Photo-generated current is important in solar cell current equation. The value of photo-generated current cannot be labeled in the conventional single origin coordinate system, and a novel dual-origin coordinate system is designed. This article provides a process in how to mark the Kirchhoff's law in a dual-origin coordinate system; besides, the current items (photo-generated current, diode current, shunt resistance current) and the voltage items (diode voltage, series resistance voltage) are also shown. A dual-origin coordinate system clearly points out that the difference between photo-generated current and short-circuit current will increase with increasing short-circuit current. This difference can be ignored only in low light, but it must be considered in high light. The proportions of output power and internal friction power may change with photo-generated current. Assuming that the output power is greater than the internal friction power, the solar cell should not be used to work under high light. A dual-origin coordinate system can provide an adaptation for solar cell study.