If you're thinking about the environmental impact of your BOINC crunching, it'll help to consider (1) the time of day, and (2) the time of year.
There are different demands on the electricity grid depending on the time of the day. You mention that your electricity comes from renewable resources, so your grid may work differently, but crunching during off-peak hours would minimize the environmental impact. Here's a Wikipedia article on the concept: https://en.wikipedia.org/wiki/Peak_demand
Depending on how big of a machine you're working with, you might consider the heat that it generates. Thinking of that heat as a resource, we would want to crunch more in the wintertime when we need to heat our space, and crunch less (or even not at all) when it's so hot outside that we need to cool our space.
You'll find your own balance of course, but it's good that you are considering the impact of your choices - after all, a smaller environmental footprint is one of the primary advantages of Gridcoin over Proof-of-Work and even other Proof-of-Stake based cryptocurrencies.
One last note - you can also reduce the CPU usage percentage in BOINC to be less than 100% when considering your hardware lifespan. I'm sure others have already written about tweaking settings in greater detail.
Good stuff! In terms of the 100% renewable I believe my supplier buys the electric wholesale from windfarms and solar only, naturally the actually electric I use comes from all sources that supply the grid.
That being said yeah offpeak times would be any time after 8pm to 9am here in the UK I think. My computer tends to be on throughout the day anyways hense the constant idle crunching but rarely on at night.
I might switch to 24/7 again, I currently run 75% cpu 75% of the time. My computer is also watercooled so it's temps never really get that high but it's really good at heating the room.
Peak demand
Peak demand is an historically high point in the sales record of a particular product. In terms of energy use, peak demand describes a period of simultaneous, strong consumer demand.