Monthly trade. Set up for example to sell 50 food per month, the price will go down to 0.69 credits on the first day of the month then come back to 0.70 immediately after.
Okay, but in my current game (at 2298) I'm maxed out on food districts w/ a comfortable but not overlarge surplus (around 120 I think), and I'm producing over 2000k energy from energy districts, but 900 from trade (w/ 225 CG from the other 1/2 of my trade), with a net gain of about +60 per month, meaning that I'm using almost all of my energy production.
Not only is there absolutely *no* way to produce the kind of food I would need to make up approx. 2k energy. Even if the market held steady at .7 (which I don't think it would w/ monthly trades close to 3k food), the approx 2857 food necessary to trade in 2000 energy at .7 trade value is a ratio of 1.4. Now, factor in the 30% trade cost, or, let's say reasonable best case scenario, 10% (because you got galactic market trade capital and you've got import/export agenda or some other buffs that knock another 10% off). Your cost now becomes 3142, which is a 1.57 ratio of conversion. That is slightly worse than the ratio of difference between base food and energy production 6 to 4.
So assuming the markets could sustain a 3k monthly trade of food into energy, on it's face my late-early game energy needs would be better met with energy districts than trading in food for energy.
Your argument in favor of this is based on the added benefits you would get from agrarian idyll and other food focuses in your empire build. And that would make sense.
Except I don't think there's any way to produce enough food. I would need 3k in 2298, meaning I would need 10-30k food for trade by mid and late game, not mention the upkeep on the massive population I would need on all those agri-worlds. Seems unlikely.
And, I don't believe the market would sustain that level of purchase (unless it does. I mean, if the market can hold price with dumping 3-15k food per month, then I guess it *might* work, but in that case, I would contend the market is broken and should be fixed). I am skeptical that your ratio of 1.57 would recover from even 3-5k monthly food trades. There may be a way to use this strategy to great effect early game, with a very specific build. But in general, it looks quite clearly like a sub-optimal (if not outright bad) way to design an economy in this game.
So, I think you're pretty significantly mistaken in your assertion that generator districts are useless and that food can be used as a substitute on a large scale and as the foundation of an empire's energy acquisition.