Learning From Your Laptop

New Advances

Desktop computers are not very conducive to being powered by the sun. While it is possible, it isn't exactly easy. The DC to AC converter has to be very clean otherwise your desktop PC can do some pretty strange things. On the other hand, laptop computers are (almost) perfect for being powered by the sun. They run on DC at voltages that photovoltaic generation and storage can easily be tailored to.

It is rather standard that most laptops operate on 4.8 volts DC and require anywhere from 2.2 amps to 5 amps. The older the laptop you have, the higher this amperage rating will be. The brand new laptops are using considerably less power than their immediate predecessors. Coming home and plugging your laptop into a battery fed by the sun all day is a novel experience.

A Simple System

The first order of business is to create enough power during an average day in New York to use my laptop for an hour or two during the evening. This means I would need 4.8 volts at 2.2 amps for two hours or a total of 4.4 amps in one day. Pak Rat Electronics sells very nice amorphous silicon panels for $50 each. They are conservatively rated at 10 watts. I have found them able to produce .700 amps at 17 volts on average in the spring and summer. Even at .600 amps at 14 volts (the manufacturers specs.) we are in business.

If two panels are used and sun is received for 4.5 hours a day at an average of .600 amps at 14 volts at the end of the day we have stored 5.4 ampere-hours in the battery. Any small marine battery will do as long as it has a capacity of greater than 15 amp-hours. Sears has many good batteries in this range

Using the above calculations we would have, over the long run, a surplus of 1 ampere-hour a day if we used the laptop for two hours each evening. This 1 amp-hour number is a little high because of the thermodynamics of energy transfer: lose a little on the way into the battery, lose a little on the way out.

The second problem to conquer is that of voltage mismatch. The battery is charged up at close to 13 volts and the laptop only requires 4.8 volts. A 5 volt regulator would work very well and to be safe we should procure one with a 5 amp rating. If your laptop consumes 5 amps, you should consider a 7 amp regulator.

Many of the electronic parts mail order houses carry voltage regulators for 5 volts at 5 amps. Mouser Electronics is such an outfit. If the mail isn't fast enough, Radio Shack sells a regulator for 5 volts at 1 amp (#276-1770). Since one regulator would not supply the 5 ampere rating we need, five could be wired in parallel for a current rating of 5 amps. This regulator, or paralleled group of regulators, is wired between the battery and the laptop. Essentially the regulator assures that no more than 5 volts ever reaches your laptop.


The system should have the following safety measures. An 8 ampere charge controller, and two 10 amp circuit breakers. Pak Rat Electronics sells a sturdy 8 amp controller (#20-210) that will make sure that your battery is never overcharged by your panels. This becomes more of a concern as the amperage size of your battery decreases. A 10 amp circuit breaker should be wired between the panels and the battery. In addition, a 10 amp circuit breaker should be placed between the battery and the laptop.

Hardware Costs

The panels were $54 each including extruded frames. The battery should cost about $30 for a 20 amp-hour size. The charge controller was $45.


The battery feed to the laptop must terminate in a power connector that your laptop accepts. Unfortunately there is no standard for these plugs. Look at the one on the end of the AC power cord that came with your laptop. You might get lucky by shopping around at the local computer stores. A more reliable source would be the laptop manufacturer. It has been my experience that they are more than happy to send you a plug by itself when you offer to pay for it.


Charging your laptop is a little more tricky and I will try to cover this in a later article.