On Taipower's own billing worksheet, a household used 350 kilowatt-hours between 13 September and 11 November. Summer prices officially ended on 30 September, yet eighteen of those sixty days still priced on the summer table. The bill is one meter read and one total on the receipt, but the utility runs two seasonal calculations and weights them by the calendar.
The bill that straddles October
Taipower defines summer rates for homes from 1 June through 30 September each year. Outside those dates, a separate non-summer table applies. That switch looks clean on a wall calendar.
Residential meters, however, rarely close on 30 September. Bimonthly routes start and stop on rolling dates tied to meter batches. The company's published example starts on 13 September and ends on 11 November: sixty days, 350 kWh on the dial.
Only the first eighteen days, 30 percent of the period, sit inside summer. The other forty-two run under October and November prices. A reader who divides the total charge by two and compares it with a neighbour's July bill is comparing unlike things before the arithmetic even begins.
The surprise is not that autumn is still muggy. It is that one printed total stitches together prices from two different columns on the government rate sheet, then scales them by how many nights on the calendar still said "summer."
What most explainers skip: two-month reads
Since July 1985 Taipower has read most apartments every two months to cut visits and paper bills. Roughly half of accounts are picked up in odd-numbered months and half in even ones, further split into meter batches whose windows hover near sixty-one days.
The bill shows two months of consumption in a single total. Press releases still ask consumers to halve the amount for a "monthly" mental model, which hides the real mechanic: tiers, not wording, are what double.
That structure predates seasonal pricing. Summer rates arrived in February 1989, when the utility began charging more for June through September to reflect costlier peaking generation. The staggered read dates did not move to match.
Spokesperson Wu Chin-chung told Taiwan's fact-checkers in 2022 that the seasonal scheme had run more than thirty years at that point, modelled on utilities in Japan, the United States, and Britain. The read cycle is older still. Whenever a batch closes on 5 October or mid-June, the company inherits a built-in straddle.
A route that opens in late May can likewise catch the first summer days before the window closes in July. The arithmetic changes; the pattern does not.
Step one: double the ladder, keep the price
Residential service uses six progressive tiers. Under the 2024 table the first 120 kWh each month cost NT$1.68 whether or not the month is summer. The next band, up to 330 kWh, jumps to NT$2.45 in summer and NT$2.16 off-season, a NT$0.29 spread at tier two. Higher bands widen further: tier three charges NT$3.70 against NT$3.03 in the children's rate explainer Taipower keeps on its education site.
For bimonthly accounts the published thresholds double: 240 kWh instead of 120, 660 instead of 330, and so on. The rate schedule's footnote states plainly that when users are billed every two months, each bracket's kilowatt-hour cap is multiplied by two.
Taipower argues doubling does not, by itself, increase what you pay versus monthly metering. You consume twice the energy before hitting the next step. The confusion shows up later, when one bill must apply two seasonal price tables at once.
Think of the tiers as a ladder printed twice as tall on the same wall. You still climb one rung at a time, but you travel farther in kilowatt-hours before the price per step rises. Seasonal pricing swaps the height of some rungs between summer and winter without moving the ladder's shape.
None of that appears on the paper bill as a line item. The receipt shows one usage total and one dollar total. Only the back-office worksheet runs the ladder twice.
MOEA's historical rate book shows the summer premium on the second tier staying remarkably flat even as absolute prices moved in 2012, 2016, and 2024. That stability is one reason the autumn straddle feels invisible: the calendar effect is often smaller than the jump between tier one and tier two once a household crosses roughly 330 kWh per month.
International visitors sometimes assume Taiwan bills like European time-of-use markets. Domestic law instead locks most homes into volumetric tiers. The day split is therefore not about rewarding midnight laundry. It is about reconciling two administrative calendars: the meter reader's and the energy ministry's.
Step two: cut the period by days, not by usage
With more than fifteen million customer accounts, Taipower says it cannot align every meter read with 1 June, 1 October, or a cabinet rate vote. When a billing window crosses a boundary, staff allocate the period's kilowatt-hours in proportion to days inside each tariff window, not by guessing which hours ran the air conditioner.
The English-language tariff FAQ describes the same ratio method used when parliament changes prices mid-cycle. Taipower also calls day-proration a routine tool among power companies adjusting tariffs.
Those parliament-approved tables themselves sit under Article 49 of Taiwan's Electricity Act, which requires Taipower to draft rates from a central formula, win approval, and publish the result. A revision such as April 2024 therefore becomes another boundary inside whatever sixty-day window is open when the new numbers take effect.
High-voltage commercial users face a slightly wider summer window, 16 May through 15 October, so factories can see seasonal splits on different dates than flats. Ordinary homes stay on the progressive non-time schedule Taiwan's fact-checkers contrast with viral "five-times daytime" hoaxes.
The chart tracks the gap between summer and non-summer prices in the 121-330 kWh band. MOEA archives show the spread near NT$0.34 per kWh in 2012, settling at NT$0.28 from 2016 through 2023 before the April 2024 revision nudged it to NT$0.29.
Run the official arithmetic
Back to the 350 kWh autumn bill. Taipower applies doubled tiers to the full read twice, once with summer unit prices and once with non-summer, then multiplies each subtotal by the day share.
In the FAQ formula the company charges the first 240 kWh at the tier-one price, then the next band up to 660 kWh at tier-two rates, for the entire 350 kWh batch. It computes that subtotal as if the whole period were summer, multiplies by 18/60, and repeats with winter prices multiplied by 42/60.
Using the 2023 worked rates in its FAQ, the summer slice runs roughly NT$753. The non-summer slice runs about NT$1,022. Combined, the example lands near NT$1,775 before fees and taxes, or about NT$296 per month if you insist on a monthly average.
Under 2024 prices the tier-two gap is slightly wider, but the shape is unchanged: most of the invoice still reflects non-summer kilowatt-hours even when September stays hot enough for air conditioning every night. That is why neighbours on different meter batches can see different "summer shares" on the same calendar weekend even when their air conditioners ran equally hard.
When the calendar still bites in November
The same day logic fired on 1 April 2024, when MOEA approved an average 11 percent rate increase. Homes under 330 kWh per month saw only a 3 percent unit bump, but anyone whose bimonthly window crossed the revision date received a blended bill.
Taipower's 2024 messaging noted that about two-thirds of households stay below 330 kWh monthly in summer, while only 2 percent exceed 1,000 kWh. Press summaries attributed roughly three quarters of higher summer spending to extra consumption and about a quarter to seasonal unit prices. Usage still dominates, but proration decides which table governs each slice.
一般家戶都是採「非時間電價」，依照用電量的級距計算電價，跟時段無關
Social-media posts insisting that daytime power will cost five or ten times midnight rates remain false for default households. Optional time-of-use plans exist for large loads willing to shift consumption. They do not replace the day-split summer mechanic for typical apartments.
Fairness critics argue that spreading kilowatt-hours evenly across days punishes people who only cranked the AC during late-September heatwaves. Taipower's reply, repeated in tariff guidance, is logistical: synchronized reads on every policy date are not operable at national scale, and the day split matches how the utility already blends price revisions.
Children's education copy on Taipower's site walks through a 350 kWh month in July versus April to show how the same total kilowatt-hours produces different bills purely from the seasonal column, before bimonthly or proration effects enter. That exercise is useful because it isolates one variable. Real bills layer both.
Heavy users who cross 1,000 kWh per month see the steepest rungs: NT$8.46 per kWh in summer at tier six on the 2024 table, against NT$6.63 off-season. Taipower's own press briefings say only about 2 percent of homes reach that band in summer. For everyone else, the seasonal split matters less than total consumption, but the split still appears whenever a read straddles 1 October.
Landlords who charge a flat per-kilowatt-hour markup on top of Taipower rarely replicate these mechanics. Tenants comparing a simplified rental rate with the regulated utility statement are often looking at two different models of the same dial.
Eighteen days in September
So the November bill that still "feels like summer" is not a clerical mistake. It is the published 350 kWh example doing what the rules require: eighteen September days priced on the summer ladder inside a single read that ends after Mid-Autumn Festival.
Taipower has priced those late-September days that way since bimonthly metering outpaced the seasonal calendar. The numbers on the receipt are the weighted sum of two complete tier calculations. They are not half summer and half winter. They are two full summers and two full winters scaled by the clock.
The worksheet never prints those two weighted subtotals on the envelope. Understanding the split is the only way to reconcile a mild October with a charge that still carries summer DNA in the official Taipower example.
Eighteen days in September, still on the summer ladder, inside one November statement. That is the whole answer hiding behind a single total. The published 350 kWh window is the cleanest place to see it work in practice.