One Peak Reading Can Hold Your Basic Charge for a Year

There are months when you used less electricity and the basic charge did not move. You take one piece of equipment out of service and the bill does not budge. In most cases there is a single reason.
What sets the basic charge is not how much you used this month, but a maximum demand reading recorded once in a particular past month.

What is maximum demand?

Maximum demand is the largest of the power values recorded during a month (kW). The basic charge is calculated by multiplying that value by a unit price. So even if you reduce your usage, the basic charge does not come down while that one point stays where it is.

It is easy to confuse with kWh, which means usage, but they are different in character. kWh is the total accumulated over a month; maximum demand is the intensity at the single highest point within it. The total and the peak move independently. Cut your operating days and the total falls, but if you run everything at once on the days you do run, the peak stays where it was or even rises.

Is maximum demand measured in 15-minute intervals?

Yes. This is not a screen setting but the metering interval the Terms of Supply prescribe. Article 6, item 10 of the Basic Terms of Supply defines maximum demand as follows.

In English, in substance: the power accumulated in 15-minute intervals by a meter capable of measuring maximum demand.

The power factor is different. Article 42(2) of the same Terms prescribes that it be calculated from values accumulated in 30-minute intervals. Both values come from the same meter, but the measuring interval differs.

[Basis: Basic Terms of Supply, Article 6, item 10 and Article 42(2), in force 2026.07.01]

Translated into practice, it means this. Because the average over one of those 15-minute windows becomes the month's highest value, even a load that spikes for only a few minutes stays on the bill if it falls inside that 15-minute window. The 96 points a day that PowerPlanner shows are exactly these 15-minute windows.

Which month's peak becomes the basis for the charge?

Not every month. Article 68(1) of KEPCO's Basic Terms of Supply sets the billing demand as follows.

In English, in substance: for a customer with a meter capable of measuring maximum demand, the billing demand is the largest maximum demand among the December, January, February, July, August and September readings within the preceding 12 months including the current metering month, plus the current month's reading; and where that largest maximum demand is below 30% of contract power, 30% of contract power becomes the billing demand.

So the months that can qualify are three (the December, January and February readings), three more (July, August and September), and the current month — seven in all. The remaining March, April, May, June, October and November readings do not qualify. They come in briefly while that month is the current month, and then drop out.

As a direct consequence of this provision, a peak recorded while commissioning equipment in April or October does not stay on later bills. Conversely, a peak recorded in August follows you all year.

[Basis: Basic Terms of Supply, Article 68(1), in force 2026.07.01]

※ This article is about customers to whom Article 68(1) of the Basic Terms of Supply applies (mainly high-voltage and above). For customers under Article 68(2), the billing demand is simply the contract power, so the above does not apply to them. And for low-voltage there is one more layer.

※ A low-voltage customer can fall under Article 68(2) even with a maximum demand meter fitted. Low-voltage customers with contract power of 20 kW or more may have a maximum demand meter installed (Terms of Supply, Article 38(4); the customers this covers are set by Article 24(2) of the Implementation Rules), but what Article 68(1) points to is only Article 38 (2) and (3). So even with a meter fitted, they go under Article 68(2). Which side your plant falls on is best confirmed with your local KEPCO branch. [Basis: Basic Terms of Supply, Article 38(4) and Article 68(1); Implementation Rules of the Basic Terms of Supply, Article 24(2), in force 2026.07.01]

※ The December, January, February and July, August, September readings set out here are the metering months designated by the Terms of Supply, not a seasonal classification. Under time-of-use tariffs, summer runs from 1 June to 31 August, and September belongs to spring and autumn (1 March to 31 May, 1 September to 31 October). Read the two classifications on top of each other and they do not line up. [Basis: Basic Terms of Supply, Appendix 3, item 1, in force 2026.07.01]

Why does last summer's peak raise this year's bill?

Because the qualifying window is the preceding 12 months. A value recorded last August stays on the candidate list right up to this August's meter reading, and if no higher value appears in any month in between, that value becomes the billing demand every month.

When we compared all 12 sites on our own SaaS, "the highest value across the whole preceding 12 months" and "the highest of the seven candidates under Article 68" were the same at every site. That is because the annual maximum was already concentrated in the July and August readings or the December, January and February readings (observed July 2026, on our own production data, n=12. This does not generalise to all sites).

Put another way, the few days recorded in those six qualifying months determine the basic charge for the remaining months too. It means that concentrating your savings effort on those few days is far more efficient than spreading it evenly across the year.

Why doesn't the basic charge fall even when I use less?

Because there is a floor. Under the proviso to Article 68(1) above, if the largest maximum demand among the candidates is below 30% of contract power, 30% of contract power becomes the billing demand. However little you use, 30% of contract power is built into the basic charge.

[Basis: Basic Terms of Supply, Article 68(1); Implementation Rules of the Basic Terms of Supply, Article 49, in force 2026.07.01]

So a plant that has cut operations until its maximum demand falls below 30% of contract power will not save a single won on the basic charge by pushing the peak down further. What such a plant has to touch is not the peak but contract power itself. Which of the two you should be managing at your own plant is settled in the difference between contract power and maximum demand.

Which time bands' peaks do we actually have to manage?

Only peaks in the mid-peak and peak time bands count. Maximum demand recorded in the off-peak band (22:00–08:00) and on public holidays does not enter the billing demand calculation. Conversely, Saturday peaks do count.

Billing demand is calculated under Article 68 on the greater of the maximum demand in the mid-peak band and in the peak band. Maximum demand on a public holiday is metered as off-peak and drops out; a Saturday that is not a public holiday is metered as mid-peak and stays in. This applies to contract types that have time bands.

From a peak management point of view, this proviso changes three things:

• There is no need to chase peaks from night working. However high the reading at three in the morning, that value does not stay on the basic charge. Installing equipment to catch a night-time peak is spending money where it has no effect.
• Load shifting pays twice. Moving a daytime peak into the night not only lowers the energy charge unit price but also takes the basic charge's own reference value out of the calculation.
• You cannot treat Saturday like a holiday. The habit of piling equipment onto Saturday overtime means that peak lands on the bill exactly as recorded. Temporary public holidays are also treated as weekdays, not holidays.

[Basis: Basic Terms of Supply, Appendix 3, item 1, proviso items 1 to 3, in force 2026.07.01]

Where do I find our plant's maximum demand?

You check it in two places: the electricity bill and KEPCO's PowerPlanner. The bill states the value used in this month's calculation, and PowerPlanner lets you see when and in what shape that value was created, as a load curve.

※ We could not find material published by KEPCO on how many months of past load curves PowerPlanner will show, and we have not measured this ourselves on a low-voltage account. What is clear, though, is that it is not kept indefinitely. If you do not take it now, there will be a stretch you cannot get back.

The specific route for downloading the data is set out in how to get your factory's power use data from KEPCO.

What should we do first to lower the peak?

Before buying equipment, look at when things overlap. A peak usually comes not from one large machine but from several machines coinciding at the same moment. Untangling the overlap costs nothing; bolting on equipment costs money. Reverse the order and you lose.

The first thing to look at is the morning start-up all at once. Switch on the compressors, chillers and air handling at the same time as the shift begins and the load lands all at once at that moment. Stagger the start times by a few minutes each to reduce how many machines coincide, and the day's peak comes down. To look at the time-band unit price structure alongside this, see the off-peak, mid-peak and peak time bands; and if you are considering buying a saving device, we recommend reading do electricity saving devices work? first.

※ The provisions above were checked character by character against the current text KEPCO publishes. We downloaded and confirmed the Terms of Supply PDF from KEPCO's information disclosure posting 「(약관) 기본공급약관 및 시행세칙(260801 시행)」 (the Terms of Supply and the Implementation Rules; registered 2026.07.30), and the last amendment to the Terms is dated 25 June 2026, in force 1 July 2026. The months used in the calculation (the December, January, February, July, August and September readings plus the current month) and the 30% of contract power floor appear in the same wording in Article 49 of the Implementation Rules of the Basic Terms of Supply as well.