This database first appeared on Ancestry last year with 139 million entries, listing everybody in a particular area who had the right to vote. With the new update there are now more than 159 million entries.
The database is compiled using an optical character recognition technique. That always means errors. I found my paternal grandfather every year from 1929, when the family moved to London, to 1939, except one. My maternal grandfather appeared from 1924 to 1960, with gaps. Expect problems with incorrect addresses, especially street names. Its essential to check the image to find out what the record really reads and who else of voting age is in the household.