On-peak hours and what to move in a Phoenix summer
The hours a Valley utility prices highest sit in the late afternoon and evening, which is the same stretch the cooling load peaks in, and which household loads can move.
Short answer
On-peak hours are the hours a utility prices a kilowatt-hour higher, and on the Valley's two large utilities they fall in the late afternoon and evening. That is also the hottest part of a Phoenix day: in a normal July the temperature is at or above 100 °F for the eight hours from 1 p.m. to 9 p.m.
A day in a Phoenix summer has a shape, and the shape is the same whether you measure it in degrees or in kilowatts. The temperature climbs from a minimum around dawn to a maximum in the mid-afternoon, and it does not fall far before midnight. The electricity a household uses for cooling follows that curve almost exactly, because a house is a box trying to stay below the outside air.
The afternoon and evening are the hours both curves peak in, and they are also the hours the Valley’s two large utilities price differently on some of their plans. That is not a coincidence. A utility’s peak is set by its customers’ simultaneous demand, and in Phoenix that demand is the weather.
What on-peak actually means
On-peak is a pricing idea, not a physical one. On a plan that uses it, the same kilowatt-hour costs more when the grid is busiest, usually on weekday late afternoons and evenings, and less at other times, including overnight and often weekends.
Three things vary, and all three are published by the utility rather than by anyone else: the hours themselves, the months in which the summer split applies, and whether a demand charge sits on top of the time-of-day prices. APS and SRP each publish their own current plans on their own sites, and the two are not the same. This page quotes no window from either one. A window printed here would be wrong for some readers on the day it was printed, and the reader would have no way to know.
What can be said honestly, and what this page is about, is the climate half: which household loads are tied to the weather and therefore land in the busy hours whatever the plan says, and which ones are tied only to habit.
The shape of a summer day
The normal temperature at each hour of a July day, Phoenix Sky Harbor
The shaded band marks the eight hours whose normal temperature is at or above 100 °F. Both the day's maximum and the grid's busiest hours sit inside it.
Typical temperature by hour, NOAA NCEI U.S. Climate Normals 1991–2020, hourly summaries, Phoenix Sky Harbor (USW00023183). Hours are local standard time: Arizona is on MST all year and never moves its clocks. The eight-hour hundred-degree run is this publication's count on the hourly normals, not a published figure.
Read the curve from left to right and the problem assembles itself. At midnight a normal July day is still at 92.5 °F. It reaches its minimum of 85.6 °F at 6 a.m. (which is the only point in the day when the air is doing anything for a house) and it is back above 90 °F by 9 a.m. The maximum, 104.8 °F, lands at 4 p.m., and the temperature is still 100 °F at 8 p.m. and 94 °F at 11 p.m.
That late fall is the part that matters for anything scheduled by the clock. There is no cool evening in July. The hours most households think of as the relief (after dinner, after the children are in bed, after the sun is down) are still in the nineties, so the cooling equipment keeps running and the grid keeps carrying it.
Midnight to 6 a.m.: the block that never gets cheap in load terms
The normal July low is 84.5 °F. Nothing about the overnight hours in a Phoenix July is cool, and the household load proves it: a house that has been held at an indoor temperature all day does not release much of it overnight, because the air outside is not cooler than the indoor target. This is why the summer here is a twenty-four-hour problem rather than an afternoon one, and it is why so much of a summer bill is spent between midnight and dawn.
The loads that run in this block are the ones that have nowhere else to go. Pool pumps on a night schedule, irrigation controllers, electric water heaters recovering from evening use, and every appliance that simply does not get switched off.
6 a.m. to 1 p.m.: the last comfortable stretch
This is the window the weather gives back. The temperature climbs from 85.6 °F to 101.6 °F over those seven hours, and the household load has not yet peaked, so the grid is carrying less at nine in the morning than it will at six in the evening.
It is also a short window, and a household’s morning is short too. That is the practical tension with any plan that prices these hours lower: the cheap hours arrive when the house is being emptied, and they are gone before the washing has finished.
1 p.m. to 9 p.m.: the eight hours everything coincides with
This is the stretch the shaded band marks. The normal temperature is at or above 100 °F for all eight hours. The building itself is gaining heat faster than at any other point in the day, the pool is at its warmest, and any cooling equipment in the Valley is being asked for the most it will be asked for all year.
Nothing in that paragraph depends on a rate schedule. It is the climate of the place, and it is why the Pool Month-by-Month Planner puts July and August in a category of their own: a pool pump that has to run a certain number of hours a day can run them in several different places on the clock, and the pool does not care which.
9 p.m. to midnight: falling, and still in the nineties
Ten o’clock is 95.7 °F and eleven is 94.0 °F. The house is still losing heat to nothing, because the outsides of the walls and the roof are still hotter than the inside, and the air conditioning is still cycling.
For a household trying to work out which habits sit in the busy hours, this is the block that surprises people. The evening is not a refuge; it is a slightly cooler version of the afternoon.
Which loads follow the weather and which follow the clock
| Load | What sets when it runs | Can the timing be chosen? |
|---|---|---|
| Refrigerated air conditioning | The outdoor temperature and the indoor target | No. It runs when the house needs it |
| Evaporative cooler | The outdoor temperature and the moisture in the air | No, with one exception: the fan can be staged differently |
| Pool pump and cleaner | The household’s own schedule and the filtration the pool needs | Yes: the hours are a choice |
| Washing machine and dryer | The household | Yes |
| Dishwasher | The household, within its own drying and delay settings | Yes |
| Oven, range and microwave | Meal times | Barely |
| Electric water heater | When hot water is drawn, and how the tank recovers | Partly |
| EV charging | When the car is at home and how far it has to go | Yes, almost entirely |
| Irrigation controller | A schedule the household sets, usually overnight in summer | Yes |
| Refrigerator, freezer, network equipment | Nothing. They are continuous | No |
The two rows at the top and the bottom are the point of the table. The largest single load in a Phoenix house follows the weather and cannot be rescheduled, and the small continuous loads cannot either. What sits between them (the pool, the laundry, the dishwasher, the car) is where a household’s timing is actually a choice.
What moving a load does, and what this page will not say
Moving a load changes when the electricity is bought. Whether that changes what the household pays depends on the plan, the season, the hours and the amount of load moved, and this site does not estimate it. There is no figure on this page for money saved, because a figure like that would be a guess about one household’s plan presented as a fact about everyone’s, and a guess is not what this site is for.
What this site can do is put the household’s own routine next to the hours. The Peak-Hours Habit Planner takes a rough idea of when the heavy appliances run and lays it against a typical summer peak window, then links out to APS’s and SRP’s own current plan pages rather than storing a copy of them. Understanding a summer electric bill takes the other half of the arithmetic (how a bill is assembled and why two identical houses get different ones) and if the question is why the evening is the way it is at all, why Phoenix nights stay hot is the climate behind it.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, hourly summaries, Phoenix Sky Harbor (USW00023183)
The typical temperature at every hour of a July day, including the 4 p.m. peak of 104.8 °F, the 6 a.m. minimum of 85.6 °F, and the eight hours at or above 100 °F.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, monthly summaries, Phoenix Sky Harbor (USW00023183)
The normal July low of 84.5 °F and the normal count of days at or above 100 °F by month.
- Arizona Public Service: residential pricing pages
That APS publishes more than one residential pricing option and that some price a kilowatt-hour differently at different hours. No APS rate or window is reproduced on this page.
- Salt River Project: residential price plan pages
The same two facts for SRP, whose plan hours are set out in its own filed schedules. No SRP rate or window is reproduced on this page.