The short answer is, if you don’t measure using a complete circuit that includes a load, your power (and therefore energy produced) measurements will be meaningless if not zero.
Here is the long answer:
The energy sensors manufactured by Vernier measure two quantities at the same time: the potential difference between two points (also called the voltage) and the amount of current flow from one point to the other. Then the software multiplies these two values together to determine the power produced. The power produced is the amount of energy being produced per unit of time. Without a load, the power will be extremely low or zero.
What happens if you don’t connect a load at all? This is also called an open circuit, and it is what you have when there is no connection for current to flow through. It is the opposite of a complete circuit. In this case, the sensor measures the voltage between the two points, but measures a current of zero. With a current of zero, there is no power produced. If you are using the Go Direct Energy Sensor with a wind turbine, an LED on the sensor body (marked with an exclamation point in a triangle) will flash red to indicate that you are trying to measure power without a load. (Unfortunately, this does not happen when using a solar panel. See Why doesn't the warning LED of my Energy Sensor come on?.)
A different situation is to connect the load terminals of the energy sensor directly to each other with a wire. This is called a short circuit. In a short circuit, current flows easily but there is no accompanying increase in voltage. In this case, the voltage stays very low, so the power measurement is correspondingly low.
It is an interesting exercise to determine the load value that gives the best possible power output for your wind turbine or solar panel. This will vary from generator to generator and solar panel to solar panel, and it also varies depending on the light source for the solar panel (see What is the best light source to make solar panels work indoors?).
In the “Internal Load” setting, the Go Direct® Energy Sensor (GDX-NRG) uses and an internal 30 Ω resistor and does not need to be connected to an external load. (This is the simplest configuration.) An external load can be a resistor such as those used in Vernier Resistor Board (VES-RB) and Vernier Variable Load (VES-VL), or an LED or a small water pump such as KidWind Small Water Pump with Tubing (KW-PUMP).