I guess I should have said "currently" -- very few grids are 100% renewable; some offer the option to individual customers (hi EBCE!) but usually what this means is that the operators purchase enough power from renewable sources to cover that customer's daily use, but crucially it's not always at the time of use. For example, the user might be supplied with power from a fossil fuel power plant at night, but then extra solar power is purchased during sunnier periods to compensate.