If you are using the book Renewable Energy with Vernier, order code REV, $48, the experiments are written for use with the Vernier Variable Load.
The Vernier Variable Load, order code VES-VL, $64 is a linear potentiometer that allows the user to vary resistance between 6 Ω and 225 Ω. The Vernier Resistor Board, order code VES-RB, $18 is a small circuit board with seven resistors (10 Ω, 15 Ω, 20 Ω, 30 Ω, 39 Ω, 51 Ω, and 100 Ω) and locations for connecting to each using alligator clip wires.
Both the Vernier Variable Load and the Vernier Resistor Board can be used in the experiments included in Renewable Energy with Vernier. Only one experiment, Current and Resistors (number 5) requires resistors, and this experiment can be completed with either the Vernier Resistor Board or a small collection of resistors purchased at a hobby or electronics store. Experiment 9, Effect of Load on Wind Turbine Output, can be done with a Vernier Resistor Board instead of the Vernier Variable Load, if students connect the resistors in series to create the different resistance values. Most other experiments can be carried out connecting to just one resistor on the Vernier Resistor Board instead of using the Vernier Variable Load.
Teachers may decide to purchase the Vernier Resistor Board due to the price difference or because it has no knobs or other pieces that might break off with hard use. Or, teachers may prefer the greater variation in resistance values possible with the Vernier Variable Load and the ability to reach greater resistance values without wiring resistors in series.