How much solar output lands in a time-of-use peak? Six utilities' peak hours, checked hour by hour

The measurement: one array's year, sorted into peak windows

We ran NLR's PVWatts version 8 model (formerly NREL's) through its public interface on October 30, 2026 for an 8 kW DC roof-mounted array in Denver, Colorado: standard modules, a fixed 20 degree tilt facing due south, PVWatts' default 14 percent system losses and a DC to AC ratio of 1.2, on the model's typical-year weather file for that location. The hourly results give the modelled AC output for each of the year's 8,760 hours. We then added up the output that falls inside each utility's peak window.

Denver is the worked location because it is the run we have hourly data for, and it sits in a utility territory with a published window of its own. The shape of the solar day is similar across the continental United States: output climbs from sunrise, peaks around solar noon and falls away through the afternoon. What changes from city to city is the total and, more subtly, where solar noon falls on the clock, which depends on where a city sits inside its time zone. Denver's longitude puts its solar noon close to the middle of the Mountain time zone, so its curve is a fair general example; a city near the western edge of its zone peaks later on the clock and puts a little more output into an evening window, and one near the eastern edge puts a little less.

Read the first row against the last. On this roof, about 11 of every 100 kWh the array makes arrive during a 4 to 9 p.m. peak, while about 88 arrive in the eight hours before it. A 4 to 9 p.m. window is not a small, expensive slice of the solar day. It is the part of the day after the panels have mostly finished.

Shorter and weekday-only windows catch even less. The Xcel and PG&E E-TOU-D rows come out the same, about 3.3 percent, because this array makes essentially nothing after 8 p.m. even in June: once a window starts at 5 p.m., the extra evening hour adds load to the window but no solar. The Con Edison row shows the opposite design. An afternoon super-peak from 2 to 6 p.m. over the summer weekdays alone catches about 9.1 percent of the whole year's output from the same roof, more than double the 4.0 percent the 4 to 9 p.m. window catches over the same months. Where a peak starts matters far more than how long it runs.

These shares are for one flat, unshaded example roof on typical-year weather. A roof-specific PVWatts run with your own tilt, azimuth and shading will move them, and the method on this page is the one to repeat with it.

The daylight saving trap that halves the answer

One technical step changes the result more than any other. PVWatts runs on the simulation engine of NLR's System Advisor Model, and SAM reports hours in local standard time with no daylight saving adjustment. Our source for that is a forum reply, not PVWatts' own documentation: a moderator on NLR's System Advisor Model support forum wrote in December 2021 that times are in local standard time given the weather file's time zone, with no adjustment for daylight saving time. The PVWatts v8 API documentation does not mention daylight saving, but the saved Denver run agrees with the reply: June output peaks around 11 a.m. to 1 p.m. on the model's clock, centred on solar noon in standard time at Denver's longitude. A utility's 4 p.m. on a July afternoon is 3 p.m. standard time, which is when the array is still producing strongly.

So for every hour between the second Sunday in March and the first Sunday in November we moved the window one hour earlier in the model's data before adding it up. Skip that step and the 4 to 9 p.m. share for this roof reads about 5.2 percent instead of 10.6 percent, half the true figure. Anyone checking an installer's time-of-use savings claim, or building their own spreadsheet from a PVWatts download, should ask this question first. Arizona does not observe daylight saving time, so a Phoenix window needs no shift.

The hourly averages show the shape behind the shares. In June the array still averages about 3.09 kW in the hour starting at 4 p.m. and 2.20 kW in the hour starting at 5 p.m., then falls to 0.97 kW and 0.21 kW over the next two hours. In December the hour starting at 4 p.m. averages 0.11 kW, and nothing at all comes after it. A winter 4 to 9 p.m. window on this roof is almost entirely a window with no solar in it.

Peak windows as six utilities print them

The windows in the measurement table come from each utility's own residential rate page, read on October 30, 2026, except Xcel Energy's Colorado schedule, where the utility's plan page did not render and the hours below are as reported by the Denver Gazette on June 4, 2026. The table keeps the hours, which the production measurement needs, and prints only the two price pairs the worked examples further down use. Rates change, often more than once a year, and current prices, fixed charges and plan comparisons belong to rate coverage: EnergyBS covers electricity rates and plans, and its time-of-use strategy guide covers how to choose one.

Three patterns stand out. First, the California, Arizona and Colorado windows all start at 4 p.m. or later, after a south-facing array is past its best hours. Second, seasons are not symmetrical: SCE's winter schedule makes the solar hours the cheapest power of the day, so self-consumed solar avoids a cheap purchase in winter and a more expensive one in summer. Third, Con Edison's structure is the outlier, with peak pricing from 8 a.m. to midnight and a summer super-peak squarely inside productive solar hours, a reminder that a time-of-use rate is not automatically bad news for solar.

Two caveats from the pages themselves. SCE's published prices are for customers who take both delivery and generation from SCE, and SDG&E publishes separate delivery-only prices for customers of a community choice aggregator, so a CCA customer's real price is the sum of two schedules. APS adds one solar-specific line: a Grid Access Charge applies to customers with rooftop solar. Fixed and solar-specific charges sit outside the hourly arithmetic here, but they belong in any bill comparison, and the true-up bill guide shows how monthly charges and annual settlement interact.

Why the export rule decides what the window costs you

On a flat rate, a solar kilowatt-hour is worth the same whenever it is made. On a time-of-use rate, the same kilowatt-hour can have three different values depending on where it goes: used in the house at once, exported to the grid, or, in the evening, replaced by a kilowatt-hour bought at the peak price. How those values relate is set by your export rule, and the window decides how much energy sits in each bucket.

There are three broad cases. Where net metering credits each exported kilowatt-hour at the retail price of the period in which it leaves the meter, a midday export earns the off-peak price and an evening import costs the peak price, so exports and imports no longer cancel one for one even though the programme is still called net metering. Under net billing, as in California's successor tariff, exports earn a separate and usually lower credit, so the most valuable use of a solar kilowatt-hour is to use it at home. And under net metering on a flat rate, the window does not apply at all, which is why moving an existing solar account onto time-of-use is a decision to model before agreeing to it.

Which case you are in is written in your export tariff, not in the rate plan. The export compensation section collects the rules by structure and state, and California homes on the successor tariff, which the California Public Utilities Commission applies to interconnection applications submitted on or after April 15, 2023, should read the California net billing guide. For a home grandfathered on an older rule, changing rate plans can carry its own risk, set out in the grandfathering guide.

One exported kilowatt-hour against one evening kilowatt-hour

The first case is easy to underestimate, so put numbers on it with SCE's TOU-D-4-9PM summer weekday prices from the table: $0.33 off-peak and $0.59 on-peak. If an export at 1 p.m. is credited at the off-peak price, it is worth $0.33, and it cancels only $0.33 divided by $0.59, about 0.56 of a kilowatt-hour bought at 7 p.m. A household that exports exactly as many kilowatt-hours at midday as it imports in the evening is not at zero; on these two prices it still owes $0.26 for every evening kilowatt-hour.

The Xcel Colorado prices as reported make the gap proportionally wider: about 7.9 cents off-peak against about 21.3 cents on-peak, so a kilowatt-hour valued at the off-peak price buys back about 0.37 of a peak kilowatt-hour. These ratios are illustrations of period-priced crediting, not a statement of how either utility credits exports today; your export tariff says which price an export earns. Under net billing the gap is usually wider again, because the export credit is set separately from the retail price. That is why time-of-use turns the question from how much the roof produces into how much of it the household uses at once.

Self-consumption: the energy that never meets the window

Every solar kilowatt-hour used in the house as it is produced avoids a purchase at whatever price applies at that hour. With about 87.6 percent of this array's output arriving between 8 a.m. and 4 p.m., and about 52.3 percent between 10 a.m. and 2 p.m. alone, the loads that can run in those hours decide how much of the array is worth the full retail price. Loads that can move into solar hours without changing how the house lives are the obvious first step:

• Timed appliances. Dishwashers, washers and dryers with a delay start can run at midday instead of after dinner.
• Water heating. An electric or heat pump water heater on a schedule can heat the tank in the early afternoon and coast through the peak.
• Pre-cooling. Running air conditioning harder from 1 to 4 p.m. and letting the house drift during the peak uses the building as a thermal store. It works best in well-insulated homes and dry climates where evenings cool.
• Pool pumps. A pump on a timer is one of the easiest large loads to move entirely into production hours.
• Electric vehicle charging. A car parked at home during the day can charge from the roof; a commuter car usually cannot, and charges overnight in off-peak hours instead. Sizing solar for an EV works through that load.

No list of moved loads touches the evening baseline: lighting, cooking, entertainment and refrigeration still draw during the window, and a south-facing array is mostly finished by then. That remainder is what west-facing panels, a battery, or a different plan are each trying to reach.

An evening the panels cannot reach

A labelled illustration with every input printed. A Denver household on Xcel's time-of-use plan has the 8 kW south-facing array from the measurement table. PVWatts gives 12,760 kWh a year, of which about 420 kWh arrive inside the 5 to 9 p.m. weekday window. Suppose the house uses 9 kWh in that window on each of 2026's 261 weekdays (holidays not removed), about 2,349 kWh a year. Even if every solar kilowatt-hour inside the window were used at home, about 1,929 kWh would still come from the grid, from a battery, or from loads moved out of the window.

Priced at the reported summer on-peak price of about 21.3 cents, that remainder is about $411 a year before any export credit is set against it; at a lower winter on-peak price it would be less. That is the evening cost an annual production figure does not show, and the number to compare with a battery's price or the value of moving loads. Replace each input with your own tariff, PVWatts run and evening use.

Facing west, a battery or a shorter window: three ways at the remainder

Turning panels toward the west moves output later in the afternoon and lowers the annual total, because a west plane misses the strongest midday sun. How much it lowers the total and how much it adds to an evening window depend on the roof's tilt, the latitude and the shading, so this page prints no rule of thumb; we have a modelled run only for the south-facing case. Where a home has a genuine choice of planes, run PVWatts for each with the actual tilt and shading, sum the output inside your window as described below, and price both the gain inside the window and the loss outside it. The production estimate guide shows how to rerun a quote's model plane by plane, and sizing from your bill shows where the annual total comes back into the decision.

A battery is the direct answer to the evening remainder: it stores midday surplus and discharges it in the window. Whether that pays is a question of the peak-to-off-peak spread, round-trip losses and the battery's price, worked in full in the battery decision guide, including the spread a plan must have before a daily cycle earns anything. The measurement on this page supplies one input: on a south roof, the energy a battery would need is available, because so little of the output was used in the window to begin with.

A plan with a shorter window is the third lever, and the table shows why it is not a free choice. SCE's TOU-D-5-8PM narrows the summer peak to three hours but prices those hours higher than the 4 to 9 p.m. plan prices its peak. The Denver array makes about 3.3 percent of its output in weekday 5 to 8 p.m. hours, against 10.6 percent in an every-day 4 to 9 p.m. window, so the narrow window is a bet on the household's evening habits, not on the panels. Plan rules can also restrict the choice: SCE's TOU-D-PRIME, for example, requires enrolling customers to confirm they own or lease an EV, a residential battery or an electric heat pump.

How to run the check for your own roof

A rate page and a free PVWatts run answer most of the questions above, if they are worked in a fixed order. Have your bill and your utility's plan page open:

1. Find the plan code on your bill, not the plan's marketing name. E-TOU-C, TOU-D-4-9PM and TOU-DR1 each have their own page.
2. Write down the peak hours, then the days they apply to. An every-day window and a weekdays-only window are different: the second drops about two days in seven.
3. Write down the season months. Summer and winter can change the window, the prices, or both, and some plans add a super off-peak period in solar hours.
4. Check who sells you generation. A community choice aggregator customer pays two schedules.
5. Find how exports are credited in your export tariff: at the retail price of the period, at a separate export rate, or by another formula.
6. Run PVWatts for your roof with hourly output selected, download the results, and move the window one hour earlier for the daylight saving months.
7. Add up the output inside the window and divide by the year's total. That is your version of the measurement table, and the figure to hold against any time-of-use savings claim.

The last two steps need a spreadsheet, and they are worth doing once. They replace a general claim about solar and peak hours with the number for your roof: how many kilowatt-hours a year land in the window, and how many do not.

Before you judge a solar quote on a time-of-use plan

• Write down your plan code, peak hours, peak days and season months from the utility's own page.
• Find in your export tariff whether exports earn the period's retail price, a separate export rate, or something else.
• Ask any installer for the hourly production model, not just the annual total.
• Add up modelled output inside your window, with the window moved for daylight saving time.
• Price the evening remainder at your peak price before comparing a battery, a west plane or a plan change.
• Check plan eligibility rules, and whether a switch affects a grandfathered export tariff.

Edge cases that move the result

• Weekend mid-peak hours. SCE prices summer weekend 4 to 9 p.m. as mid-peak, not off-peak, so an analysis of weekdays alone misses part of the evening cost.
• Holidays. Most plans treat named holidays as off-peak. It matters little for solar but changes evening costs.
• Cloudy peak seasons. The typical-year weather file stands in for many years. A run of overcast summer afternoons lowers window output below the table.
• Trackers. Ground mounts with single-axis tracking stretch output later into the afternoon than a fixed roof array; the table applies to fixed arrays.
• Position in the time zone. Two cities with the same weather can differ in window share because their solar noon falls at different clock times.
• Expansions. Adding a west-facing plane to an existing south array changes the combined shape and can affect an older export tariff, which the expansion guide covers.