In football as in other sports, the home field advantage has a measurable effect that is taken into account in these Elo ratings as well. If the Elo formulas were applied without any home field advantage adjustment, home teams would win a lot more Elo points than away teams. To counter this effect, home teams have to be rated artificially higher until the expectation to win or lose Elo points is balanced between home and away sides. The following graph shows how many points the home team would win on average depening on how big the adjustment is.
Empirical analysis shows that the home advantage is around 90 Elo points for modern domestic games. For modern European Cup games it is 120. [When I analysed the whole database, I discovered that in the past - especially in the 1970s - the home advatage was much more significant than it is nowadays. Click here for details] When team A plays team B at home, with team B having 90 Elo points more than team A, the probability for either team to win are the same.