When can't I use P = I²R instead of P = V²/R?

You can technically use either formula in any DC circuit, but practical considerations matter:

Avoid P = I²R when:

• Current is difficult to measure accurately
• The circuit has multiple parallel branches
• You're working with high-impedance circuits
• Current fluctuates significantly

Avoid P = V²/R when:

• Voltage varies unpredictably
• You're working with series circuits where current is constant
• Measuring voltage would require multiple test points

Important note: In AC circuits, use RMS (root mean square) values for voltage and current to get accurate power calculations.

The limitation isn't the formula itself—it's about having accurate data for the variables you're using.