When I looked at Shell, all their EFL's said the buyback was at RTW - Real-Time Wholesale prices that update every 5 minutes with supply/demand and are published by ERCOT. They average ~3c kWh but can spike up to 500 ¢/kWh during peak demand periods. The EFL you posted earlier had it at a fixed rate but I couldn't find that on their website.
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So this is basically the same for me as long as I keep my "Free Night Time" usage at greater than 50% of the monthly billed. In other words, I pay for all at 33c kWh but half is free so it's actually 16.5c kWh. The more used at night the better it is for now.