The diagram below shows how the a mini cell solar is wired to an INA2019 current sensor. The load is a fixed resistor. The INA219 measured the current in mili Ampere (I) and the voltage over the load (V). The power of solar energy in electric power is:

$$P(W/m^2) = \frac{(IxV)}{\text{(area of solar cell)}}$$

Solar energy can be expressed in several terms from photon energy to energy stored in battery. The first one is the potential energy and the latter is the useful term for end-users. In between these terms are a few losses needed to be accounted:

- Loss from photo energy to electron energy. That is where efficiency of solar photovotaics counted. The commerical type can archieved an efficiency in low 20% to mid 20's percent. Reports from NREL indicated that exceeding 40% conversion is viable.
- Loss to convert electron energy to a stable voltage ready for charging (or directly using). Solar charging are varied which Max Power Point Tracking (MPPT) is the best charge controller. More about MPPT.
- Loss due to resistance of the battery and other joints from solar panel to battery.
- Temperature, tilt angle of the solar panel can be factored in for losses.

Using a fixed resistor as the load can be a great point for debate since the power (P) is the product of current (I) and voltage across the load (V). The choice of 1-ohm resistor is an attempt to stay close in the range of internal resistance of various types of battery. Also, the location of placing the solar is critical. My location is better to measure direct sunlight from noon the afternoon. In the morning, most solar energy is in diffused form.

Last update: September 2020