Every 6 tiles of soylent will generate approximately 1 energy per minute. With the ERN portal energy upgrade, every 4 tiles of soylent will produce 1 energy in a minute. Note that there's also efficiency loss based on the time it takes a packet to get to the tower, but at the absolute worst that's only a 20% drop in income.

This table lists how many seconds towers take to pay for itself once operational depending on the situation, and includes the miners for comparison. The EC is the ERN portal's energy collection upgrade.

So basically if you place a tower that can make 121 soylent and it survives for at least 27 seconds, it has paid for itself (a good guideline might be 30 seconds to take into account distance inefficiency). If it died at any point past that, it would have been worth it to build for energy generation. Note that the ERN portal's energy generation upgrade doesn't reduce the initial payoff time as much as you might think, because most of that time is waiting for the soylent to accumulate.