When playing Stellaris, I often find myself with a lot of energy credits. The most obvious way to spend them is to run the economy at a deficit for a few months, but it's not clear to me how I can safely and reversibly do this, or what advantages it would give me. Also, the game constantly nags me if I try. There are some things which cost energy credits in lump sums, but I haven't seen many big ones besides diplomacy, which can be hit or miss depending on the neighbors' attitudes. The end result is that I usually sit at the credit cap once I've been running my empire for long enough, and the game then nags me for "wasting" the surplus.
What are some strategically reasonable things to spend energy credits on in bulk?