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Real-time electricity usage in Texas is graphed by ERCOT, the Electric Reliability Council of Texas. Today’s graph can be found here, with a screen-grab below:Load Forecast vs. Actual

Note the difference between the lines and the bars. There are three lines: yesterdays forecast for today, today’s forecast for today, and today-actual. According to the help menu, the lines are for the "system load". I quote: "The System Load (green line on graph) represents the current system load. It updates at 15 minutes after each hour." (the green line is hiding under the blue one). Fine. Very interesting.

What about the bars? Two sets of bars: forecast vs. actual. I quote: "The Current-Day HSL (lighter blue bars on graph) represents the current HSL. It updates at 7 minutes after each hour." Oh, what’s "HSL"? I quote, it is the "Hourly System-wide Load (HSL)" Super duper dandy.

My question is: what’s the difference between the system load, and the hourly system-wide load? Why aren’t these exactly the same, up to rounding errors? I can’t imagine how to integrate, differentiate, average, Laplace transform or apply any other tricks, that would explain this difference, that would explain how they are related.

I do notice that the difference between these two is approximately equal to the solar+wind generation. But only approximately: at 6AM, there is a 20GW difference in this graph, whereas combined wind+solar was generating 12GW. So that’s not it.