How should I spend energy credits?

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?

For spending credits directly I am going to have to agree with the wiki. Clearing tiles and buildings robots takes a small amount of energy, and is obviously not always available. Terraforming consumes a large amount but is also situational. Trade deals are likely the most reliable way to spend a large amount of energy.

There is an alternative solution for using excess energy. Ships in orbit of a friendly planet or spaceport (especially with upgrades) have reduced maintenance costs. You can build up a particularly large fleet and keep it docked to maintain positive income. When you send the fleet out to annex your neighbor, you will have a deficit until you dock fleet again. This would seem to meet your earlier plan for a reversible method of running a deficit. As a bonus, a large fleet is always of strategic benefit.

In general, try to avoid running a large surplus or large deficit.

If your fleet is docked, it's upkeep costs are lowered. If you think your surplus is too large, try undocking your fleet. If the surplus becomes a deficit, you're fine. You have enough energy to finance a war.

If I have a large surplus that I'm having trouble spending down, this is what I do:

• Stop building power plants or mining stations that produce energy.
• If I have power plants that are built on tiles that don't have any energy, replace them. You can't have excess research.
• Build more ships. I have the energy, so I might as well project some force.
• Give my sectors energy, if they're not at the cap.
• Make 10-year trade deals where I give away energy monthly. This can be for anything. Get some extra minerals. Get some star charts. Form some active sensor links. Get some strategic resources. Deals that are favorable to your trading partners will increase their opinion of you, possibly paving the way for additional treaties. I will often just gift a bunch of energy to my protectorates.

While the trade deals are running, my empire continues to expand. This reduces the surplus over time, especially since I'm not building new power plants. By the time 10 years are up and my deals expire, my surplus has been reduced to a more reasonable level. If it's still too high, make some more deals and keep avoiding additional energy production.