In general, when using an energy sensor to measure the power output of a small solar panel such as the KidWind 2V/400mA Solar Panel (KW-SP2V), you will get higher power measurements with a specific resistance load compared with all other amounts of load. We have done some experimenting and have determined that the appropriate resistor value for maximum power output differs depending on the light source. For our test results, see What is the best light source to make solar panels work indoors?.

If you are using a different solar panel, such as an older one or one from a different manufacturer, or you are using different light bulbs than the ones we performed tests with, our test results may indicate resistance values that are not ideal for your situation. So here is how to determine the appropriate load resistance for your solar panel and light source.


  • Either Vernier Energy Sensor (VES-BTA) and a data collection interface and software such as Logger Pro,  Graphical Analysis app, Graphical Analysis Pro, or Go Direct® Energy Sensor (GDX-NRG) and Graphical Analysis, Graphical Analysis Pro, LabQuest App (LabQuest 2 or LabQuest 3 only).
  • Vernier Variable Load (VES-VL) or Vernier Resistor Board (VES-RB) or a selection of resistors, from 5 ohms to 100 ohms
  • Wires with alligator clips on both ends for making connections
  • Solar panel
  • Light source


  1. Set up the solar panel and light source in the way you expect students to set them up.
  2. Launch the software and connect your sensor to the software. If you are using the Vernier Energy Sensor, follow the steps to zero the sensor. If you are using the Go Direct Energy, zeroing is unnecessary.
  3. Set up the software data collection mode to Selected Events. You can skip naming the event and providing units.
  4. Set up a single graph in the software to display Power vs. Resistance.
  5. Connect the smallest load resistance. If you know about parallel circuits, you can make a smaller resistance by putting two small resistances in parallel.
  6. Start data collection and turn on the light source, if it is a lamp. Wait about 15 seconds.
  7. Click or tap the Keep button in the data collection software to collect a data point.
  8. Change the resistance to another value, wait 10 seconds, and click or tap the Keep button to collect a second data point.
  9. Continue collecting data as in step 8 for a range of resistances. Make sure you have found the resistance at which peak power is produced by collecting data for resistances that are higher and lower than where you think the peak is. It is sometimes helpful to keep several data points for a given load resistance.