How the peak moved into the evening
Two curves run through every summer day here. Solar output rises with the sun, peaks in the early afternoon, then falls off a cliff as the sun sets. Demand does something different: it climbs through the afternoon heat and holds high into the evening, because a house that baked all day keeps asking its air conditioner for more long after dark. On the hottest days the regional peak now tends to land in the early evening rather than at midday.
Grid operators track the difference between total demand and the solar and wind feeding in, and they call it net load (the demand left over for everything that is not weather-driven renewable power). As more rooftop and utility-scale solar comes online, net load in the middle of the day keeps dropping, while the evening net load stays stubborn. The result is a steep ramp in the early evening, when the sun leaves but the heat, and the demand, do not.
The honest math on the evening gap
Consider the scale, with the usual caution about numbers that move. Regional summer demand in the Phoenix area runs into the thousands of megawatts (a megawatt is roughly enough to power several hundred homes at once), and both major utilities have reported record or near-record peaks in recent hot summers, per their own public filings. Solar can cover a large share of the afternoon, but its contribution at 7 in the evening in late June is close to nothing, because the sun is nearly down.
That is where batteries come in. A battery charged on cheap midday solar can discharge for roughly four hours in the evening, which happens to match the shape of the ramp fairly well. Both SRP and APS have added utility-scale storage and have more under contract, at last public report. The honest limit is duration: four hours covers the sharpest part of the evening, but a long, hot night can outlast it, and that is when older resources still get called.
Who is counted on to fill it
In rough order, the evening lineup looks like this. Solar and its paired batteries take the first shift. Natural gas plants, which can ramp up and down quickly, cover the ramp and the late night. And underneath all of it sits Palo Verde, the nuclear station west of Phoenix that runs around the clock and was, at last public report, the largest US power plant by annual output. It does not chase the peak; it provides a steady floor the rest of the system is built on.
Uncertainty is real here and worth naming. Monsoon season, roughly July through September, drags thunderstorms and cloud cover across the state, which can cut solar output for hours with little warning and push demand around as humidity spikes. How much storage gets built, how fast, and whether it is the four-hour kind or something longer, are open questions in front of regulators and utility boards right now. Anyone who tells you the evening gap is solved is selling something.
Worth watching this month
1. Watch SRP's publicly elected board meetings this fall for any storage or generation decisions, since that board, not a state commission, sets SRP's direction.
2. Track the Arizona Corporation Commission's docket for APS, the regulated utility whose rate and resource filings run through the commission.
3. Check the EIA Hourly Electric Grid Monitor for Arizona's balancing authorities to see how late in the day the regional peak actually landed this summer.
4. Note the end of monsoon season, roughly late September, which usually eases the sharpest demand days but does not flatten the evening ramp.