Here is my cheat sheet for the NABCEP Entry Level exam, enjoy!
Units

 1 Meter = 3.28 feet
 Degrees Fahrenheit: °F = (9/5 x °C) +32
Degrees Celsius: °C = (°F – 32) x 5/9  1 HP= 746 Watts
 1 Sun= 1000 Watts/meter2
1 Peak SunHour= 1kWh/m2= 3.6 MegaJoules/m2= 1/86 Langleys  kilowatt 10 ^{3}
 Megawatt 10^{6}

Gigawatt 109
Ohm’s Law
Ohm’s Law:

 V= I x R;
 I= V/R;
 R= V/I
 V= I x R;
Where: I= Current, the flow of electricity measured in Amps
V or E= Voltages, the potential to make electricity; measured in Volts (V)
R= Resistance, it resists the flow of electricity (larger diameter wire= less resistance); measured in Ohms (Ω)
Power
Power is measure of Watts (W):

 P= V x I;
 V= P/I;
 I= P/V
 P= V x I;
Watt is the measure of electric power, or amount of work. One ampere of current flowing at a potential of one volt produces one watt of power.
Annual PV System Performance

 [(system wattsSTC)x(sunhours)x(derating factor) x 365 x Acorr]= # of kWh per year.
Acorr= 1, azimuth correction from due south if PV array is due south
Use 365 days for one year, or 30.5 days for the average month.
May need to divide by 1000 to convert to kilo watts.
Module Efficiency
Module Efficiency is calculated by dividing Power Output by the Power Input and adjusting for module area

 [Pout/(Pin x Module Area)]
Since the Power Input that results in the nameplate Power Output rating is 1000 Watts per each square meter of area, Efficiency is defined as the ration of Power Output to Power Input.
Temperature Correction Factor
% Change in module maximum power voltage (Vmp) as a function of cell temperature=

 [(operating cell temperature C° – 25°C) x (0.5%/C°)]
National Electric Code Table 690.7
Ambient Temperature (°C) 
Correction Factors 
Ambient Temperature (°F) 

25 to 10 
1.06 
77 to 50 
9 to 0 
1.10 
49 to 32 
1 to 10 
1.13 
31 to 14 
11 to 20 
1.17 
13 to 4 
21 to 40 
1.25 
5 to 40 
Very accurate information, easy to understand. A must for the exam!
The study guide is a must have too! http://greencredential.com/nabcepstudyguide/