Power vs Energy: The Difference Between kW and kWh

Part 4 of 23 in Cobler's Electricity Fundamentals series. New here? Start with the course map.

Two numbers on a TNB commercial bill trip up almost everyone. One is measured in kWh, the other in kW, and they look close enough that people assume they mean the same thing with a typo somewhere. They do not. The gap between power vs energy is the single most useful distinction in electricity, and once you see it, half of your bill stops being a mystery.

Here is the whole idea in one line: power is a rate, energy is a quantity. Power (measured in kilowatts, kW) is how fast energy is flowing right now. Energy (measured in kilowatt-hours, kWh) is how much flowed over some stretch of time. Everything else in this article is just that sentence, worked out in numbers you can check.

What is the difference between power vs energy?

Power is the speed of energy transfer; energy is the total delivered. The cleanest way to feel the difference is your car dashboard.

The speedometer reads your rate: 90 km/h. It tells you nothing about how far you have gone. The odometer reads your total: 40,000 km. It tells you nothing about how fast you are going right now. You can sit at 90 km/h for a minute or for six hours, and the speedometer never changes while the odometer climbs the entire time.

Power is the speedometer. Energy is the odometer. A kettle rated at 2 kW is drawing energy at a fixed rate whenever it is on, the same way 90 km/h is a fixed speed. How much energy it actually uses depends entirely on how long you leave it running.

Water works the same way. A tap open at 10 litres per minute is a flow rate, like power. The 30 litres sitting in your bucket afterward is a volume, like energy. Rate times time gives you the quantity: 10 litres per minute for 3 minutes is 30 litres. Kilowatts times hours gives you kilowatt-hours. That is literally what the "-hour" in kilowatt-hour is doing.

How do you calculate kWh from kW?

Multiply the power in kilowatts by the time in hours. That is the entire formula: kWh = kW × hours.

A 2 kW appliance running for 30 minutes uses 2 kW × 0.5 hours = 1 kWh. A 1 kW appliance running for a full hour also uses 1 kWh. A 100 W bulb (0.1 kW) left on for 10 hours uses 0.1 × 10 = 1 kWh as well. Same energy, three completely different power levels, because the slower ones simply ran longer.

This is where the famous confusion lives. People say "kilowatts per hour" when they mean their consumption, and it is meaningless. A kilowatt is already a rate. Saying "kilowatts per hour" is like saying "kilometres per hour, per hour," which describes acceleration, not distance. Your bill does not charge for kilowatts per hour. It charges for kilowatt-hours: kilowatts multiplied by hours, not divided.

Can a high-power device cost less than a low-power one?

Yes, and this catches people out constantly. What you pay for energy depends on power and time together, so a hungry device used briefly can cost less than a modest device left on all day.

Run a 2,000 W kettle for 3 minutes to boil water: 2 kW × 0.05 hours = 0.1 kWh. Now leave a single 15 W standby light on for a full day: 0.015 kW × 24 hours = 0.36 kWh. The standby light, which draws less than one percent of the kettle's power, quietly burns more than three times the energy over the day. The kettle looks dramatic on the speedometer and barely moves the odometer. The little light does the opposite.

This is why chasing high-wattage appliances is often the wrong instinct. A 3.5 kW air-conditioner matters not because of its rating but because it runs for eight hours. Time is the multiplier that decides the bill.

Why does a TNB commercial bill charge for both kW and kWh?

Because the grid has two separate costs, and they answer to two different numbers. Your energy charge pays for the electricity you actually consumed. Your demand charge pays for the size of the pipe TNB had to build to serve your worst moment. One is the odometer, the other is the speedometer, and RP4 bills both.

The energy charge is straightforward: every kWh you consume, metered and priced in sen per kWh (a general medium-voltage rate, the supply level most commercial buildings sit on, is around 29.83 sen/kWh). Use more energy, pay more. This is the part everyone expects.

The demand charge is the part that surprises people, and it is pure power, not energy. Under the RP4 tariff that took effect on 1 July 2025, TNB unbundled the old single maximum demand charge into a Capacity Charge plus a Network Charge, both billed per kW of your maximum demand. For a commercial medium-voltage site that comes to RM89.27 per kW each month (RM29.43 capacity + RM59.84 network), rising to RM97.06 per kW on the time-of-use tariff (whose per-kWh energy rate shifts between peak and off-peak hours, while this per-kW charge stays flat).

Maximum demand is not your total usage. It is the highest 30-minute average power your site hit at any point during the billing month. One bad half-hour, every chiller and compressor and pump kicking in together at 9am, can set a peak of 500 kW and cost you roughly RM45,000 for that month regardless of how careful you were the other 29 days.

Why should the grid care about a single half-hour? Because wires, transformers and substations have to be sized for the peak rate they will ever carry, not the total energy they eventually deliver. A road built for one rush-hour surge still needs those lanes even if it sits empty at 3am. TNB sizes the connection for your speedometer's highest reading, then charges you to reserve that capacity. That is the demand charge. Cutting it means flattening your peaks, which is a scheduling and monitoring problem long before it is a hardware one.

If you want to see exactly where these lines sit and how they add up, we walk through a full bill in how to read your TNB electricity bill.

A quick sense of scale

Energy in kWh is easy to picture once you have a reference. One kWh is 3.6 megajoules (the joule is the underlying SI energy unit, and we untangle why energy carries two names in Part 5). In everyday terms, 1 kWh runs that 1 kW heater for an hour, or ten 100 W bulbs for an hour. A litre of petrol, for contrast, holds about 9.5 kWh of chemical energy, which is why liquid fuel still beats batteries on raw energy density.

Power has its own reference point. One horsepower is about 746 watts, so a "5 hp" motor is roughly 3.7 kW. Horsepower was a sales unit James Watt invented to compare his steam engine to the animal it replaced, and the watt now does the same job across every domain. We tell that story in Part 6.

One more thing: kW has siblings

For a simple resistive load like a heater, power really is just kW. But motors, chillers and transformers introduce a twist. The current they draw does not line up neatly with the voltage, so some of it sloshes back and forth without ever doing useful work. The total power they pull from the grid (measured in kVA) is therefore larger than the useful power they turn into work (kW), and the reactive component (kVAR) accounts for the gap. The three do not relate by simple subtraction; they combine as a right triangle, where kVA² = kW² + kVAR². That is why an operator sees three power figures where a homeowner sees one, and it is why TNB adds a power-factor penalty on top of the two charges above.

Those three are a family, and they get their own article. For now, hold onto the foundation: kW is the rate, kWh is the quantity, and your bill charges for both because the grid pays for both.

This is Part 4 of 23 in Cobler's Electricity Fundamentals series. Previous: What Is an Electric Circuit? Series vs Parallel. Next: Joules vs kWh: Why Energy Has Two Names.

Peaks are a monitoring problem before they are a hardware one. CobiNeural tracks your maximum demand in real time and flags the half-hours that set your bill. Book a demo and we will show you where your kW and kWh are actually going.