The Epoch of Reionization ( EoR ) 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds .
We have used semi-numerical simulations to study how this affects the error predictions for the EoR 21-cm power spectrum .
We expect { SNR } = \sqrt { N _ { k } } for a Gaussian random field where N _ { k } is the number of Fourier modes in each k bin .
We find that non-Gaussianity is important at high { SNR } where it imposes an upper limit [ { SNR } ] _ { l } .
For a fixed volume V , it is not possible to achieve { SNR } > [ { SNR } ] _ { l } even if N _ { k } is increased .
The value of [ { SNR } ] _ { l } falls as reionization proceeds , dropping from \sim 500 at \bar { x } _ { { H~ { } { \sc { i } } } } = 0.8 - 0.9 to \sim 10 at \bar { x } _ { { H~ { } { \sc { i } } } } = 0.15 for a [ 150.08 { Mpc } ] ^ { 3 } simulation .
We show that it is possible to interpret [ { SNR } ] _ { l } in terms of the trispectrum , and we expect [ { SNR } ] _ { l } \propto \sqrt { V } if the volume is increased .
For { SNR } \ll [ { SNR } ] _ { l } we find { SNR } = \sqrt { N _ { k } } / A with A \sim 0.95 - 1.75 , roughly consistent with the Gaussian prediction .
We present a fitting formula for the { SNR } as a function of N _ { k } , with two parameters A and [ { SNR } ] _ { l } that have to be determined using simulations .
Our results are relevant for predicting the sensitivity of different instruments to measure the EoR 21-cm power spectrum , which till date have been largely based on the Gaussian assumption .