The median dark-of-moon zenith sky brightness above La Palma, measured from ING archive imaging data (about 500 images), taken during solar minimum (1994-6), well away from the ecliptic and galactic planes, is given below in mags/square arcsecond:
Dates U B V R I Notes
1994-6 22.0 22.7 21.9 21.0 20.0 LP archive data
22.4 22.9 21.9 20.9 Other dark sites
The rms errors on the estimated medians are probably less than 0.15mag in B,V,R (more in U,I for which there are fewer data), showing that the ING sky is as dark (within the errors) as other good sites. These values supercede the estimates given in the Observers' Guide, which were based on very few data.
The moonless sky is about 0.5mag brighter during the 3-4 years flanking solar maximum (due to additional daytime excitation of the airglow by coronal EUV). It is also significantly brighter at low galactic or ecliptic lattitudes, at low elevation, or under some dusty conditions (there is a tradeoff between scattering and extinction).