It’s the same thing. Say you have a bill of 120€/year. On average that comes to 10€/month (120€ divided by 12 months). If you have save 10% a month you save 1€, which after 12 months becomes a saving of 12€. Now do that yearly: 10% of 120€ is… 12€.

10% of saving for July's or August monthly bill will give less than 10% saving for the year as in the winter solar output is lower.