If you want to be more environmental, use power when the current carbon intensity of the grid is low. Effectively you are then providing demand-side management to reduce overall carbon dioxide production.
https://carbonintensity.org.uk/
What you really want to know is what the marginal carbon intensity is. In other words, if I add demand now, what form of generation will supply that additional demand? This is somewhat different from what the current mean carbon intensity is.