I was going to start my how to build your own solar Christmas lights with the solar panel, but then I realized how foolish that would be.  The type of solar panel you use would depend on the battery you are charging.and the size and type of battery would depend on the size of the lighting system you are trying to make.  So the whole system actually depends on the solar Christmas lights display you design. If you are looking to do something of the size pictured here you are going to need more than just one battery.

Now in order to determine what you need you will need to know a bit about electricity. Now I apologize to the electrical engineers out there but for this to make sense I need to make sure everyone is on the same page so I’m going to explain this as if I were talking to my mother.

First of all lets look at the big 3, current, resistance, and voltage. To relate these I will use the analogy of a brook. Current is measured in Amps and in our analogy it is the amount of water flowing in the brook.  Resistance is measured in Ohms and it is represented in our analogy by the rocks that tend to resist the flow of water in our stream.  Voltage is the push and it would be represented by the steepness of our stream.  A stream running down a mountain side would represent high voltage while a stream meandering across a plain would represent low voltage.

In our solar Christmas lights the voltage comes from the battery which pushes the current down the wires to the lights which will be our resistance. These things will determine the size of the battery needed because because batteries have a rating called Amp-Hours or AH.  Basically this means that the battery has enough electrons to flow 1 amp every hour for that many hours.  To fit this into our analogy it is like if you had a 100 gallon barrel and a hose that drained it at a rate of 10 Gallons an hour. We could say that the barrel had 10 gallon hours because the barrel would drain in 10 hours. We got this by dividing the 100 gallons by the 10 gal an hour. In the same manner if you know the amps that will be drawn by your lights then you can tell how long your lights would be powered by the battery by dividing the amp hours by the amps drawn. For example If your battery is rated at 15 AH and you have a light string that draws 3 amps you could run the lights for 15/3 or 5 hours.

The famous formula for this is V=IR where I is the current in amps, R is the resistance in Ohms and V is the voltage. If you know the voltage and you know the resistance of your bulbs then you can figure out the amps your system should draw by dividing the voltage by the resistance.

However there is an easier way, since your lights are likely to tell you the watts that each string uses. Now a watt is the volts times the amps or W=VA. That means that your volts (most likely 12) times your amp hours will equal your watt hours. Now all you do is divide this by the watt rating on you light strings and you know how long they will last. An example of this would be, if your battery is rated 15 Amp Hours and it is 12 volts then you would have 15 x 12 = 180 watt hours. If your light string says it is 18 watts then you would be able to run the lights 180/18 or 10 hours.

In my next post we will go shopping and do this for a sample system.  Now hopefully this is starting to make sense but I’m sure a real life example will help.