Reduce Your Electricity Bill: A Worked TNB Bill Exercise | Cobler
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Capstone: Read and Reduce Your Electricity Bill
A worked exercise to finish the course. Aina reports to the owner of our example office tower: sort the September 2026 TNB bill by what drives each line, value a saved kWh at the marginal rate, and price three options against a shoplot's LV bill.
Tan Kok XinUnderstanding Your Electricity Bill
Part 7 of 7 in Cobler's Understanding Your Electricity Bill course. New here?See the course page.
At the start of this course, the owner of the office tower handed Aina, the tower's facility executive, the September 2026 TNB (Tenaga Nasional Berhad) bill for RM310,532 and asked two questions: "Why is it so high, and what can we do?" Part 6 explained the last unexplained line, so she can now read every line of the bill. This final part is the report she writes back to answer the owner, worked through as an exercise.
To read and reduce an electricity bill, sort each line by what drives it, then value each option only by the lines it really changes. For the tower, on the MV (medium voltage) non-domestic General tariff, a saved kWh is worth 33.50 sen, and a kW taken off the month's highest half hour is worth RM89.27. For the café owner in the shoplot, on LV (low voltage) non-domestic General, only kWh matter. The tasks below use both buildings' September 2026 bills and value three options: lowering the tower's maximum demand by 100 kW, moving a load out of the 2pm to 10pm Time-of-Use window, and keeping the power factor above 0.85.
How to use this exercise
Each task gives you some information and asks a few questions. Try the questions first with a calculator and a notepad. The Worked answer comes straight after each task so you can check your working.
Both buildings are fictional, and their numbers are consistent across Cobler's courses. Tariff figures are the RP4 rates (the TNB tariff structure in force since July 2025) for Peninsular Malaysia, and the Automatic Fuel Adjustment (AFA) figure is for September 2026. All ringgit figures are before the KWTBB levy and tax, as in Part 3.
You need four ideas from earlier parts:
From .
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Maximum demand (MD) is the highest 30-minute average kW in the month. On MV General it counts at any hour. From Part 4.
The ToU peak window and the power factor surcharge. From Part 5 and Part 6.
Task 1: Sort each bill line by what drives it
Here are the two September 2026 bills from Part 3, line by line.
Office tower (MV General): energy RM149,150; capacity RM47,088; network RM95,744; retail RM200; AFA RM18,350. Total RM310,532 for 500,000 kWh and an MD of 1,600 kW.
Shoplot (LV General): energy RM2,162.40; capacity RM706.40; network RM1,185.60; retail RM20.00; AFA RM293.60. Total RM4,368.00 for 8,000 kWh.
For each building, group the lines into three piles: lines that follow kWh, lines that follow maximum demand, and fixed lines. How much is in each pile?
Which piles can Aina influence, and which can the café owner influence?
Worked answer
For the tower:
Follows kWh: energy and AFA, RM149,150 + RM18,350 = RM167,500 (54% of the bill).
Follows maximum demand: capacity and network, RM47,088 + RM95,744 = RM142,832 (46%).
Fixed: retail, RM200 (under 0.1%).
For the shoplot:
Follows kWh: energy, capacity, network and AFA, RM2,162.40 + RM706.40 + RM1,185.60 + RM293.60 = RM4,348.00 (99.5%). On LV, capacity and network are charged per kWh, so they sit in this pile.
Follows maximum demand: nothing. An LV bill has no per-kW charge.
Fixed: retail, RM20 (0.5%).
Aina can influence two piles: the kWh pile, by using less energy, and the demand pile, by lowering the month's highest half hour. Together they are all but RM200 of the bill. The café owner can influence only the kWh pile, which is nearly the whole bill. The retail charge does not change for either of them.
So the answer to "why is it so high?" has two parts. The tower used 500,000 kWh, and nearly half the bill came from a single half hour at 1,600 kW. As Part 4 showed, the tower's load factor is about 0.43: its peak is well above its average, which makes the demand pile large.
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The tower's bill has two piles Aina can influence; the shoplot's bill has one.
Task 2: What is one saved kWh worth?
The level 12 east office floor of the tower (lighting, sockets and the supply fan of air handling unit AHU-12A) uses 7,392 kWh a month, as worked out in Read a Building's Electrical Story, the capstone of our Electricity Fundamentals course.
What do those kWh add to the tower's bill? Use the lines that follow kWh.
Aina's first draft divided the bill by the kWh to get an average cost of 62.1 sen/kWh, and multiplied by that. What answer did she get, and why is it wrong for valuing a saving?
What is one saved kWh worth to the café owner?
Worked answer
On MV General, a kWh adds to the energy line (29.83 sen) and the AFA line (3.67 sen in September 2026). That is 33.50 sen for each kWh, the marginal rate: the change in the bill for one more or one fewer kWh.
Energy: 7,392 × 29.83 sen = RM2,205.03
AFA: 7,392 × 3.67 sen = RM271.29
Together: 7,392 × 33.50 sen = RM2,476.32 a month.
The average rate is RM310,532 ÷ 500,000 kWh = 62.1 sen/kWh (62.1064 sen more exactly). At that rate, 7,392 kWh comes to about RM4,590.91, which overstates the answer by about RM2,114.59. The average includes the capacity and network lines, and those fall only if the month's highest half hour falls. If the floor used fewer kWh, the MD would fall only if the saving happened to reduce the load in that one 2:30pm half hour, and only the bill's MD reading can confirm that. So Aina values kWh savings at 33.50 sen, and adds a demand saving only when the peak really falls.
For the shoplot, every line except retail follows kWh. A saved kWh removes 27.03 + 8.83 + 14.82 + 3.67 = 54.35 sen from the energy, capacity, network and AFA lines. The shoplot's average of about 54.6 sen/kWh (RM4,368 ÷ 8,000) is only slightly higher, because only the RM20 retail charge is left out.
Worth knowing: The average cost of a kWh on a bill is not what a saved kWh is worth. On the tower's bill the average is 62.1 sen, but a saved kWh removes only 33.50 sen, because nearly half the bill follows the peak, not the kWh. On the shoplot's LV bill the two are almost the same.
Task 3: Find the half hour that sets the demand charge
The tower's monitoring system logs the energy used in each half hour. Here is part of Thursday 17 September 2026, a hot weekday afternoon. The month's highest reading falls on this afternoon.
Half hour
Energy in the half hour (kWh)
12:00 to 12:30
710
12:30 to 13:00
695
13:00 to 13:30
725
13:30 to 14:00
755
14:00 to 14:30
785
14:30 to 15:00
800
15:00 to 15:30
790
15:30 to 16:00
770
16:00 to 16:30
745
16:30 to 17:00
715
Turn each reading into an average demand in kW.
What is the maximum demand, and when was it set?
What does this MD cost for the month on MV General?
Worked answer
Each reading is energy over half an hour. Average power = energy ÷ time = kWh ÷ 0.5 h, which is the same as kWh × 2. For example, 710 kWh in half an hour is an average of 1,420 kW. The ten averages are 1,420, 1,390, 1,450, 1,510, 1,570, 1,600, 1,580, 1,540, 1,490 and 1,430 kW.
The highest half hour is 2:30pm to 3:00pm at 1,600 kW (800 kWh × 2). Because no other half hour in September is higher, the month's MD is 1,600 kW. (For this exercise, assume the logged half hours line up with the periods the TNB meter uses. The TNB meter's own MD register is what appears on the bill.)
On MV General, the capacity and network lines together are RM89.27 per kW: 1,600 kW × RM89.27 = RM142,832, the demand pile from Task 1.
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The tallest half-hour bar sets the maximum demand. The shaded band is the ToU peak window, which does not matter on the tower's General tariff, where a half hour at any time of day counts.
Task 4: Option one, lower the maximum demand by 100 kW
Aina's first option is to hold the tower's highest half hour to 1,500 kW. Part 4 put a figure on this; here she checks it against the peak-day log and the bill lines. The building management system (BMS) would briefly hold off flexible loads as demand nears the limit: car park ventilation fans, the water transfer pumps that refill the roof tanks, and a small reset of the chilled water temperature. The loads run a little later instead, so the load is moved, not removed.
On the peak day in Task 3, which half hours are above 1,500 kW, and how many kW must be held off in each?
Which bill lines change, by how much, and what is the new total?
What would it be worth over a year, and what has to be true for that figure to hold?
Worked answer
Five half hours are above 1,500 kW: 1:30pm (1,510 kW), 2:00pm (1,570), 2:30pm (1,600), 3:00pm (1,580) and 3:30pm (1,540). The BMS must hold off 10, 70, 100, 80 and 40 kW in those half hours. Held for half an hour each, that is (10 + 70 + 100 + 80 + 40) × 0.5 = 150 kWh, which is used later in the afternoon instead.
Only the two per-kW lines change:
Capacity falls by 100 × RM29.43 = RM2,943, to RM44,145.
Network falls by 100 × RM59.84 = RM5,984, to RM89,760.
The saving is RM8,927 for the month (the same as 100 × RM89.27). The new total is RM310,532 − RM8,927 = RM301,605, about 2.9% lower.
The energy and AFA lines stay the same, because the kWh were moved, not saved. Retail stays at RM200.
If the peak is held 100 kW lower every month, the saving is 12 × RM8,927 = RM107,124 a year. That holds only if no half hour in any month goes above the target. One hot afternoon that the BMS misses sets that month's MD, and the saving for that month is lost.
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Holding the tower to 1,500 kW means trimming five half hours; the 150 kWh held off are used later, so only the demand lines fall.
Task 5: Option two, move a load out of the 2pm to 10pm window
Aina's second idea came from Part 5. She could move a flexible load, such as the water transfer pumps, so that it runs before 2pm or after 10pm on weekdays.
The tower is on MV General. What does moving the load save on the energy and AFA lines?
When would the move save anything at all?
Would switching the tower to MV ToU make moving loads worthwhile? Part 5 found that ToU would cost the tower about RM6,172 more a month.
The café owner asks the same question about the shoplot, which is open from 8am to 10pm every day.
Worked answer
Nothing. On a General tariff every kWh costs the same at every hour (29.83 sen energy plus 3.67 sen AFA on the tower's bill). Moving a kWh from 3pm to 11pm leaves both lines unchanged.
Only if the move lowers the month's highest half hour. Then it saves RM89.27 for each kW taken off the peak, but that is option one again. Aina counts it once, in Task 4, not twice.
No. On MV ToU, each kWh moved from the peak window to off-peak saves 31.32 − 27.23 = 4.09 sen. In September 2026, ToU would have cost the tower RM12,464 more on demand and RM6,292.40 less on energy, so RM6,171.60 more in total. Just to break even, the tower would have to move about RM6,171.60 ÷ RM0.0409 ≈ 150,900 kWh a month out of the window. That is over nine-tenths of the 164,000 kWh it uses there, while its offices are occupied until 6pm. The demand line would also stay RM7.79 per kW higher on ToU for whatever peak is left in the window. The tower stays on General, and option two is worth nothing on its own.
On LV General, moving a load saves the café owner nothing either, and there is no demand line to lower. On LV ToU, the capacity and network rates per kWh are the same as on LV General, so only the energy rates differ. The LV ToU rates sit 1.49 sen above and 2.60 sen below the LV General rate, the same gaps as on MV, so Part 5's break-even applies: ToU lowers the bill only if more than about 36% of the shoplot's kWh fall outside the window. Weekday mornings until 2pm and all of Saturday and Sunday are off-peak, but the café's evenings are inside the window. The owner needs a month of half-hour data from the meter to check the share before switching.
Task 6: Option three, keep the power factor above 0.85
The tower's September bill shows a power factor of 0.92 and no surcharge. Aina's third option is to keep it that way: check the power factor on every bill and have the capacitor banks maintained, so that a failed step does not go unnoticed.
What is this option worth on the September 2026 bill?
What would each 0.01 below 0.85 cost the tower in a September-sized month?
Does the same apply to the shoplot?
Worked answer
Nothing on this bill. At 0.92, the tower pays no surcharge, so there is no line to reduce. The option protects against a charge rather than reducing one.
Below 132 kV, the surcharge is 1.5% of the energy, demand and AFA charges for each 0.01 below 0.85 (Part 6). Those charges are RM149,150 + RM142,832 + RM18,350 = RM310,332. So each 0.01 below 0.85 would cost 1.5% × RM310,332 = RM4,654.98 a month. Part 6 worked out the cost at a power factor of 0.80.
TNB's published rule does not say how the surcharge applies to LV accounts, and the shoplot's September bill has no power factor line. The café owner should check the bill rather than assume either way.
Aina's report to the owner
Aina's report answers the owner's two questions in plain terms. The bill is high because the tower used 500,000 kWh, worth RM167,500 at 33.50 sen each, and because one half hour at 2:30pm on 17 September set RM142,832 of capacity and network charges. Of the three options, only lowering the peak changes this month's bill. She also adds the shoplot as a contrast, because the café owner asked her the same question.
Option
Office tower (MV General)
Shoplot (LV General)
Lower the highest half hour by 100 kW
RM8,927 a month; RM107,124 a year if held every month
Nothing: no per-kW charge
Move a load out of 2pm to 10pm
Nothing on its own; counts only if it lowers the peak (then it is option one). ToU would cost about RM6,172 more a month
Nothing on LV General; on LV ToU only if more than about 36% of kWh are off-peak
Keep power factor above 0.85
Nothing today (PF 0.92); avoids RM4,654.98 a month for each 0.01 below 0.85
No published LV rule; check the bill
Use fewer kWh
33.50 sen per kWh saved, plus RM89.27 per kW if the peak also falls
54.35 sen per kWh saved
Her recommendation is to start with demand limiting through the BMS, keep the monthly power factor check, stay on the General tariff, and look for kWh savings valued at 33.50 sen. The savings above are before KWTBB, which is 1.6% of the kWh and kW charges (Part 3), so the real saving would be slightly higher.
Where to go next
If one task gave you trouble, go back to the part it draws on:
If all six tasks made sense, you have finished the course. The owner's next question will be whether demand limiting is worth what it costs to set up, and that is the subject of our Energy Management: The Economics of Saving Energy course. It takes the tower's options further: how to measure a baseline, prove a saving, and judge a project with payback and discounted cash flow. For how a BMS holds a demand limit in practice, see Load Shedding and Demand Limiting with a BMS.
Check your understanding
A tenant's lighting upgrade in the tower saves 2,000 kWh a month, and the bill's MD does not change. What does the bill fall by? 2,000 × 33.50 sen = RM670 a month, from the energy and AFA lines. Valuing it at the 62.1 sen average would give about RM1,242, which wrongly counts capacity and network charges that did not change.
The café owner hears that the tower could save RM8,927 a month by lowering its peak. Would staggering the café's air conditioners to lower its peak save money in the same way? No. The shoplot is on LV General, which has no per-kW demand charge; capacity and network are charged per kWh. Its bill falls only if it uses fewer kWh, at 54.35 sen for each kWh saved.
Recap: Sort the bill by what drives each line. On the tower's MV General bill, RM167,500 follows kWh, RM142,832 follows the month's highest half hour and RM200 is fixed; on the shoplot's LV General bill, everything except RM20 follows kWh. Value a saved kWh at the marginal rate (33.50 sen for the tower, 54.35 sen for the shoplot), not the bill's average, and add RM89.27 per kW only when the peak really falls. Holding the tower to 1,500 kW saves RM8,927 a month. Moving a load out of the 2pm to 10pm window saves nothing on a General tariff unless it lowers the peak, and ToU would cost the tower about RM6,172 more a month. Keeping the power factor at 0.92 costs nothing on today's bill but avoids RM4,654.98 a month for each 0.01 below 0.85.
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Tan Kok XinUnderstanding Your Electricity Bill
