# Confusion about how to calculate signal to noise ratio(SNR)?

I searched a lot but I didn't find a good answer. According to wikipedia SNR page, SNR can be calculated by both ratio of Power of clean signal to its noise Power either ratio of variance of clean signal to variance of its noise, but actually power and variance are two different things so I am confused how we can use both of them equally. I should mention that I speak about two finite signal, first the original one and second the noisy one and want to calculate the SNR. Thank you