# Formulas for the NABCEP Entry Level Exam

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 Sun-Hour= 1kWh/m2= 3.6 MegaJoules/m2= 1/86 Langleys
• kilowatt 10 3
• Megawatt 106

#### Ohm’s Law

Ohm’s Law:

• V= I x R;
• I= V/R;
• R= V/I

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

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 watts-STC)x(sun-hours)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