Some basic abbreviations
Electromotive Force = Volts = V or rarely, E
Current = Amps = I
Resistance = Ohms = R
A common analogy is Volts is like water pressure, low or high.
Amps - like a small tube, or a firehose, a little flow or a LOT
Ohms - a restriction in the water line, or a kink in the hose, slows the flow down
Power = a measure of work energy = Watts = W or P (Also handy to convert from different voltage scales - 100W at 5V (20 amps) is the same as 100W at 120V (0.833A))
Formulas (secret electrical incantations): 
The power triangle (power formula) The magic triangle can be used to calculate all formulas of the “electric power law”. You hide with a finger the value to be calculated. The other two values show then how to do the calculation.
Shortcut - the Ohms Law Triangle V over I * R
This simple formula describes the relationship between voltage, current and resistance:
Voltage = Current X Resistance or V = IR
Current = Voltage / Resistance or I = V / R
Resistance = Voltage / Current or R = V / I
Ohms law can easily be remembered using the triangle below. Cover Up the value you want to find, and treat letters next to each other as multiplications, and letters above each other as divisions. Try it!
Formula Wheel (uh oh - no more secrets now ! ) http://www.sengpielaudio.com/calculator-ohm.htm
All of the above is valid only for DC circuits and AC circuits that contain only resistive loads, like incandescent lights and simple heaters. AC circuits with reactive loads like motors, transformers, florescent lights, require one to consider the phase angle offset between voltage and current which makes the math more complex. :)
And when working with over 30 Volts, keeping one hand in your pocket will help prevent a fatal current flow thru your heart, thereby possibly saving your life.
Working with batteries or heavy currents, remove metal jewelry - if it shorts and arc welds itself to something, it will get very hot, with your little finger trapped inside!
And, least I forget - House Wires, are commonly Black, White and green. Always remember the Black Death. That is the color for the HOT wire - Black, if the place was wired right. It should always be connected to the toughest thing to touch, the shorter of the 2 slots in an outlet, or the deep center contact of a lightbulb.
Building wiring may also involve wires of many other colors, all of which are likely to be hot at least part of the time. Also it is entirely possible (and legal) for white or green wires to be hot. The best rule is to assume all wires are hot until you prove otherwise by testing.
There is a simple wattage calculator here
http://shop.altenergystore.com/Calculators/OffGridCalculator.html
that can help you determine size of panels you may need.
You can buy simple voltage testers here:
http://www.thinkgeek.com/stuff/41/lebedev.shtml
From http://www.angelfire.com/pa/baconbacon/page3.html