  The late-time integrated Sachs-Wolfe (ISW) signal in the CMB temperature
anisotropies is an important probe of dark energy when it can be detected by
cross-correlation with large-scale structure surveys. Because of their huge sky
area, surveys in the radio are well-suited to ISW detection. We show that 21cm
intensity mapping and radio continuum surveys with the SKA in Phase~1 promise a
$\sim5\sigma$ detection if we use tomography, with a similar forecast for the
precursor EMU survey. In SKA Phase~2, the 21cm galaxy redshift survey and the
continuum survey could deliver a $\sim6\sigma$ detection. Our analysis of the
radio surveys aims for theoretical accuracy on large scales. Firstly, we
include all the effects on the radio surveys from observing on the past
lightcone: redshift-space distortions and lensing magnification can have a
significant impact on the ISW signal to noise ratio, while Doppler and other
relativistic distortions are not significant. Secondly, we use the full
information in the observable galaxy angular power spectra $C_\ell(z,z')$, by
avoiding the Limber approximation and by including all cross-correlations
between redshift bins in the covariance. Without these cross-bin correlations,
the ISW signal to noise ratio is biased.
