<Dd> P = I 2 R . (\ displaystyle P = I ^ (2) R .) </Dd> <P> However, if the current is a time - varying function, I (t), this formula must be extended to reflect the fact that the current (and thus the instantaneous power) is varying over time . If the function is periodic (such as household AC power), it is still meaningful to discuss the average power dissipated over time, which is calculated by taking the average power dissipation: </P> <Dl> <Dd> <Table> <Tr> <Td> P avg (\ displaystyle P_ (\ text (avg)) \, \!) </Td> <Td> = ⟨ I (t) 2 R ⟩ (\ displaystyle () = \ left \ langle I (t) ^ (2) R \ right \ rangle \, \!) (where ⟨...⟩ (\ displaystyle \ langle \ ldots \ rangle) denotes the mean of a function) </Td> </Tr> <Tr> <Td> </Td> <Td> = R ⟨ I (t) 2 ⟩ (\ displaystyle () = R \ left \ langle I (t) ^ (2) \ right \ rangle \, \!) (as R does not vary over time, it can be factored out) </Td> </Tr> <Tr> <Td> </Td> <Td> = I RMS 2 R (\ displaystyle () = I_ (\ text (RMS)) ^ (2) R \, \!) (by definition of RMS) </Td> </Tr> </Table> </Dd> </Dl> <Dd> <Table> <Tr> <Td> P avg (\ displaystyle P_ (\ text (avg)) \, \!) </Td> <Td> = ⟨ I (t) 2 R ⟩ (\ displaystyle () = \ left \ langle I (t) ^ (2) R \ right \ rangle \, \!) (where ⟨...⟩ (\ displaystyle \ langle \ ldots \ rangle) denotes the mean of a function) </Td> </Tr> <Tr> <Td> </Td> <Td> = R ⟨ I (t) 2 ⟩ (\ displaystyle () = R \ left \ langle I (t) ^ (2) \ right \ rangle \, \!) (as R does not vary over time, it can be factored out) </Td> </Tr> <Tr> <Td> </Td> <Td> = I RMS 2 R (\ displaystyle () = I_ (\ text (RMS)) ^ (2) R \, \!) (by definition of RMS) </Td> </Tr> </Table> </Dd>

How to calculate rms value of ac voltage