TL;DR: At ~30 to 40 energy per 1 metal from overdrive, depending on how much you value their buildpower.

With caretaker spam getting more common these days I just added caretakers (and different versions of windgens) to my magical overdrive table (inspired by ddaba/Kyubey) to show their metal production efficiency compared to other energy sources in combination with overdrive. I felt the urge to share this information to protect my head from facepalming while spectating future games.

If the table below is not clear, read this:
[Spoiler]
(click image for larger version)

Note that numbers given are rounded and do not account for energy cost for building construction. Also note that because of how overdrive works, wind generators' efficiency is slightly below their average output, except when the average is 2.5.

To answer the question: the exact number is 40.25 energy per 1 metal gained from overdrive (according to wolfram). That's when Singularities and Caretakers both take 715.555.. seconds or ~12 minutes to pay back. Energy:metal ratios of grids can be estimated by looking at their colors in eco-view. For exact numbers check out the tooltips of buildings connected to the grid.

Finally, of course this is not an "ultimate guide to ecoing". There's many more things to consider: Solars have hp, Singularities explode, Caretakers have buildpower, cheaper buildings complete faster and produce earlier, just to name a few examples. As such the number can probably be rounded down to somewhere between 30 and 40, perhaps even lower. The table above is merely supposed to provide a few numbers people might be interested in and to maybe give players a few new ideas.

Yes 733.333 is the time they'd take if you only count their metal production, however they produce energy as well. Now, even if their energy output can not be directly converted into metal via overdrive, it's still available. That means other energy sources like solars do not have to spend their energy output on shields, cloak, construction, etc. and can instead add their energy to overdrive.

In other words, the times shown for caretakers are only correct as long as you have sufficient energy available for overdrive through other structures. I decided against simply writing "733" because having energy available for OD is very likely to be the case in your "average", non-crazy game.

Side note: Your energy bar blinks if you "waste" energy. "Waste" is a number shown in the bar's mouseover tooltip and only exists if your team's energy storages are full and you produce more energy than can be consumed by overdrive and other things, which means you need to connect your pink grids (or use more energy via cloak/shields/repair/resurrection).

Could you add Tidals to the chart?

Yup, "Windgens' average output is close to the Fusion to Mohogeo range." and tidal is pretty much a windgen on lowest possible average, thus a Fusion. I'm afraid of adding more columns because the table already looks a bit messy at first glance as is and I don't want to scare away people before they even take a closer look. If anybody has an idea on how to make the table feel less overwhelming/chaotic please let me know. Should I just remove a few rows or perhaps try to draw a graph of some sort?

This seems about right. Even in a big team game you should generally stop at orange, 15x. At about 20x it starts going red, IE "This is bad now". Red overdrive means "Stop overdriving" and it stops getting much redder at about 30x.

I think by the time you're getting to 40x, even in the silliest FFA's, it's time to attack/reclaim/get more mexes in your grid/etc rather than go "Oh, caretakers now!".

You always want to build caretakers up to and probably beyond your buildpower, but there comes a point where excess buildpower is kind of useless. If you have targets for reclaim, repair, or can use up reclaim spurts from the battlefield sure, make more caretakers. But there comes a point where extra BP is worth very little (Though this is indeed another reason to have as much excess BP as is reasonable).

I was going to ask that earlier myself, but the I thought that the OD grid colors actually do that...

Saktoth: I thought the point of spamming caretakers in FFA or what have you was for the minute metal income they give you, not for their buildpower. Is this mistaken?

The people who can't read the table.

Only when their minimal output is equal to their maximum: 2.5E-2.5E. Otherwise see When to spam windgens.