Joules, Kilojoules, and Kilowatt-Hours

What you’ll learn

Convert energy units without confusing energy with power.

បម្លែងឯកតាថាមពលដោយមិនច្រឡំជាមួយអានុភាព។

Meaning and use

A kilowatt-hour is energy transferred by one kilowatt for one hour. It is a product, not a power per hour. Joules and kilowatt-hours measure the same kind of quantity in different units. A stated device power can be used with elapsed time to find energy if the appropriate average power is known.

គីឡូវ៉ាត់ម៉ោងជាថាមពលដែលផ្ទេរដោយអានុភាព 1 kW ក្នុង 1 h។ វាជាផលគុណ មិនមែន kW/h ទេ។ Joule និង kWh ជាឯកតាថាមពល។

Formulas

Energy in kWh

\[E_{\mathrm{kWh}}=P_{\mathrm{kW}}t_{\mathrm{h}}\]

Conversions

\[1\,\mathrm{kJ}=1000\,\mathrm{J},\qquad1\,\mathrm{kWh}=3.6\times10^6\,\mathrm{J}\]

Symbols and units

Symbol	Meaning	Unit or restriction
E	Energy transferred	J, kJ, or kWh
P	Constant or interval-average power	W or kW
t	Duration	s with W; h with kW for kWh

Conditions and limits

• Use matching power and time units in each form.
• A rating is not always the actual average power; use the value specified in the model.
• កុំសរសេរ kWh ជា kW/h។

Worked examples

An energy amount

A 2 kW device operates at that power for 3 h.

1. Calculate \(E=2\times3=6\) kWh.
2. Convert \(6\times3.6\times10^6=21600000\) J.

6 kWh = 21.6 MJ

kW គុណ h ផ្ដល់ kWh ដែលជាថាមពល។

From watts to kilowatts

A 500 W device runs at that power for 2 h.

1. Power \(500\,\mathrm{W}=0.5\,\mathrm{kW}\).
2. Energy \(0.5\times2=1\) kWh.

1 kWh

បម្លែង W ទៅ kW ដោយចែក 1000។

Common mistakes

Instead of	Use	Why
kWh is a unit of power.	kWh is a unit of energy.	Power multiplied by time gives energy.
1 kWh = 1000 J.	1 kWh = 3600000 J.	Multiply 1000 W by 3600 s.

Practice

Solve these before revealing the worked solutions. Include units where relevant.

សាកល្បងដោះស្រាយមុនបើកមើលចម្លើយ។ សូមបង្ហាញជំហាន និងឯកតាដែលពាក់ព័ន្ធ។

1. Find energy for 1.5 kW used for 2 h.
2. Convert 2 kJ to J.
3. Find energy for 100 W over 10 s.

Answers and steps

Show solutions / បង្ហាញដំណោះស្រាយ

1. 3 kWh

   1.5 × 2 = 3.

2. 2000 J

   Multiply by 1000.

3. 1000 J

   100 × 10 = 1000; W × s gives J.

Quick recap

• kWh measures energy.
• Keep matching time and power units.
• Use actual or specified average power for the interval.