How much effect does a SSL actually have over SEO?

[SKP Global]

They found that HTTPS is moderately correlated with higher search rankings on page 1 of Google. There are other studies out there that have found minor correlations as well, like this one from Moz. The SSL certificate can significantly improve the user experience of your visitors.

[SarahDavis]

Using SSL certificates for your websites does not drastically improve your websites' SEO rankings. Even so, they do help boost the rankings of your website.