According to an embodiment, an image forming system includes: an image forming apparatus; a peripheral device; and a first control unit. The image forming apparatus includes a first voltage detecting unit that detects voltage of a first power source supplied by an external power source, and outputs as first voltage value; and a first current detecting unit that detects current of the first power source, and outputs as first current value. The peripheral device includes a second voltage detecting unit that detects voltage of a second power source supplied by an external power source, and outputs as second voltage value; and a second current detecting unit that detects current of the second power source and outputs as second current value. The first control unit calculates total power value of the image forming system based on: the first voltage value; first current value; second voltage value; and second current value.