The formula assumes the inputs are valid
A calculator cannot detect damaged insulation, an incorrect meter range, a short circuit, or a mislabeled supply.
Power ratings need margin
A resistor or speaker at its exact calculated power limit has no reserve for heat, peaks, or component tolerance.
Series and parallel are physical connections
The result is only correct when the actual wiring matches the selected arrangement.
Codes and manufacturer limits still apply
Wire size, enclosure, grounding, protection, and mains voltage work require rules beyond a basic formula.
The calculator is one part of the job
Use the result to support a decision, then compare it with the drawing, material, tool, product instructions, and actual conditions.