Off-Peak, Mid-Peak, Peak: Where You Shift Load Decides the Saving

If the energy charge box on your bill has three lines, those three lines are off-peak, mid-peak and peak. It is the same 1kWh, but priced differently according to when you used it. This is one of the few places where you can reduce the bill without changing any equipment.

What are off-peak, mid-peak and peak?

They are time bands divided by how heavily national electricity demand is concentrated. When demand is concentrated, even expensive generators have to run, so the unit price is high; in the small hours it is low.

Under Article 38(2) of the Basic Terms of Supply, only contract types where a time-of-day meter is installed are in scope, and among industrial types those are (A) II and (B). If the energy charge on your bill is a single line, you are not in scope.

What hours do the bands run from and to?

The authoritative source for the time bands is not the tariff table but item 1 of Appendix 3 of the Basic Terms of Supply (the seasonal and time-of-day classification table) (in force 1 July 2026). Summer and spring-autumn share the same bands, and only winter differs.

The lengths of the three bands are the same regardless of season — off-peak 10 hours, mid-peak 8 hours, peak 6 hours. All that changes with the season is where they sit. What changed and why is in the time-of-use tariff reform explained.

Do the time bands differ by season?

They do. And the season is divided by date, not by billing month. The boundaries of the seasons are set by the same item 1 of Appendix 3 of the Basic Terms of Supply.

If your meter reading date is not the first of the month, one bill will contain two seasons mixed together. For high-voltage large-capacity customers the split is by actual consumption either side of the season boundary date; otherwise it is by the ratio of days (Article 48(9) of the Implementation Rules).

How much do the unit prices differ?

Considerably, by season. This is where the most common misunderstanding arises — the widely quoted "peak is twice off-peak" is the summer unit price, and in spring and autumn that gap shrinks to a third of it. Winter sits in between.

These are for Industrial (B) high-voltage A, Optional II, from Appendix 1 of the Basic Terms of Supply (the monthly electricity tariff table). The gap between peak and off-peak is KRW 113.0 per kWh in summer (1.97 times), KRW 81.6 in winter (1.66 times) and KRW 34.9 in spring and autumn (1.30 times). Move the same 1kWh and the effect varies by more than threefold depending on the season.

Unit prices are published separately for each contract type, supply voltage and optional tariff, so you need to check the tariff table for your own conditions.

How much do you save by moving operations?

The consumption you moved × the difference in unit price, and moving the same amount produces amounts that vary by more than threefold depending on the season. As an example, move 1,000kWh a month from peak to off-peak and — in summer that is (229.0 − 116.0) × 1,000 = KRW 113,000 a month, while in spring and autumn it is (150.9 − 116.0) × 1,000 = KRW 34,900 a month. These are numbers you get with no capital investment and no reduction in consumption, but converting the summer figure into a full-year result will greatly inflate the saving.

Does the basic charge also change with where you move to?

It does. A peak moved to the night or a public holiday drops out of the billing demand calculation altogether, while a peak moved to Saturday is captured as it stands. That is because, under the proviso to item 1 of Appendix 3 of the Basic Terms of Supply (in force 1 July 2026), billing demand takes only the maximum demand in mid-peak and peak hours, and public holidays are metered as off-peak hours.

The savings are measured against peak, on Industrial (B) high-voltage A, Optional II unit prices. Move the same load and the saving comes in two layers if it goes to the night or a public holiday, and one layer if it goes to Saturday or to daytime mid-peak. Load-shifting advice in circulation usually counts only the first layer.

There are two traps. The public holidays meant here are the holidays under 「관공서의 공휴일에 관한 규정」 (the Regulation on Holidays for Government Offices), and temporary public holidays are treated as weekdays, so a peak from special working on a temporary public holiday is reflected as it stands.

Also, this special treatment applies only to contract types with time bands (General and Industrial (A) II and (B), and Educational (B)). The calculation procedure is in how the basic charge is calculated.

In spring and autumn there is also a weekend discount tariff. Article 67-7 of the Terms (weekend discount tariff) provides the basis, and Article 48-7 of the Implementation Rules sets out the detail.

This scheme has an end date. Under the Supplementary Provisions to the Terms (2026.3.16), paragraph 2, item 3, it has effect from the date of commencement until 31 December 2030, and whether to extend it is reviewed every three years, analysing take-up of weekend demand shifting and the effect on easing output curtailment. If you are planning capital investment on the assumption of the discount, you have to put this end date into your payback period.

Is running at night always better?

No. There are three things to look at together.

First, the off-peak unit price went up with the reform of 16 April 2026. On Industrial (B) high-voltage A, Optional II, off-peak rose from KRW 110.9 to KRW 116.0, up KRW 5.1 per kWh in every season. Peak, by contrast, came down, but by different amounts in different seasons — in summer from KRW 245.9 to KRW 229.0, down KRW 16.9, and in spring and autumn from KRW 164.1 to KRW 150.9, down KRW 13.2 (the deferral calculation formula in the Supplementary Provisions to the Terms, 2026.3.16, paragraph 2, item 4 states the same differences). For a plant with a large night-time share, this reform has made things worse.

Second, from 22:00 to 08:00 the following day is the leading power factor band, so falling short of the benchmark only adds a surcharge to the basic charge, with no discount available. Increase night-time operation and the power factor charge moves with it.

Third, night-time labour costs and the cost of organising shifts may exceed the saving.

Where do we check our plant's usage pattern?

The starting point is downloading the time-of-day consumption curve from KEPCO PowerPlanner. On what we have confirmed from a high-voltage account, we were able to obtain 96 points a day, that is a curve at 15-minute intervals (our own measurement, June 2026).

How to obtain it is in how to get your KEPCO usage data.