How to Convert Energy and Power Units

Use energy converter for J, kJ, MJ, cal, kcal, kWh, BTU. Use power converter for W, kW, MW, hp. Both run in the browser; neither asks for an account.

Energy: joules and the aliases

1 J = 1 N·m = 1 W·s.

Useful aliases:

• 1 cal (thermochemical) = 4.184 J
• 1 kcal = 1 food Calorie = 4184 J
• 1 kWh = 3.6 MJ = 3.6 × 10⁶ J (exact)
• 1 BTU ≈ 1055.06 J (International Table BTU is 1055.05585262 J)

A nutrition line of 500 kcal is 500 × 4184 = 2.092 MJ, or 0.581 kWh. That is the chemical energy on the label, not the electrical energy of a microwave. A utility bill in kWh is electrical energy delivered, not food.

If a lab quote is in calories and the report template is joules, convert energy. If the quote is 500 W, you are already on the other page.

Power: watts and horsepower

1 kW = 1000 W. A kettle marked 1500 W is 1.5 kW of electrical power while it is on. It is not 1.5 kWh unless it runs for one hour.

Horsepower is the leftover:

• Mechanical hp ≈ 745.7 W
• Metric hp (PS, CV) = 735.49875 W

A 100 hp (mechanical) engine is about 74.57 kW. A 100 PS engine is 73.55 kW. Marketing that says “100 hp / 75 kW” is rounding; check which hp they used. Electric motors are usually rated in kW already. Do not convert a 5 kW motor to “5 hp.” 5 kW is about 6.7 mechanical hp.

Time is the bridge, not a unit swap

Energy = power × time.

E (J) = P (W) × t (s)

E (kWh) = P (kW) × t (h)

You cannot put 1500 W into the energy converter and get a truthful kWh without saying how long the kettle ran. That is not a limitation of the website. It is the definition of the units.

The inverse also bites: a battery in kWh is a store of energy. A charger in kW is a power. “This 10 kWh battery is a 10 kW battery” is how people undersize inverters.

One numeric check: a 1500 W kettle for 4 minutes

P = 1500 W, t = 4 × 60 = 240 s.

E = 1500 × 240 = 360000 J = 360 kJ.

In kWh: 1.5 kW × (4/60) h = 1.5 × 1/15 = 0.1 kWh.

In kcal (if you insist): 360000 / 4184 ≈ 86.0 kcal. That is a small chocolate bar of electrical energy, which is why “boiling water costs almost nothing per cup” and “the monthly kWh still adds up” can both be true.

Confirm 360000 J → kWh on the energy converter: 0.1. Confirm 1500 W → kW on the power converter: 1.5. If you somehow got 1500 kWh, you treated watts as if they were already an hour-scale energy unit.

Calculations versus conversions

Kinetic energy ½mv² produces joules from kilograms and metres per second. That is how to calculate kinetic energy, then optionally the energy converter if the rubric wants kJ. Electrical power P = VI or mechanical P = Fv is how to calculate power, then the power converter if you need hp.

Temperature of the water is not an energy unit. Use the temperature converter for °C/°F/K, then q = mcΔT if you are doing calorimetry. ΔT in kelvin equals ΔT in °C.

What to write down

Label every result J, kWh, W, or hp, and say which horsepower. 0.1 in a lab notebook will be reused as 0.1 kW. Utility-scale talk (MWh, MW) is the same SI prefixes; a 50 MW plant running 2 hours delivers 100 MWh, not 50 MWh unless the duration is one hour.

The unit converters hub sits next to these two pages so you can leave energy and open pressure or mass without pretending joules convert to pascals.