 75 watt panels and using a series diode which usually comes with each unit, combine the power to form a 150w power source.
The DC voltage from the solar panels would then feed current to a large 12v battery.  To insure that the battery is not overcharged, a quality battery charge control box would need to be inserted between the two solar panels and the battery.  I like the kind of product which also shows the condition of the battery as the type shown below.
75 Watt Solar Panel  
Next, a DC to AC inverter will be needed to convert the 12 volts from the battery into 120vac for the computer and any other AC loads.  The switch mode power supplies used in modern computers should work will with the cheaper modified sine type inverters.  These units are quite efficient and generate a voltage waveform which looks like 160v positive and negative pulses, lasting about 6 milliseconds.  This averages out to produce the needed 120vac and also has the expected 160 volts peak voltage, which many power supplies expect.
I think a pretty standard 150 watt modified sign wave inverter should work for this application.  The added power margin might be nice for short term extra power needs.  The inverter could draw power from a medium size 12v lead acid battery.

Finally, the writer will need a rechargeable battery.  Although he may only need 600 watt-hours, the battery should be rated at twice this figure, to insure a long life.  That would put it at 1200 watt-hours.  Divide that by 12 and you get 100 Amp-hours.  So, a good 12v 100 Amp-hour sealed lead acid battery might be what he should get.

150 Watt Inverter
12v 100 Amp-hour Battery

May 2011     Issue 17

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