If you work around electrical systems, utilities, mechanical equipment, or building energy models, you have seen the confusion: someone says a device "uses 10 kW per hour" and everyone nods politely. The phrase is wrong, but it is common. The real issue is not grammar. The issue is that energy and power are different physical quantities, and confusing them changes conclusions about sizing, performance, and cost.
The units are closely related, which is why people slip. A kilowatt (kW) and a kilowatt-hour (kWh) look similar. They share letters and they both show up on invoices. But they are not interchangeable. One is a rate. The other is a quantity.
Power is instantaneous rate
Power tells you how fast energy is being used or produced at a given moment. When a motor is rated at 5 kW, that is a statement about how much work it can do per unit time, or how much electrical power it draws at a given operating point.
Power units include:
- watts (W)
- kilowatts (kW)
- horsepower (hp)
Converting between them is straightforward, but you should always keep the physical meaning clear. A power rating does not tell you how much energy is used over a day. It only tells you the rate when operating.
Energy is accumulated work
Energy is what you get when power acts over time. It is the accumulated quantity. If you run a 5 kW heater for 2 hours, the energy used is:
Energy = power x time
5 kW x 2 h = 10 kWh
Energy units include:
- joules (J)
- kilojoules (kJ)
- megajoules (MJ)
- kilowatt-hours (kWh)
- BTU (British thermal units)
Utility bills often use kWh because it is convenient for electricity consumption. Thermal systems might use BTU or MJ. Engineers often switch between these depending on discipline. The conversion is fine. The confusion is treating power and energy as the same thing.
The simple mistake: "kW per hour"
People say "kW per hour" when they mean one of two things:
- They mean kWh, which is energy.
- They mean a change in power over time, which is a ramp rate. That can be a real quantity, but it is not what most people intend.
A ramp rate might matter in grid operations or in motor starting analysis. For example, a generator might ramp power output at 2 MW per minute. That is a rate of change of power. But a household appliance does not "use kW per hour."
In most engineering work, if you see "kW per hour" in a report, it is a signal to slow down and clarify what the author meant.
A practical example: equipment sizing versus energy cost
Consider an electric heater rated at 12 kW. That rating helps you size wiring and panels. It does not tell you the monthly cost. Cost depends on operating hours and usage pattern.
Suppose the heater runs on average 3 hours per day during a cold season month. The energy consumption is:
12 kW x 3 h/day = 36 kWh/day
Over 30 days, that is 1080 kWh. If electricity costs $0.15 per kWh, the monthly operating cost is about $162. If someone mistakenly treats 12 kW as "12 kWh" and multiplies incorrectly, they can underestimate cost by a large factor.
Power factor and apparent power: the next layer of confusion
In AC electrical systems, you will also see kVA (apparent power) and kVAR (reactive power). These are not the same as kW. Motors and transformers are often rated in kVA because current and heating relate to apparent power. The utility may bill demand in kW.
If you are converting and comparing these quantities, confirm:
- Are you dealing with real power (kW) or apparent power (kVA)?
- What is the power factor at the operating condition?
A conversion tool can translate units. It cannot guess whether your 50 "kW" should have been 50 "kVA".
Sanity checks that prevent the worst mistakes
- Use time explicitly. If you are talking about energy, write the hours or seconds.
- Separate sizing from costing. Size with power, cost with energy.
- Check magnitudes. A home might use tens of kWh per day, not tens of kW continuously.
- Be suspicious of "per hour" phrases. They are often a sign of mixed concepts.
The mildly funny part is that people mix up kW and kWh even though utilities print the difference on every bill. The less funny part is that engineers sometimes inherit those mistakes in project documentation. Treat power and energy as separate quantities and the confusion disappears.